The workshop of the APS Topical Group on Hadronic Physics (GHP) provides an important opportunity for the advancement and dissemination of knowledge related to QCD and hadron physics. This workshop should interest physicists working in fields such as the partonic structure of hadrons and nuclei, heavy-ion physics, lattice gauge theory, jet physics, spectroscopy, and QCD at finite temperature and density. This includes research associated with current and future facilities such as Jefferson Lab, RHIC, the Electron-Ion Collider, Fermilab, and many other facilities around the world.
This workshop will provide a forum to discuss recent results in experiment and theory, and thereby help to enhance communication within our diverse hadron physics community. This workshop will be held remotely and immediately precedes the APS April Meeting which is also virtual.
The call for abstracts is now closed.
Topics for the workshop include:
Electroweak probes
Extreme matter and neutron star collisions
Hadrons in nuclei
Hadron spectroscopy
Hadron tomography and hadronization
Heavy flavor and jet production
Neutrino-hadron interactions
New physics and discrete symmetry violation in hadron physics
Nonequilibrium dynamics
Nucleon and nuclear spin physics
Origin of hadron mass
Physics of the quark-gluon plasma
Quantum information for hadron physics
Small systems and collectivity
Transverse and longitudinal structure of hadrons
Ultraperipheral Collisions
The contact email address for this workshop is: ghp2021@anl.gov
Originally this workshop was to be held as an in-person meeting in Sacramento, CA.
co-host: Dave Gaskell and Tim Hobbs
co-host: Ian Cloët and Dave Gaskell
Understanding the properties of nuclear matter and its emergence through the underlying partonic structure and dynamics of quarks and gluons requires a new experimental facility in hadronic physics known as the Electron-Ion Collider (EIC). The EIC will address some of the most profound questions concerning the emergence of nuclear properties by precisely imaging gluons and quarks inside protons and nuclei such as their distributions in space and momentum, their role in building the nucleon spin and the properties of gluons in nuclei at high energies. In January 2020 the EIC received CD-0 and Brookhaven National Laboratory was selected as site. This presentation will highlight the experimental equipment especially the flow down from physics to detector requirements and the integration into the accelerator and give an update on the status of the EIC project.
Collisions of heavy ions at ultrarelativistic energies at RHIC and at the LHC are an excellent tool to reach extreme temperatures and/or baryon densities, where the QCD matter undergoes a phase transition to a state of deconfined quarks and gluons, called the quark-gluon plasma (QGP). One of the many probes which can provide insight into the properties of this medium, are measurements of anisotropic flow. They are sensitive in particular to transport coefficients of the QGP, and the initial state of a heavy-ion collision, essential for proper theoretical description of the subsequent medium evolution.
Characteristic flow features believed to originate in this collectively expanding medium were also observed in high-multiplicity collisions of small systems, such as pp or pA collisions. It is still under debate whether their origin lies in the creation of a medium with similar nature as the one created in heavy-ion collisions.
In this talk, I will discuss recent results in this area. I will highlight their improving ability to provide precise constraints to theoretical calculations of large systems and to help to pin down the origin of the observations in small systems and implications to our current understanding of the QGP. I will conclude with an outlook to the potential future investigations in this topic.
The vast majority of hadrons are not stable with respect to the strong interactions, and are seen as resonant enhancements in the scattering amplitudes of the lightest stable hadrons. Recent developments have enabled the determination of hadron resonance properties from lattice QCD, a first-principles approach whereby finite volume spectra are computed numerically, that are then used to constrain the infinite volume scattering amplitudes. A summary of the approach and several examples will be presented for resonances seen in coupled-channel scattering amplitudes in scalar ($J^P=0^+$), axial-vector ($J^P=1^+$) and exotic ($J^{PC}=1^{-+}$) quantum numbers.
Precision tests of the Standard Model provide excellent opportunities to discover beyond-the-Standard-Model (BSM) physics. The current experimental program is very rich encompassing searches for CP-violation, probes of the Majorana nature of neutrinos, to the stability of atomic nuclei. The interpretation of this program requires a solid understanding of how BSM physics, typically written at the level of elementary fields, manifests at the level of complex systems such as hadrons and nuclei. I will discuss recent developments in this understanding and the corresponding interplay of the particle, hadronic, and nuclear physics communities.
co-hosts: Ian Cloët and Dave Gaskell
co-host: Astrid Hiller Blin
The GlueX experiment at Jefferson Lab aims to study the light meson spectrum with an emphasis on the search for hybrid exotic mesons. A tagged photon beam, with energies in the range 3—11.6 GeV, and linearly polarized near 8.5 GeV, is incident on a hydrogen target inside a detector with near-complete neutral and charged particle coverage. The experiment has completed its first phase of data taking, producing orders of magnitude more data than previous photoproduction experiments in this energy regime. A good theoretical description of the production mechanisms will be needed to interpret any potential signals for exotic mesons. Photoproduction data on conventional mesons are very useful for this purpose, where polarization observables and the energy and t-dependence of production cross sections provide complementary information. We present new, high-statistics extractions of polarization observables and cross sections in photoproduction of vector mesons at GlueX and discuss the implications for our understanding of the production mechanisms.
The GlueX experiment studies the light meson spectrum and searches for hybrid and exotic mesons. As part of its program, photoproduction cross sections of $\eta$ mesons have been measured in the $\gamma + p \rightarrow \eta + p$ reaction in a new, previously unexplored kinematic regime at beam energies up to 11 GeV, and production polar angles down to 8$^\circ$. The new cross section results will be presented in regions corresponding to t- and u-channel exchange. The $\eta$ mesons have been identified through their $\gamma \gamma$ decay channel which has a branching ratio of 39.41%. Besides the high energy data, an additional set of low energy (3 to 5.5 GeV) photoproduction data has been collected that are overlapping with previously published data. The new cross section results at lower beam energies will be compared to previously published data and all will be compared with recent model calculations.
With a need to understand the physics attributed to the light-quark meson spectrum, multiple past experiments have searched into the possibility of non quark-antiquark (exotic) quantum numbers. The search for exotic hybrid mesons at the GlueX experiment has advanced past previous experiments' data collection with the initial phase of data accumulation being completed during 2018; higher statistics are being viewed along with larger acceptances for all final states. A strong interest has been placed on both the $\pi^0 \eta$ and $\pi^0 \eta'$ systems due to their strong possibility of containing the desired exotic quantum numbers. By comparing both of these channels, the role of flavor symmetry could also be highlighted, allowing for a better understanding of meson production mechanisms. Preliminary results will be shown for $\gamma p \rightarrow \pi^0 \eta^{(_{'})}p \rightarrow 4\gamma \pi^+ \pi^- p$, utilizing all of the GlueX Phase-1 data as well as comparison to observations seen in previous experimental (COMPASS) and theoretical (JPAC) results of the same channels.
The GlueX experiment in Hall D, Jefferson Lab aims to map the meson spectrum, with a focus on exotic mesons and hybrids which are not allowed in a simple quark-antiquark model. In this talk, we present efforts to characterize the $\omega \pi^{0}$ decay channel of the $b_{1}(1235)$ meson as precursor to a search for the $\pi_{1}(1600)$ exotic hybrid meson candidate, which is predicted to decay dominantly to $b_{1} \pi$.
The cross-section of the $\omega \pi^{0}$ channel has been extracted as a function of photon beam energy and is of the order of 1 $\mu$b, in agreement with previous measurements. The differential cross section as a function of momentum transfer indicates the presence of two production processes based on a changing slope around 1 $GeV^{2}/c^{2}$. The analysis steps, corrections and systematic errors will be presented, along with preliminary amplitude analysis of the neutral and charged $\omega \pi$ final state.
A proposal for a secondary beam of neutral kaons in Hall D at Jefferson Lab was recently approved by the JLab PAC. The experiment will use the GlueX experimental setup to study hyperon spectroscopy by measuring both the differential cross sections and self-analyzed polarizations of the produced $\Lambda$, $\Sigma$, $\Xi$, and $\Omega$ hyperons using the GlueX detector at the Jefferson Lab Hall D covering $W$ up to 2500 MeV. The proposed facility will also study the strange meson sector through measurements of the $K\pi$ system up to invariant masses of 2 GeV. In this talk, the proposed experimental program will be discussed along with the status of the facility.
co-host: Fatma Aslan
The distance, z, along the lightcone that the quark fields that enter the correlator describing the Parton Distribution Functions (PDFs) are separated by is a natural variable for separating the non-perturbative (long-distance) and perturbative (short-distance) physics. We study how the behavior in z of a PDF can be mapped out given its Mellin moments. Pseudo PDFs describe the nucleon matrix elements of quark field operators that are separated by a space like distance z. These are calculable in lattice QCD and as z^2 approaches zero, pseudo PDFs approach the actual PDFs. Complementary to lattice efforts, we study the behavior of pseudo PDFs as a function of z^2 in a spectator diquark model. We also extend the study to Generalized Parton Distributions (GPDs), which involves taking into account an extra degree of freedom because of the non diagonal nature of the hadronic matrix element in the case of GPDs.
The non-perturbative part of the cross-section of high-energy processes may be expanded in terms of the process's large energy scale. This gives rise to a tower of distribution functions, labeled by their twist (mass dimension minus spin). The leading twist (twist-2) contributions have been at the center of experimental measurements, theoretical investigations, and lattice QCD calculations. It has been recognized that twist-3 contributions to distribution functions can be sizable and should not be neglected. However, it is challenging to disentangle them experimentally from their leading counterparts, posing limitations on the structure of the proton.
Calculating the x-dependence of PDFs and GPDs from lattice QCD has become feasible in the last years due to novel approaches. In this work, we employ the pioneering approach of quasi-distributions proposed by X. Ji in 2013. This method relies on matrix elements of fast-moving hadrons coupled to non-local operators. The quasi-distributions are matched to the light-cone distributions using Large Momentum Effective Theory (LaMET). The approach has been extensively used for twist-2 PDFs, as well as twist-2 GPDs. More recently, we demonstrated the feasibility of the approach for twist-3 PDFs. In this talk, we present results on the first-ever lattice QCD calculation of twist-3 GPDs. The calculation is performed using one ensemble of two degenerate light, a strange and a charm quark (Nf=2+1+1) of maximally twisted mass fermions with a clover term, reproducing a pion mass of 260 MeV.
Studying the role of gluons in exclusive scattering processes is essential as new physics programs, such as an electron ion collider, are planned in unprecedented kinematic regimes. We present a parameterization of gluon generalized parton distributions (GPDs) calculated using a reggeized spectator model. This parameterization is constrained using a combination of lattice QCD form factor calculations and extracted deep inelastic parton distributions. We evolve our parameterization at leading order in $Q^2$ to the scale of experimental data using a perturbative QCD evolution framework. Through Fourier transform of the transverse momentum transfer $\Delta_{T}$, we can develop images of the transverse spatial distribution of the gluons. Understanding the behavior of gluon GPDs is a first step towards extracting the gluon contribution to deeply virtual Compton scattering (DVCS) and timelike Compton scattering (TCS) observables.
Using the QCD equations of motion in the quark sector, Lorentz symmetry and discrete symmetries, one can derive an expression of partonic angular momentum in which the total, J, orbital, L, and spin, S components can be associated with specific twist two and twist three Generalized Parton Distributions. Within this approach, the different terms are derived using nonlocal matrix elements only, emphasizing the role of transverse momentum-spin correlations. I will discuss the angular momentum decomposition obtained in this approach for both the longitudinal and transverse cases.
co-hosts: Bjoern Schenke and Julia Velkovska
Collider data shows that elliptic flow in high-multiplicity proton-nucleus and nucleus-nucleus collisions emerges as a response to the spatial ellipticity characterizing the energy density deposited in the transverse plane by the collision process. For collisions at small multiplicities, however, elliptic flow is expected to receive as well an important contribution from the anisotropic structures of the full energy-momentum tensor of the system, beyond the mere energy density. I show that, in the color glass condensate effective theory of high-energy QCD, these structures emerge from the correlation of gluon fields in the earliest stages of the collision, so that obtaining evidence of their manifestation in experiments is of the utmost importance. A recent study within the IP-Glasma+MUSIC+urQMD framework shows that signatures of these primordial anisotropies can be found experimentally in $v_2$-$\langle p_t \rangle$ correlations, whose sign at low multiplicities depends on whether or not the full structure of the IP-Glasma energy-momentum tensor is included in the simulations. I review these results, and use them to argue that the discovery of the primordial anisotropic flow of heavy-ion collisions is a simple matter of defining the right correlations that magnify its signatures.
It is widely known that the quark-gluon plasma (QGP), a state of deconfined quarks and gluons under equilibrium, is generated in relativistic heavy-ion collisions. Compared to the heavy-ion, small colliding systems such as nucleon-induced collisions were regarded as a reference for the heavy-ion collisions. Since around 2010, however, it has been reported that the QGP might be formed even in the small colliding systems according to various experimental data. It is still being discussed whether the signals truly are consequences of the QGP formation. We approach this problem by building a dynamical model which is capable of performing p–p to A–A collisions within a unified manner to acquire comprehensive understandings of the observed data. Motivated by one of the important experimental results, strangeness enhancement reported by the ALICE Collaboration [1], we apply the core–corona picture to a dynamical initialization of hydrodynamic simulations of the QGP. The core–corona is a two-component picture of the thermalized and non-thermalized matter. Under the conventional core–corona picture, high-density regions in which matter reaches thermal equilibrium are referred to as core while low-density regions in which thermalization is not achieved are referred to as corona. In our framework, initially produced partons dynamically generate QGP fluids as they traverse vacuum/medium. Generated fluids are regarded as an initial state of hydrodynamics while surviving partons go through string fragmentation. Due to the dynamical separation of core and corona, our model, the dynamical core-corona initialization (DCCI) [2, 3] is applicable for the entire multiplicity regime from small to large colliding systems. In this talk, I highlight some recent results in proton-proton (pp) and lead-lead (PbPb) collisions from an updated version of our model (DCCI2). I mainly discuss how each contribution from core and corona affects multiplicity-dependent bulk observable in small systems. Also, I discuss the existence of corona possibly affect transverse momentum distribution and anisotropy even in PbPb collisions.
[1] J. Adam et al., Nature Phys., 13:535{539, 2017.
[2] Y. Kanakubo et al., PTEP, 2018(12):121D01, 2018.
[3] Y. Kanakubo et al., Phys. Rev. C, 101(2):024912, 2020.
Collisions of heavy nuclei at relativistic energies create the quark-gluon plasma, the phase of matter that existed in the first few microseconds after the big bang. One of the major pillars of heavy-ion physics today is the study of so-called small systems, consisting of collisions of a light nucleus on a heavy nucleus or even of two light nuclei. In this talk we will give a short overview of a variety of results on small systems from the Relativistic Heavy Ion Collider at Brookhaven National Lab.
Hot QCD medium effects were extensively studied in heavy nucleus-nucleus collisions while the measurements in proton-nucleus (pA) collisions were primarily to study cold nuclear effects. However, the observations of the collective flow of light flavor particles in both proton-proton (pp) and pA collisions raised the question whether the hot medium effects are negligible in small-collision systems. Other explanations for the observed collectivity were also proposed, based on initial momentum correlations. Heavy flavor quarks are almost entirely created via the initial hard processes. They evolve with the whole system, providing unique opportunities to probe both initial-state physics and in-medium effects. We will discuss the measurements for collectivity and productions of heavy flavor particles, including both open heavy flavor hadrons and quarkonia, in pp and proton-lead collisions at the LHC.
Understanding the onset of collective flow from small to large heavy ion collisions is crucial for the study of the smallest QGP droplet. As particle multiplicity decreases in smaller systems, flow signal becomes smaller and nonflow background is larger. The nonflow correlations can be significant even when imposing a large pseudorapidity gap between particles when using two-particle correlations in small systems. The experiments at the Large Hadron Collider have developed two methods for subtracting nonflow contribution in two-particle correlations. The methods have also been tested at the lower collision energy at the Relativistic Heavy Ion Collider. For multiparticle correlations, using multiple subevents has been shown to remove much more nonflow compared to the standard cumulant method in small systems. Directly looping over particle azimuthal angles could remove most of the nonflow when pseudorapidity gaps are applied between particles. I will review the nonflow subtraction techniques for both two particle and multiparticle correlation methods in this talk.
co-host: Ramona Vogt
The electroweak model operates with only a few fundamental parameters. Extracting them from high-precision measurements in various systems and at different kinematics requires computing quantum corrections (both universal and process-specific) to leading-order SM predictions with an adequate precision. Recent advances in computing process-specific electroweak box corrections led to establishing a 3$\sigma$ deficit in the unitarity of the Cabibbo-Kobayashi-Maskawa mixing matrix in the top row, $|V_{ud}|^2+|V_{us}|^2+|V_{ub}|^2=0.9985(5)$, which may be a hint to BSM physics. Electroweak boxes depend on fine details of strong interaction and hadronic structure, and are sensitive to a wide range of energy scales. Modern calculations combine theory inputs valid at each scale (perturbative and lattice QCD, dispersion relations and nuclear models), together with experimental measurements. I will give an overview of recent developments and open questions.
The emergence of gravitational wave observatories provides an exquisite opportunity to probe the nuclear equation of state at a few times nuclear saturation density with mergers involving neutron stars. If nucleons possess interactions beyond those prescribed by the Standard Model, then these modify the equation of state – and thus macroscopic neutron star properties – in a way that may be detectable in these events. These new forces also modify a number of hadronic processes, including η and η’ decays, such that they can be probed at, e.g., JLab. In this talk, I will discuss what neutron star observations say about the possibility of quark-coupled new physics and how current and future measurements of rare meson decays can test these new interactions.
The MUon Scattering Experiment (MUSE), which takes place at the PiM1 beamline of the Paul Scherrer Institut (PSI), aims to simultaneously measure elastic $ep$ and $\mu p$ scattering in order to determine the proton charge radius. However with the beamline and kinematics available to the experiment, MUSE has a broad physics reach. As the experiment uses both positively and negatively charged leptons, a precise two photon exchange measurement can be performed for both electrons and muons in the $0.002 < Q^2 < 0.08$ (GeV/c)$^2$ and $0.26 < \varepsilon < 0.94$ regime. The experiment has both a LH$_2$ target and a carbon target, allowing for a variety of precise cross section measurements. With access to $\pi^{\pm}$ in the beam it is also possible to measure absolute and relative elastic pion cross sections to high precision with the MUSE detector. In this talk the physics reach of MUSE and projected uncertainties for the measurements will be discussed.
The η-Meson is a unique tool to study low energy QCD phenomena and to test the corre- sponding theoretical models and predictions.
The isospin violating decay η → π+π−π0 is driven by the strong force and allows a probe of the light quark masses, via measurement of the corresponding decay amplitude. This amplitude is accessible either via a Dalitz plot or partial wave analysis. The latter method allows for a direct calculation of the quark mass ratio Q, whereas the parameters from the Dalitz Plot analysis give insights into the decay dynamics and can be compared to theoretical calculations.
These η-decays have been produced and measured in the reaction γp → pη with the GlueX experiment, located at the Thomas Jefferson National Laboratory. GlueX finished its first phase of data taking in the latter part of 2018.
This talk will discuss the status and prospects of the η → π+π−π0 Dalitz Plot analysis using the GlueX Phase I data set, as well as the application of partial wave analysis.
The Hall D Jefferson Lab Eta Factory (JEF) will collect 500 pb${^{-1}}$ of data between 8 and 12 GeV at incident photon-beam energies with the GlueX apparatus and an upgraded Forward Calorimeter. The JEF physics program addresses new physics and symmetry violation in hadron physics, including hadron decay dynamics. The search for sub-GeV dark particles will be performed in three different processes: (1) Primakoff-like process e.g. $\gamma p \to a p$ via $t$-channel photon exchange for axion-like particles (ALPs) coupling to photons, (2) Compton-like process e.g. $\gamma e^- \to A^{'} e^-$ for dark photons ($A^{'}$) coupling to the Standard Model charged current, and (3) $\eta^{(')}$ meson decays e.g. $\eta^{(')} \rightarrow B^{'}\gamma$ for leptophobobic gauge vector boson ($B^{'}$) coupling to the baryon number. The study of C violating $\eta^{(')}$-meson neutral decays e.g. $\eta^{(')}\rightarrow 3\gamma$ aims to further constraining CVPC new physics. The data set will enable precision tests of low-energy QCD via $\eta \rightarrow \gamma\gamma\pi^0$ which is sensitive to higher order terms of chiral perturbation theory. Among the important aspects of hadron decay dynamics are Dalitz plot analyses, in particular for the decay $\eta \rightarrow 3\pi$ which gives access to the quark mass ratio. The talk will present the physics objectives and status of the JEF experimental program.
co-host: Tim Hobbs
Collinear Parton Distribution Functions of the nucleon continue to be a very active field of theoretical and experimental research. Because of their universality, they can be extracted from - and affect the interpretation of - a wide range of experiments: hadron collisions, Drell-Yan and W production, and Deep Inelastic Scattering. A rich experimental program is underway at high-energy colliders like the LHC and RHIC, as well as fixed-target experiments at COMPASS/AMBER and Jefferson Lab. In the future, new data from the EIC will vastly augment the kinematic reach of the existing data. I will give a short overview of the present experimental status, with a few selected examples of recent and upcoming measurements.
We present a new global QCD analysis of spin-averaged and spin-dependent PDFs from high-$x$ DIS and $W$ production data using a Monte Carlo approach. This analysis includes the first extraction of the helicity-dependent antiquark asymmetry $\Delta \bar{u} - \Delta \bar{d}$ using $W$ production data in $\vec{p} p$ collisions at RHIC. We also focus on the high-$x$, low-$W$ region, where effects from power corrections, such as target mass corrections (TMCs) and higher twists, are particularly important, especially for the $d/u$ PDF ratio. We quantify the effects on the extracted PDFs from various theoretical treatments of the power corrections and cuts on the experimental kinematics.
We present an impact study of future EIC measurements on our knowledge of PDFs, using the JAM Monte Carlo global QCD analysis framework. We study the effect of EIC pseudo-data for polarization asymmetries on quark and gluon helicity distributions in the proton. An overview of the impact of future inclusive DIS and parity-violating DIS data on unpolarized PDFs is also shown.
Two new extractions of the QCD coupling constant at the Z pole, $\alpha_S(m_Z)$, will be presented from detailed comparisons of inclusive W and Z hadronic decays data to state-of-the-art perturbative Quantum Chromodynamics calculations at next-to-next-to-next-to-leading order (N$^{3}$LO) accuracy, incorporating the latest experimental and theoretical developments. In the W boson case, the total width computed at N$^{3}$LO is used for the first time in the extraction. For the Z boson pseudo-observables, the N$^{3}$LO results are complemented with the full two- and partial three-loop electroweak corrections recently made available, and the experimental values are updated to account for newly estimated LEP luminosity biases. A combined reanalysis of the Z boson data yields $\alpha_S(m_Z) = 0.1203 \pm 0.0028$, with a 2.3\% uncertainty reduced by about 7\% compared to the previous state-of-the-art. From the combined W boson data, a value of $\alpha_S(m_Z) = 0.101 \pm 0.027$ is extracted, with still large experimental uncertainties but also reduced compared to previous works. The levels of theoretical and parametric precision required in the context of QCD coupling determinations with permil uncertainties from high-statistics W and Z boson samples expected at future $e^+e^-$ colliders such as the FCC-ee, will be discussed in detail.
We develop a non-perturbative model for valence PDFs based on the quark interactions in the mean field of the nucleonic interior. The model is based on the separation of valence 3q system from the residual system which is the source of the mean field. The PDFs are calculated within effective light-front diagrammatic approach which allows to introduce light-front valence quark and residual wave functions. The model allows us to obtain a new relation between the position of the peak of xq$_{\mathrm{V}}$(x) distribution of the valence quark and the effective mass of the residual system, m$_{\mathrm{R}}$: x$_{\mathrm{p}}$ ~ 1/4(1 - m$_{\mathrm{R}}$/m$_{\mathrm{N}})$ and explains the difference in the peak positions for d- and u- quarks due to expected larger residual mass in the case of valence d- quarks. We evaluated the Q$^{\mathrm{2}}$ dependence of the mass of the residual system and its effective size which gives a new insight on the effects of the QCD evolution on mean field of the nucleon. The evaluated wave functions of valence 3q- and residual systems can be used in calculations of other observables such as nucleon form factors, generalized and transverse momentum distributions.
co-host: Dave Gaskell
Radiative transitions between strongly stable hadrons and hadronic resonances offer a direct probe of the charge distribution within the resonance and can provide insightful revelations on the composition of hadronic resonances. This talk presents a toy-model investigation on the determination of 1 to 2 infinite-volume transition amplitudes from finite-volume matrix elements as would be performed in lattice QCD calculations. While the necessary formalism exists, there remains a challenge: most resonances couple to multiple two-hadron channels. Due to the reduction of rotational symmetry and the absence of asymptotic states in finite-volume Euclidean space-time, there is no one-to-one correspondence between the desired 1 to 2 infinite-volume coupled-channel transition amplitudes and the readily available finite-volume matrix elements of external currents. We show that by using parameterizations of the amplitudes that satisfy analyticity and unitarity, reasonable reconstruction of the underlying amplitude given sufficient effort is possible.
Based on work in progress in collaboration with
R. Briceño, J. Dudek, B. Slimmer
(for the Hadron Spectrum Collaboration)
Electroweak form-factors provide information about the internal structure of hadrons; however, short-lived strong resonances are challenging to probe experimentally. In this talk, I will outline a procedure to calculate the elastic form-factors of resonances directly from QCD. This approach relies on computing finite volume matrix elements via lattice QCD, and a formalism to relate these to infinite volume transitions mediated by single local current. After mentioning the key aspects of this formalism, I will describe the decomposition of the two-hadron transitions into generalized form-factors and known kinematic functions. This decomposition allows for a rigorous extraction of elastic form-factors of two-hadron resonances.
The neutron is a cornerstone in our depiction of the visible universe. Despite the neutron zero-net electric charge, the asymmetric distribution of the positively- (up) and negatively-charged (down) quarks, a result of the complex quark-gluon dynamics, lead to a negative value for its squared charge radius. The precise measurement of the neutron's charge radius thus emerges as an essential part of unraveling its structure. The measurement of the neutron charge radius presents challenges, since no atomic method is possible and the electron scattering method suffers from severe limitations due to the absence of a free neutron target. Thus, it’s extraction has been based so far on the measurement of the neutron-electron scattering length, where low-energy neutrons are scattered by electrons bound in diamagnetic atoms. The results of this method exhibit tension, pointing to an underestimation of the underlying systematic uncertainties inherent to the extraction technique. Recent experimental and theoretical progress allows to extract the neutron charge radius from electron scattering experiments, thus making possible to address these discrepancies, and open up the path for a further improvement of the neutron charge radius determination. Results and prospects on the extraction of the neutron charge radius will be presented in this talk.
Calculations of nucleon structure using lattice QCD are approaching the level of precision necessary to make important impacts on experimental programs. This talk will explore multiple theoretical approaches to calculations of the nucleon axial charge. We present numerical evidence and show that the various approaches have significantly different dependence on excited-state, yielding to ground-state plateaus that are observed to be multiple fm closer to the source location. Additionally, the results further provides evidence supporting the notion that lattice QCD calculations are able to keep excited-state contamination for nucleon correlation functions under control by demonstrating consistency in ground state observables from the different approaches presented today.
The striking discrepancy in the proton form factor ratio, $\mu_p G_E^p/G_M^p$, measured using unpolarized and polarized techniques is still not resolved. The proposed explanation is that hard two-photon exchange (TPE) is responsible. Hard TPE is difficult to calculate without significant model dependence, and has generally not been included as a radiative correction. Furthermore, three recent experiments found only a small contribution but were limited to relatively low $Q^2$ where the discrepancy is not clear. A new test beam experiment, TPEX@DESY, would use a planned extracted beam at the DESY test beam facility together with a liquid hydrogen target and high precision lead tungstate calorimeters to measure hard TPE at higher beam energies. This would permit measurements in a $Q^2$ regime where the discrepancy in the proton form factor ratio is significant and where the expected hard TPE contribution is predicted to be large. The motivation and overview of the proposed measurements will be presented. The TPEX experiment is supported by DOE, NSF and DESY. This work is partially supported by the National Science Foundation under Grant 1807338.
co-host: Susan Shadmand
For the first time using lattice QCD we determine an exotic $1^{-+}$ hybrid meson appearing as a resonance in coupled-channel scattering. The calculation, performed with an exact SU(3) flavor symmetry, suggests that the hybrid has a large coupling to at least one axial-vector—pseudoscalar channel. A simple extrapolation to the physical light quark mass suggests a broad $\pi_1$ decaying dominantly through the $b_1 \pi$ mode.
based upon material appearing in
arXiv:2009.10034
A.J. Woss, J.J. Dudek, R.G. Edwards, C.E. Thomas and D.J. Wilson
for the Hadron Spectrum Collaboration
I will discuss some recent first-principles lattice QCD calculations of $DK$ and $D\bar{K}$ scattering, relevant for the enigmatic $D_{s0}(2317)$, with light-quark masses corresponding to $m_\pi = 239$ MeV and $m_\pi = 391$ MeV. The S-waves contain interesting features including a near-threshold $J^P = 0^+$ bound state in isospin-0 $DK$, corresponding to the $D_{s0}(2317)$, with an effect that is clearly visible above threshold, and suggestions of a $0^+$ virtual bound state in isospin-0 $D\bar{K}$. The S-wave isospin-1 $D\bar{K}$ amplitude is found to be weakly repulsive. There is a deeply bound $D^\ast$ vector resonance, but negligibly small P-wave $DK$ interactions are observed in the energy region considered; the P and D-wave $D\bar{K}$ amplitudes are also small.
Based on material in arXiv:2008.06432
G. K. C. Cheung, C. E. Thomas, D. J. Wilson, G. Moir, M. Peardon, S. M. Ryan,
for the Hadron Spectrum Collaboration
For the first time, we find a complex $D_0^\star$ resonance pole in elastic Isospin-1/2 $D\pi$ scattering, using Lattice QCD with a pion mass $m_\pi\approx239$ MeV and the Lüscher finite-volume quantisation condition. The resonance, which is lighter than the $D_{s0}^\star$ found on the same lattice, is strongly coupled to the $S$-wave $D\pi$ channel. We find that both $q\bar{q}$-like and $D\pi$-like constructions are necessary to interpolate the corresponding spectrum from the vacuum.
based upon work in preparation
L. Gayer, N. Lang, S. Ryan, D. Tims, C. E. Thomas, D. J. Wilson
for the Hadron Spectrum Collaboration
The situation regarding the lightest strange resonance, the $\kappa/K_0^*(700)$ has been long debated for the last few decades, and although its existence is nowadays widely accepted, the data driven determination of its parameters is not so well known. In this talk we present a precise and model-independent determination of its pole parameters, therefore proving its existence. We use both partial-wave hyperbolic and fixed-$t$ dispersion relations as constraints on combined fits to $\pi K\rightarrow\pi K$ and $\pi\pi\rightarrow K\bar K$ data. We then use the former equations to perform the analytic continuation of the isospin $I=1/2$ partial waves to the complex plane, in order to determine the $\kappa/K_0^*(700)$ and $K^*(892)$ resonances. A comparison between our dispersive scattering lengths and Lattice QCD predictions are also performed.
The feasibility of describing the LHCb J/psi-N pentaquark and DK states in terms of triangle production and final state interactions is assessed. We find that reasonable descriptions of both sets of states is possible, but that there is substantial model ambiguity, making it difficult to assign definitive interpretations to these signals, absent additional information.
co-host: Lamiaa El Fassi
We present the results that are necessary in the ongoing lattice calculations of the gluon parton distribution functions (PDFs) within the pseudo-PDF approach. We give a classification of possible two-gluon correlator functions and identify those that contain the invariant amplitude determining the gluon PDF in the light-cone $z^2\to0$ limit. One-loop calculations have been performed in the coordinate representation and in an explicitly gauge-invariant form. We made an effort to separate ultraviolet (UV) and infrared (IR) sources of the $\ln(−z^2)$-dependence at short distances $z^2$. The UV terms cancel in the reduced Ioffe-time distribution (ITD), and we obtain the matching relation between the reduced ITD and the light-cone ITD. Using a kernel form, we get a direct connection between lattice data for the reduced ITD and the normalized gluon PDF. We also show that our results may be used for a rather straightforward calculation of the one-loop matching relations for quasi-PDFs.
The pseudo-distribution formalism is one such lattice methodology capable of extracting light-cone distributions from matrix elements of suitably constructed Euclidean non-local operators of a spacelike extent. Leveraging the distillation spatial smearing program, we extract the unpolarized isovector valence quark PDF of the nucleon via a direct 1-loop matching of the computed Ioffe-time pseudo-distribution and model PDFs. We benchmark the efficacy and systematics inherent to this choice by also extracting the PDF from the matched light-cone Ioffe-time distribution. The tempering of excited-states and improved spatial sampling afforded by distillation lead to higher-quality Ioffe-time distributions relative to the literature, thereby bolstering confidence in extracted PDFs; comparisons with several phenomenological determinations are also made. Prospects of the pseudo-distribution paradigm when applied to the off-forward case will be discussed.
We investigate the so-called Lorentz invariance relations from the standpoint of the proper definitions of partonic correlations functions resulting from factorization; that is in light the proper treatment of ultraviolet divergences. We show that there are corrections to the naive Lorentz invariance relations are nontrivial even in very simple renormalizable quantum field theories. We also discuss the implications for phenomenological applications.
We revisit the derivation of collinear factorization for Deep Inelastic Scattering to more faithfully represent the partonic kinematic at sub-asymptotic energy than possible in the conventional approach. We verify the validity of the obtained factorization formula by considering a diquark spectator model designed to reproduce the main features of electron-proton scattering at large $x_B$ in Quantum Chromo-Dynamics, by comparing analytical calculations of the full process and factorized cross sections. We propose a new scaling variables that maximizes the kinematic range of validity of collinear factorization, and highlight the intrinsic limitations of collinear factorization due to the inevitably approximate treatment of four momentum conservation in factorized diagrams.
The quasi-PDF approach, proposed by Ji in 2013, is at the forefront of numerical calculation of partonic structure of strongly interacting systems from lattice QCD. This approach relies on the extraction of matrix elements of space-like operators for fast-moving hadrons. Such auxiliary quantities can be related to the light-cone PDFs through a perturbatively calculable matching coefficient. We explore the formalism of matching, for the first time, for the twist-3 PDFs $g_{T}(x)$, $e(x)$ and $h_{L}(x)$. In this talk, we address the theoretical aspects and the subtleties involved in the extraction of the matching coefficient due to the presence of the singular zero-mode contributions.
Parton distribution functions (PDFs) are important quantities in hadron physics, quantifying the momentum distribution of quarks and gluons inside hadrons. In this talk, we present a lattice QCD calculation of twist-3 $g_T(x)$ and $h_L(x)$ PDFs of the proton, that are of great interest in phenomenology, as they encode new information on quark-gluon-quark correlations, accessible experimentally in inclusive, semi-inclusive DIS and Drell-Yan processes.
In our work, we employ the quasi-distribution formalism and compute correlation functions between two boosted proton states, that are eventually matched onto light-cone distributions through perturbative matching formulae developed in Large Momentum Effective Theory (LaMET). We use an ensemble of gauge configurations with two degenerate light quarks, a strange and a charm quark $(N_f= 2+1+1)$ within the twisted mass formulation, with lattice spacing $a=0.093$ fm and pion mass $M_{\pi}=270$ MeV. The proton is boosted to 0.83, 1.25 and 1.67 GeV.
Twist-3 PDFs are compared to their twist-2 counterparts PDFs and, for the first time, the Wandzura-Wilczek approximation is tested using an ab initio calculation within lattice QCD.
co-host: Garth Huber
We present a lattice determination of pion valence parton distribution function (PDF) and electomagnetic form-factors. Our study use NNLO leading-twist perturbative matching formula to extract first few moments and reconstruct the x-dependent PDF of pion. Three mixed action ensambles including a physical pion mass with fine lattice spacings of a = 0.04, 0.06 and 0.076 fm are used to investigate the mass dependence as well as approaching continuum limit.
We present a lattice QCD calculation of the pion and kaon form factor and explore the generalized form factors of the one-derivative vector operator. We also give an update of our results of $\langle x \rangle$, $\langle x^2 \rangle$, and $\langle x^3 \rangle$. We use an ensemble of two degenerate light, a strange and a charm quark $(N_f=2+1+1)$ of maximally twisted mass fermions with clover improvement. The quark masses are chosen to reproduce a pion mass of about 260 MeV and a kaon mass of 530 MeV. We analyze several values of the source-sink time separation within the range of 1.12 - 2.23 fm to study and eliminate excited-states contributions.
Following our recent Monte Carlo determination of the pion’s PDFs from Drell-Yan (DY) and leading neutron electroproduction data, we extend the analysis by including effects from threshold resummation. At higher orders in the strong coupling, $\alpha_s$, soft gluon emissions cause large logarithmic corrections, which become important in the $q\bar{q}$ channel of the DY partonic cross section near threshold. These corrections can be summed over all orders of $\alpha_s$. However, different prescriptions exist for how the threshold resummation is implemented, for instance, using varying levels of approximation in the Minimal Prescription with cosine, expansion, and double Mellin methods. We present the Monte Carlo results of the first simultaneous fit of the valence, sea, and gluon distributions in the pion taking into account the ambiguities in the resummation calculations. We present the wide ranges of valence distributions at large $x$ and the effective behavior of the valence distribution as $x$ approaches $1$.
In this talk, we present a model-independent calculation of the $x$-dependence of pion valence PDF with the large-momentum effective theory approach. In this calculation we adopt the most up-to-date theoretical developments on the systematic corrections, which include the hybrid renormalization scheme that rigorously renormalizes the lattice matrix elements at both short and long distances, as well as the inverted two-loop matching kernel that allows for extraction of the PDF without any model assumption. Therefore, we are able to make predictions for the PDF within a range of $x\in [x_{\rm min}, x_{\rm max}]$ where the systematic uncertainties are under control. This is a firm step towards the stage of precision calculation of PDFs.
The lattice computation of PDFs is still emerging and in the process of being complementary to the experimental determinations. In the meanwhile, we discuss how the lattice PDF computations can immediately serve the purpose of understanding the nonperturbative origins of parton structures by extending it to theoretically interesting cases that are otherwise not accessible experimentally. For this I will discuss two cases: (1) Understanding the differences between the quark structures of pion and its radial excitation. (2) Understanding how the short-distance quark structure of pion is intimately connected to the infrared symmetry-breaking physics by theoretically tuning the strength of symmetry-breaking.
The analytic structure of two-body bound state amplitudes is embedded in the Nakanishi integral representation in terms of real functions. Within the rainbow-ladder truncation, we derive the integral equations for these Nakanishi spectral functions from the corresponding Bethe-Salpeter equations in Minkowski space. The resulting integral equations are then solved numerically using adaptive mesh grids with solutions verified in Euclidean space.
co-host: Vincent Cheung
Quarkonia are among the most important tools for studying Quantum Chromodynamics (QCD) in high energy hadronic collisions. Despite decades of extensive studies, we still have a limited knowledge of their production mechanism and hadronization; and carrying out as many measurements as possible in $p$+$p$ collisions over a broad kinematic region at different energies is essential to understand their production mechanisms. The PHENIX experiment has measured inclusive $J/\psi$ production as well as its angular decay coefficients at mid (|y|<0.35) and forward (1.2<|y|<2.2) rapidities in $p$+$p$ collisions at 200 GeV and 510 GeV. Recent results from these measurements will be presented.
One of the best ways to understand hadronization in QCD is to study the production of quarkonium. The color evaporation model (CEM) and Nonrelativistic QCD (NRQCD) can describe production yields rather well but spin-related measurements like the polarization are stronger tests. In this talk, we will present the first calculation of quarkonium polarization in the improved color evaporation model (ICEM) by considering all diagrams at the order of $\alpha_s^3$ and integrating over all color
Quarkonium suppression in heavy ion collisions has been used as a probe of the quark-gluon plasma (QGP) for decades. The intuitive picture of sequential suppression based on the Debye screening of the heavy quark potential is obscured by other in-medium processes such as dissociation and recombination. A natural question to ask is what we can learn about the QGP from measurements of quarkonium suppression.
In this talk, we will try to address this question using effective field theory techniques and the open quantum system formalism. We argue that when the quarkonium size is small, the interaction between quarkonium and the hot medium is weak. Then the density matrix of the heavy quark pair, as a subsystem, and the hot nuclear environment can be factorized. The time evolution of the subsystem is governed by the Lindblad equation. By applying the Wigner transform to the Lindblad equation and carrying out a gradient expansion, we derive the semiclassical Boltzmann equation and work out the leading quantum correction. The reaction rates are factorized into a quarkonium dipole transition function and a chromoelectric distribution function of the nuclear medium. For differential reaction rates, the chromoelectric distribution function is momentum dependent, defined by two electric fields connected via a staple-shaped Wilson line. For inclusive reaction rates, it becomes momentum independent and the Wilson line collapses into a straight line along the time axis. The relation between the Wilson line structures in the differential and inclusive reaction rates is similar to that between the gluon PDF and the gluon TMDPDF, except that the time here is the real time rather than the lightcone time. The construction can be easily generalized to the interaction between quarkonium and cold nuclear matter, which is of much relevance for quarkonium production in eA collisions, to be carried out in the future Electron Ion Collider.
We solve the Lindblad equation describing the Brownian motion of a Coulombic heavy quark-antiquark pair in a strongly coupled quark-gluon plasma using the highly efficient Monte Carlo wave-function method. The Lindblad equation has been derived in the framework of pNRQCD and fully accounts for the quantum and non-Abelian nature of the system. The hydrodynamics of the plasma is realistically implemented through a 3+1D dissipative hydrodynamics code. We compute the bottomonium nuclear modification factor and compare with the most recent LHC data. The computation does not rely on any free parameter, as it depends on two transport coefficients that have been evaluated independently in lattice QCD. Our final results, which include late-time feed down of excited states, agree well with the available data from LHC 5.02 TeV PbPb collisions.
The nuclear modification factors $R_{AA}$ and $R_{pA}$ of $\Upsilon$ mesons, measured from PbPb, pPb, and pp collisions at $\sqrt{s_{NN}} = 5.02$ TeV at CMS, are reported. The second-order Fourier coefficients ($v_2$) characterizing the elliptic flow of $\Upsilon$'s in PbPb are also reported. In all cases, the $\Upsilon$'s are reconstructed through their dimuon decay channel, as measured at CMS in the rapidity range $|y|<2.4$. In the presence of a quark-gluon plasma, bottomonia are expected to be suppressed, and their resulting elliptic flow signature should be very small. The $\Upsilon$ production is observed to be suppressed in pPb collisions, and substantially more suppressed in PbPb collisions. Models which incorporate sequential suppression of bottomonia are in better agreement with the data than those which only assume initial-state modification. The $v_2$ of the $\Upsilon$'s in PbPb is found to be consistent with zero.
co-hosts: Ian Cloët and Dave Gaskell
co-host: Oleg Eyser
One of the motivations for the recent upgrade of Jefferson Lab was to precisely explore the connection between the fundamental quarks and gluons of Quantum Chromodynamics (QCD)- the accepted theory of the strong force- and the effective hadron descriptions of the strong interaction. The ultimate goal being an accurate understanding of the emergence of nuclei from QCD. The key experiments of this program typically aim to study fundamental QCD processes in nuclei and measurements of properties of quark systems in the nuclear medium. Many of the early experiments that have been completed at the upgraded JLab are part of this program designed to address the connection between quarks and nuclei. We will discuss some of these new results that have been published and other preliminary results, such as the search for color transparency, the studies of the EMC effect and the charge symmetry of quark distributions. We will also highlight upcoming experiments that are part of this program.
The Beam Energy Scan (BES) program at the Relativistic Heavy-Ion Collider (RHIC) provides us a unique opportunity to study the phase structure of QCD matter at finite temperatures and densities. Phase II of the RHIC BES program will provide precision measurements for heavy-ion collisions below 20 GeV in collision energy. This talk will highlight recent theoretical progress towards a full-fledged (3+1)D dynamical description of relativistic nuclear collisions. Such dynamical frameworks play an indispensable role in mapping heavy-ion collisions to the QCD phase diagram event-by-event and extracting the equation of state and transport properties of the QCD matter at finite densities. Challenges and opportunities to access dynamics associated with critical point and first-order phase transition will be discussed when confronting the upcoming RHIC BES phase II measurements.
Unraveling the rich structure of hadrons requires studying its quark and gluon degrees of freedom including their transverse momenta and the origin of the proton spin in terms of the spin and orbital angular momenta of its constituents. To that end, a large number of experiments have been carried out over the past four decades at accelerator laboratories in the United States, Europe and Japan. Data related to transverse-momentum dependent (TMD) PDFs allow the study of spin-orbit correlations inside the nucleon and the mapping of its transverse-momentum structure, while hard exclusive data related to generalized parton distributions (GPDs) open the pathway to the 3D nucleon structure in position space. An overview of experimental results will be given with emphasis on the fixed-target experiments.
Nucleon structure is encoded in parton distribution functions (PDFs). Spin-averaged and spin-dependent PDFs contain information on the momentum distributions and spins of quarks and gluons. Measurements from collider and fixed target experiments provide rich datasets for precise determination of PDFs. In this talk, I will review the recent results and future prospects that relate to collinear PDFs.
co-hosts: Oleg Eyser and Dave Gaskell
co-host: Matthew Sievert
Neutron stars are fascinating laboratories for strong gravity, multi-messenger astronomy and nuclear physics. In this talk, I will show how neutron star mergers can probe extreme states of matter, such as the appearance of hyperons at high densities and the deconfinement of quarks. I will also present how these effects are encoded in the high-frequency part of gravitational wave signals.
Finally, I will comment on the feasibility of using neutron star mergers to study the physics of neutrino-driven bulk viscosity.
We investigate the phase transition from hadron to quark matter expected to take place in dense and/or hot matter in the general case without the assumption of chemical equilibrium. This is the case for matter produced in particle collisions, in very young, or recently merged neutron stars. In each of these cases, different conditions concerning temperature, density, strangeness and isospin are expected, all of which affect the deconfinement phase transition. We discuss and quantify how to translate future deconfinement signals from one case to another.
The observation of gravitational waves from low mass compact objects (be it black holes or neutron stars) opens up the possibility to learn about nuclear physics at baryon densities above nuclear saturation and at very low temperatures. Interestingly, introducing non-trivial structure in the speed of sound sourced by changes in the degrees of freedom (possibly quarks) of ultra-dense matter can lead to extremely heavy neutron stars with interesting features and signatures in gravitational wave observables. I will here discuss these features and signatures and describe the prospects to constrain them with current and future gravitational wave detectors.
The quark-matter phase of neutron stars is thought to be governed by short-range interactions among quark and gluons within Quantum Chromodynamics (QCD). Pressure, energy, as well as other mechanical properties of the nucleon are encoded in the QCD Energy-Momentum Tensor. The matrix elements of this tensor are connected to the Mellin moments of the generalized parton distributions which can be measured in deeply virtual exclusive scattering experiments. We establish a connection between observables from high-energy experiments and from the analysis of gravitational-wave events.
The Jefferson Laboratory parity violating electron scattering experiments PREX and CREX cleanly measure the neutron densities of 208Pb and 48Ca. This has implications for the equation of state of neutron rich matter and the structure of neutron stars. We compare PREX results to NICER X-ray telescope observations of neutron star radii and LIGO gravitational wave observations of neutron star deformabilities. We discuss the coming CREX results for 48Ca that will constrain Chiral EFT three neutron forces.
co-host: Dave Gaskell
In the past decades, the study of hadrons in three dimensions has received a lot of attention and made significant progress using both the generalized parton distributions (GPDs) and the transverse momentum dependent (TMD) parton distribution functions. In this presentation, I will discuss how the GPDs can help us understand better nuclei in terms of quarks and gluons with a specific highlight on several experimental programs. First, I will present the CLAS experiment that mesured deeply virtual Compton scattering (DVCS) off helium-4, which led to the first extraction of nuclear GPDs. Then, I will discuss the extension of this work that is being prepared for the CLAS12 experiment using a low energy recoil detector (ALERT). Finally, I will present some recent phenomenological studies toward nuclear DVCS experiments at the future electron ion collider (EIC).
The ability of current and next generation accelerator-based neutrino-oscillation measurements to reach their desired sensitivity requires a detailed understanding of neutrino-nucleus interactions. These include precise knowledge of the relevant cross sections and of our ability to reconstruct the incident neutrino energy from the measured final state particles. Incomplete understanding of these interactions can skew the reconstructed neutrino spectrum and therefore bias the extraction of fundamental oscillation parameters. In this talk, I will present new wide phase-space electron-scattering data, collected using the CLAS spectrometer at the Thomas Jefferson National Accelerator Facility (JLab), where we studied how well we can reconstruct the incident lepton energy from the measured final state particles. Disagreements with the commonly used GENIE event generator are observed, indicating a potential bias for future oscillation analyses and pointing the way for improving these event generators.
The hadronization or fragmentation, where a struck quark transforms into color-neutral hadrons, is an effective tool to probe the confinement dynamics as well as the characteristic time-scales involved in the process. These time-scales elucidate our understanding of the color-neutralization and subsequent non-perturbative formation of the observed hadrons. This talk will report the first-ever analysis of the semi-inclusive deep inelastic scattering of Λ hyperons in the current and target fragmentation regions using the accumulated Jefferson Lab CLAS6 data-sets with deuterium, carbon, iron,and lead targets. Results on the multiplicity ratios and the transverse momentum broadening will be presented along with a highlight of the upcoming CLAS12 color propagation measurements.
Color transparency (CT) is a unique prediction of Quantum Chromodynamics (QCD) where the final (and/or initial) state interactions of hadrons with the nuclear medium are suppressed for exclusive processes at high momentum transfers.
During the spring of 2018, the experiment E1206107 to measure the Proton Transparency was the first to run in Hall C at Jefferson Lab using the upgraded 12 GeV electron beam. Our experiment used the High Momentum Spectrometer (HMS) and new Super High Momentum Spectrometer (SHMS) in coincidence to measure $^{12}C(e,e'p)$ proton knockout to extract the proton nuclear transparency with additional $^1$H measurements to determine the elementary process, over the range $Q^2=8-14.2~(GeV/c)^2$. A rise in the proton transparency as a function of $Q^2$ is predicted to be a signature of the onset of CT.
This talk will summarize the results of the experiment and the status of ancillary analyses such as $1s/1p$ shell dependence on transparency.
The cross section for elastic electron-nucleus scattering is described by nuclear form factors: fundamental quantities that describe the spatial structure of the nucleus. By going to low energy and forward angle, the contributions of the magnetic form factor $G_M(Q^2)$ are minimized, which allows for an accurate extraction of the charge form factor $G_E(Q^2)$ without having to perform a Rosenbluth separation. The RMS radius of the nucleus is proportional to the slope, $dG_E/dQ^2$, as $Q^2 \to 0$, so by measuring the electric form factor of a target at very low $Q^2$, we can measure the nuclear radius.
The primary aim of my work is the extraction of the cross section from data collected in Jefferson Lab Hall A experiment E12-11-112, which measured elastic scattering from the mirror nuclei 3He and 3H at low momentum transfer $Q^2 \approx 0.1$ GeV$^2$. I will also determine the ratio of the 3He and 3H elastic cross sections since many of systematic uncertainties that enter into the absolute cross section extraction cancel in this ratio. The new results will be used to improve global fits that examine the difference between the 3H and 3He charge radii. In particular, these results will help fit the normalization of previous elastic tritium measurement and thus will dramatically improve the global fit.
co-host: David Richards
We present a determination of the spectrum of excited and exotic bottomonium using lattice QCD. Using a large basis of single-meson operators many highly excited states are identified. These include states which can be grouped into supermultiplets matching quark model expectations and additional states with exotic spin–parity–charge-conjugation quantum numbers $J^{PC}= 0^{+−}, 1^{−+}, 2^{+−}$ that cannot be formed from $q\bar{q}$ alone. States with a dominant gluonic component are identified and form a hybrid supermultiplet with $J^{PC}=(0,1,2)^{-+}, 1^{--}$ at approximately 1500 MeV above the lightest state in the system, in agreement with similar calculations in the light, heavy-light and charmonium systems.
Based upon material appearing in arXiv:2008.02656.
Sinéad M. Ryan and David J. Wilson,
for the Hadron Spectrum Collaboration
We present calculations of form factors and radiative transitions in the low-lying charmonium sector using Lattice QCD. Results for $J/\psi \to \eta_c \gamma$, $\chi_{c0} \to J/\psi \gamma$ partial widths are presented alongside other form factors. Comparisons are given to previous results in both lattice and experimental studies. Studying radiative transitions provides insights into the structure of charmonia and this study serves as a demonstration of techniques applicable to other more interesting transitions, such as those involving excited and exotic states.
Based on work in preparation by
J. Delaney, C. O’Hara, C.E. Thomas, S.M. Ryan
for the Hadron Spectrum Collaboration
We present the first calculation within lattice QCD of excited light meson resonances with $J^{PC}=1^{--},2^{--}$, and $3^{--}$. We work within exact SU(3) flavor symmetry in the singlet representation and observe two $1^{--}$ resonances (a first in LQCD), a lighter broad state and a heavier narrower state, a broad $2^{--}$ resonance decaying in both P- and F-waves which is presently unobserved in experiment, and a narrow $3^{--}$ state. We present connections to the experimental $\omega^*_J$, $\phi^*_J$ resonances decaying into the physical states $\pi\rho$, $K\bar{K}^*$, $\eta\omega$ and others.
based upon material appearing in
arXiv:2012.00518
C.T. Johnson and J.J. Dudek
for the Hadron Spectrum Collaboration
Most of the exotic quarkonium states have been observed in transitions to standard quarkonium states plus light-quark hadrons. However, so far very little is known of these transitions widths in the Born-Oppenheimer picture of exotic quarkonium, that is when exotic quarkonium are considered as heavy-quark-antiquark bound states over the spectrum of static energies for any given set of light degrees of freedom. In this talk we present the computation of the transitions for isospin I=0 exotic quarkonium to standard quarkonium with one or two light-quark mesons in the final state.The computation has two distinct parts: the heavy quark transition matrix elements, which are obtained in a nonrelativistic EFT incorporating the heavy quark, multipole and adiabatic expansions; and the hadronization of the gluonic operators into the light-meson final states. The single mesons production is obtained through the axial anomaly and a standard pi0-eta-eta' mixing scheme. Two pion and kaon production is obtained by solving the coupled Omnès problem.
Lattice QCD calculations are beginning to push comparisons between theoretical and experimental tests of the Standard Model to unprecedented precisions. In order to meet this precision goal, QED effects must be introduced. In this talk, I will present a systematic study of the spectra of hadrons using a novel technique in which power-law finite-volume effects are mitigated through the introduction of a non-zero photon mass. I will show that the effects of zero-mode and other non-perturbative contributions may be fully accounted for within this framework, and explore an isospin breaking scheme in which electromagnetic and strong isospin breaking effects are separated at leading order.
co-hosts: Bjoern Schenke and Julia Velkovska
The understanding of the QCD phase structure at finite baryon densities and the nature of the phase transition from the hadronic to the Quark-Gluon Plasma phase dominated by partonic degrees of freedom depends crucially on experimental measurements. The ongoing second phase of the Beam Energy Scan program at RHIC (BES-II) focuses on exploring the high baryon density region of the QCD phase spase with high precision measurements. The STAR experiment at RHIC has implemented several detector upgrades for BES-II including upgrades for the Time Projection Chamber (TPC), a new Event Plane Detector (EPD) and the end-cap Time Of Flight detector, that will enhance the kinematic reach and statistical precision of measurements. The BES-II also includes a fixed target (FXT) program that extends the nucleon-nucleaon center of mass energy of collisions down to 3 GeV. In this talk we will present the latest results from the BES-II program at STAR including new results of identified hadron production, collectivity, and criticality in $\sqrt{s_{NN}}$ = 3 $-$ 20 GeV Au+Au collisions at RHIC. Physics implications of these new results will also be discussed.
I will review the results obtained within the BEST collaboration and the progress towards mapping out the QCD phase diagram. While the main focus of the talk will be lattice QCD, I will also discuss the other collaboration activities and main results.
It has been proposed that the azimuthal distributions of heavy flavor quark-antiquark pairs may be modified in the medium of a heavy-ion collision. This assumption was tested through next-to-leading order (NLO) calculations of the azimuthal distribution, $d\sigma/d\phi$, including transverse momentum broadening, employing $\langle k_T^2 \rangle$ and fragmentation in exclusive $Q \overline Q$ pair production [1].
The results have been compared to $p+p$ and $p + \overline p$ data on $Q \overline Q$ azimuthal correlations [1] as well as $b \overline b$ correlations in $p+p$ collisions through their decays to $J/\psi J/\psi$, as measured by LHCb [2].
Agreement with the data was found to be excellent.
Possible cold and hot matter effects on these correlations are investigated through the effects of nuclear modifications of the parton densities, enhanced $k_T$ broadening and energy loss.
[1] R. Vogt, Phys. Rev. C ${\mathbf 98}$ (2018) 034907.
[2] R. Vogt, Phys. Rev. C ${\mathbf 101}$ (2020) 024910.
Nuclear deformation is a ubiqutous phenomenon for most atomic nuclei, reflecting collective motion induced by interaction between valance nucleons and shell structure. Collisions of deformed nucleus lead to large shape and size fluctuations in the initial state geometry, which after collective expansion, lead to a non-trivial correlation between harmonic flow $v_{n}$ and event-by-event mean transverse momentum $[p_{T}]$. Therefore detailed study of [$p_{T}$] flucutations and $v_2$ -[$p_T$] correlations could probe the quadrupole deformation. In this talk, we present a overview of [$p_T$] fluctuations and $v_{n}$-[$p_T$] correlations for n=2 and 3 from STAR and ATLAS experimental data. Centrality selection, the choice of transverse momentum range and nonflow effect will be discussed. Comparison with state-of-art model calculations is also discussed. This measurement opens up an avenue for studying nuclear structure at a much shorter time scale ($\sim 10^{-23}$s) in heavy-ion collisions.
co-host: Fatma Aslan
One common application of transverse momentum dependent correlation functions is to the interpretation of scattering cross sections in terms of the intrinsic parton structure of hadrons. In such applications, one often encounters divergent ultraviolet integrals over transverse momentum. I will discuss some current and recent efforts to deal with these divergences in ways that preserve standard hadron-structure interpretations.
We study the impact of the Electron-Ion Collider (EIC) on the phenomenological extraction of the tensor charge from a QCD global analysis of single transverse-spin asymmetries (SSAs). We generate EIC pseudo-data for the Collins effect in semi-inclusive deep-inelastic scattering for proton and He-3 beams across multiple energy configurations. We find a significant reduction in the theoretical errors for the up, down, and isovector tensor charges that will make their extraction from EIC data on SSAs as or more precise than current lattice QCD calculations. We also analyze the constraints placed by potential future data from the proposed SoLID experiment at Jefferson Lab.
We propose a new factorized approach to QED radiative corrections (RCs) in inclusive and semi-inclusive lepton-hadron deep-inelastic scattering. The method allows the systematic resummation of the logarithmically enhanced into factorized lepton distribution and fragmentation (or jet) functions that are universal for all final states. The new approach provides a uniform treatment of RCs for the extraction of parton distribution functions, transverse momentum dependent distributions, and other partonic correlation functions from lepton-hadron collision data.
We present our recent preliminary results on unpolarized Transverse-Momentum Dependent (TMD) Parton Distribution Functions (PDFs) and Fragmentation Functions (FFs) from a study of Semi-Inclusive Deep-Inelastic Scattering (SIDIS) data. The analysis is performed in the TMD factorization framework and is a key step for the study of future data from the Electron-Ion Collider. We also provide an estimate of the impact that EIC data will have on TMD extractions.
We summarize the recently published HERMES results on azimuthal single-spin (SSA) and double-spin asymmetries (DSA) in semi-inclusive leptoproduction of pions, charged kaons, protons, and antiprotons from transversely polarized protons. The comprehensive data set on SSA and DSA include the previously published HERMES results on Collins and Sivers asymmetries, the analysis of which has been extended to include protons and antiprotons and also to an extraction in a three-dimensional kinematic binning and enlarged phase space. They are complemented by corresponding results for the remaining four single-spin and four double-spin asymmetries allowed in the one-photon-exchange approximation of the semi-inclusive deep-inelastic scattering process for target-polarization orientation perpendicular to the direction of the incoming lepton beam. Among those results, significant non-vanishing $\cos{\left(\phi−\phi_S\right)}$ modulations provide evidence for a sizable worm-gear (II) distribution, $g_{1T}$. Most of the other modulations are found to be consistent with zero with the notable exception of large $\sin{\phi_S}$ modulations for charged pions and positive kaons.
co-host: Ralf Seidl
Dihadron beam spin asymmetries present powerful probes of nucleon structure and hadronization, in particular, spin-momentum correlations in hadronization. Recent measurements at CLAS12 provide the first empirical evidence of a nonzero $G_1^\perp$, the parton helicity-dependent dihadron fragmentation function (DiFF), which encodes spin-momentum correlations in hadronization. A sign change is observed, with different behavior above and below the $\rho$ resonance. Moreover, the dihadron production cross section expands in partial waves, each containing a DiFF corresponding to the interference of dihadrons of particular polarizations; it turns out that $G_1^\perp$ needs the interference with a dihadron in a $p$-wave or higher order, that is, a dihadron with a nonzero angular momentum. Asymmetry amplitudes for each partial wave can be measured using a simultaneous fitting technique. This presentation focuses on updates to dihadron beam spin asymmetries at CLAS12, from 10.6 GeV electrons scattering on a proton target, along with prospects for learning more about nucleon structure and hadronization.
In this talk I will present a study transverse polarization of lambda-hyperons in single-inclusive leptonic annihilation (SIA). We show that when the transverse momentum of the lambda-baryon is measured with respect to the thrust axis, a transverse momentum dependent (TMD) formalism is required and the polarization is generated by the TMD polarizing fragmentation function (TMD PFF), $D_{1T}^\perp$. However, when the transverse momentum of the lambda-baryon is measured with respect to the momentum of the initial leptons, a collinear twist-3 formalism is required and the polarization is generated by the intrinsic twist-3 fragmentation function $D_{T}$. Thus, while these measurements differ from one another only by a change in the measurement axis, they probe different distribution functions. Recently, Belle measured a significant polarization in single-inclusive lambda-baryon production as a function of the transverse momentum with respect to the thrust axis. However, this data can in principle be re-analyzed to measure the polarization as a function of the transverse momentum of the lambda-baryon with respect to the lepton pair. This observable could be the first significant probe of the $D_{T}$ function. In this study, we first develop a TMD formalism for lambda-polarization; we then present a recent twist-3 formalism that was established to describe lambda-polarization. Using the TMD formalism, we demonstrate that the lambda-polarization at Belle and OPAL can be described using the twist-2 factorization formalism. Finally, we make a theoretical prediction for this polarization in the twist-3 formalism at Belle.
We perform a comprehensive new Monte Carlo analysis of high-energy lepton-lepton, lepton-hadron and hadron-hadron scattering data to simultaneously determine parton distribution functions (PDFs) in the proton and parton to hadron fragmentation functions (FFs). The analysis includes all available semi-inclusive deep-inelastic scattering and single-inclusive $e^+ e^-$ annihilation data for pions, kaons and unidentified charged hadrons, which allows the flavor dependence of the fragmentation functions to be constrained. Employing a new multi-step fitting strategy and more flexible parametrizations for both PDFs and FFs, we assess the impact of different data sets on sea quark densities, and confirm the previously observed suppression of the strange quark distribution.
Hadronization, the process by which quarks and gluons form color-neutral bound states, remains poorly understood despite its widespread importance as a fundamental component of QCD. Jets present ideal environments in which to study high-energy hadronization, as they are formed by a scattered parton and contain the final-state hadrons produced during the hadronization process. Distributions of the final state hadrons in jets provide insight into hadronization dynamics and color neutralization mechanisms and help constrain the current theoretical parameterizations for describing hadronization in jets, the jet fragmentation functions. Comparisons of hadron distributions in forward Z-tagged jets, jets recoiling against a Z boson, and in midrapidity inclusive jets at LHC energies can probe differences between light-quark and gluon hadronization as forward Z-tagged jets are predominantly light-quark-initiated while midrapidity inclusive jets are predominantly gluon-initiated. In this talk, recent LHCb results of the longitudinal momentum fraction, transverse momentum with respect to the jet axis, and radial distribution of charged hadrons in Z-tagged jets will be presented and implications of these and future measurements for advancing our understanding of hadronization will be discussed.
We present predictions and projections for hadron-in-jet measurements and electron-jet azimuthal correlations at the future Electron-Ion Collider. These observables directly probe the quark transversity and Sivers parton distributions, the Collins fragmentation functions, and TMD evolution. This jet-based approach will allow us to avoid the convolution of TMD parton distributions and fragmentation functions, which limits the constraining power of SIDIS measurements. We explore the feasibility of these measurements with detector simulations and discuss key requirements.
The partonic structure of the proton is of great interest in hadron physics. Particularly the distribution of antiquarks has gained a lot of attention due to its unique sensitivity to the dynamics of the strong force (i.e. QCD).
SeaQuest at Fermilab is a fixed-target experiment to measure the Drell-Yan process in proton+proton and proton+deuteron scattering, using the 120-GeV proton beam and targets of LH2, LD2, C, Fe and W. SeaQuest's primary goal is to precisely measure the flavor asymmetry of the light-antiquark distributions ($\bar{d}(x)/\bar{u}(x)$) at large x. SeaQuest also measures nuclear effects on anti-quarks and the partonic energy loss in cold nuclear matter. It completed the data taking in 2017. The latest results will be discussed in this talk, with emphasis on the $\bar{d}(x)/\bar{u}(x)$ result that was published in February 2021.
SpinQuest is a successor to SeaQuest, using transversely-polarized NH$_3$ and ND$_3$ targets. SpinQuest aims to directly measure the Sivers asymmetry of light antiquarks via the Drell-Yan process. It is now completing construction of the experimental apparatus, and plans to start taking data in spring 2021. The status and the prospects of the experiment will be presented in this talk.
Recent analysis of unpolarzed data for Drell-Yan and SIDIS show new light of the structure of nucleai.We perform an anlysis on these data, at small transverse momentum and using the TMD factorization theorem. I will show recent progresses in this analysis which include W-boson analysis, the impact of PDF and flavor dependence.
COMPASS is a fixed target experiment in the North Area of CERN. One of the primary goals of its broad physics program is to study the transverse momentum dependent (TMD) parton distribution functions (PDFs) that describe the spin structure of nucleons. To extract observables related to TMD PDFs, COMPASS uses both Semi-Inclusive Deep Inelastic Scattering (SIDIS) and the Drell-Yan (DY) process. Results related to the quark Sivers functions are especially interesting, as these functions are expected to change sign between SIDIS and DY. Here we focus on the DY portion of the COMPASS program. In 2015 and 2018, COMPASS collected DY data by scattering a negative pion beam off a transversely polarized ammonia target. The most recent COMPASS results agree with the predicted sign flip of the Sivers function between SIDIS and DY. During the DY runs, COMPASS also recorded many J/ψ events. Single-spin asymmetries in J/ψ production may give access to the gluon Sivers function and may improve our understanding of the J/ψ production mechanism. The reconstruction of raw experimental and Monte-Carlo data, necessary to perform physics analysis, was primarily realized exploiting the parallel computing resources of the Blue Waters supercomputer at NCSA and the Frontera supercomputer at TACC.
The E1039/SpinQuest experiment is a transversely polarized fixed target experiment at Fermi National Accelerator Laboratory aiming to explore the sea quark and gluon Sivers functions via the measurement of the transverse single spin asymmetry (TSSA) for a number of physics processes including J/Psi, Psi’ and Drell-Yan production. The experiment employs a 120-GeV extracted proton beam colliding with transversely-polarized NH3 and ND3 cryogenic targets and its spectrometer is optimized to detect the oppositely-charged muon pair output of these processes. In pursuit of these asymmetry measurements, we are seeking to develop an advanced GPU-based multi-threaded framework that allows an efficient parallelization of the online data processing, which will facilitate prompt online reconstruction, optimization, and robust data quality monitoring. In this talk, I will report the status of our GPU online reconstruction project along with results estimating the anticipated accuracy of the TSSA measurement via J/Psi production from early SpinQuest production data.
We perform the global analysis of polarized Semi-Inclusive Deep Inelastic Scattering (SIDIS), pion-induced polarized Drell-Yan (DY), and W+-/Z boson production data and extract the Sivers function for u, d, s, and for sea-quarks. We use the framework of transverse momentum dependent factorization at N3LO accuracy. The Qiu-Sterman function is determined in a model-independent way from the extracted Sivers function. We also evaluate the significance of the predicted sign change of Sivers function in DY with respect to SIDIS.
co-host: Astrid Hiller Blin
In the talk, I will discuss two main aspects of the amplitude analysis of the three-body final state: angular decomposition, and parametrization of the resonance dynamics. First, I will present an application of the recently proposed method of the Dalitz-Plot Decomposition to the studies of CP violation in the LHCb experiment. Second, I will discuss the realization of the three-body unitarity constraints in the decays of the exited $\Lambda_b^0$ resonances.
The quality of the recent vector-meson photoproduction data from JLab and MAMI at the proximity of the data to the energy threshold gives access to a variety of interesting physics aspects. As an example, an estimation of the vector-meson-nucleon scattering lengths are provided within the vector meson dominance model and projected to the Yp scattering length for EIC. Due to the small size of 'young' vs 'old' vector-meson, measured scattering length is small. So, the vector-meson created by the photon at the threshold then most probably vector-meson is not formed completely and its radius is smaller than that for normal vector-meson, Therefore, one observes stronger suppression for the vector-meson-nucleon interaction.
The first calculation of two-nucleon interactions from lattice QCD dates back to over 25 years now. Even with vast increases in computational power and several algorithmic improvements since then, there remains disagreement within the community on the basic parameters that describe these interactions. This is a testament to the difficulty of the problem. Baryons suffer from a serious signal-to-noise ratio problem, significantly limiting the range of useful time-separations in the calculated correlation functions. Here we present recent results for both the $I = 0$ and $I = 1$ nucleon-nucleon interactions using SU(3)-flavor-symmetric ensembles. These calculations are performed with the distillation method and its stochastic variant. We also make use of a variational method, which can help to reduce unwanted excited states while allowing for several desired states to be extracted. Our results disfavor a bound state in both nucleon-nucleon systems. Several possibilities to explain the discrepancies within the literature will be discussed.
Using the technique of target rescattering of nucleons off protons, the reactions np -> d pi0 and pp -> d pi+ have been measured using the CLAS detector. For pure resonance productions, these reactions should be related by isospin. The total cross sections, as a function of the center-of-mass energy, W, for both reactions show a peak in the invariant mass of the deuteron and pion at about 2145 MeV. Results for the pp -> d pi+ reaction agree with previous measurements, whereas the np -> d pi0 cross sections are new. Together, these results suggest that this d* resonance has isospin 1, as predicted many years ago using SU(6) symmetry.
This study investigates a recently-observed $N\Delta$ ($d^∗$) resonance decaying to $\pi d$ final state using CLAS at Jefferson Lab, Virginia. Tagged photons with beam energies between 0.8 and 3.6 GeV were produced using the bremsstrahlung process incident on a liquid deuterium target. The final state particles detected were an energetic deuteron and a two oppositely charged pions. The $d^∗$ resonance has been seen in other preliminary analyses at CLAS. Partial-wave analysis of pion-deuteron scattering has also shown a resonance at a mass of about 2145 MeV. Preliminary differential cross section measurements of the two charged states of this resonance will be presented.
co-hosts: Bjoern Schenke and Julia Velkovska
In the almost 20 years since the discovery of the Quark Gluon Plasma, our understanding of its properties has changed dramatically. We now understand that the phase transition into deconfined quarks and gluons is a cross-over at vanishing baryon densities that appears to lead to a flavor hierarchy for chemical freeze-out. Out-of-equilibrium properties demonstrated through transport coefficients like shear and bulk viscosity, q-hat, and charge diffusion cannot be calculated directly from QCD. Rather, through a combination of effective models and Bayesian analyses, an emerging understanding of these transport coefficients is being developed. This talk will discuss the latest developments in our understanding of the QGP and potential discoveries in the years to come.
The sPHENIX experiment at RHIC is currently under construction and on schedule for first data in early 2023. Built around the excellent BaBar superconducting solenoid, the central detector consists of a silicon pixel vertexer adapted from the ALICE ITS design, a silicon strip detector with single event timing resolution, a compact TPC, novel EM calorimetry, and two layers of hadronic calorimetry. The hybrid streaming/triggered readout of the detector enables full exploitation of the luminosity provided by RHIC. The science program of sPHENIX focuses on jets and heavy flavor, observables with specific relevance to questions of the initial state in heavy ion collisions. The talk will describe the readiness of the experiment for operations, present an overview of the envisioned physics program. The science program of sPHENIX focuses on jets and heavy flavor, observables with specific relevance to questions of the initial state in heavy ion collisions. The talk will describe the readiness of the experiment for operations, present current projections of key jet and heavy flavor measurements, and discuss their potential scientific impact.
The sPHENIX detector at BNL’s Relativistic Heavy Ion Collider (RHIC) will measure a suite of unique jet and jet and heavy flavor observables with unprecedented statistics and kinematic reach at RHIC energies. A MAPS-based vertex detector upgrade to sPHENIX, the MVTX, will provide a precise determination of the impact parameter of tracks relative to the primary vertex in high multiplicity heavy ion collisions. These new capabilities will enable precision measurements of open heavy flavor observables, covering an unexplored kinematic regime at RHIC. The physics program, its potential impact, and recent detector development will be discussed in this talk.
Colliding large nuclei at velocities close to the speed of light can produce a plasma of strongly interacting nuclear matter known as quark-gluon plasma. This nuclear plasma can be caracterised by macroscopic properties such as its equation of state and transport coefficients. These properties can be constrained by systematic comparison with the large number of available measurements from the Relativistic Heavy Ion Collider and the Large Hadron Collider. In this presentation, I review the status of Bayesian constraints on the shear and the bulk viscosities of the quark-gluon plasma. I discuss the strengths of Bayesian parameter estimation as well as common misconceptions.
co-hosts: Ian Cloët and Dave Gaskell
co-host: Phiala Shanahan
A beautiful description of nature’s fundamental forces has been devised through gauge fields introducing local symmetries or conserved charges. Though classical techniques continue to provide invaluable information on the emergent properties of gauge field theories relevant to experimental programs throughout the scientific domains, some experimentally relevant parameter regimes e.g., where coherent dynamics demand exponentially large Hilbert spaces, remain beyond current or foreseeable computational capabilities. While leveraging quantum architectures directly within a computational framework is expected to be more naturally capable of exploring such parameter regimes, the inefficient utilization of Hilbert space in the presence of local symmetries demands careful considerations in the presence quantum noise. During this talk, we will discuss current strategies and perspectives for representing quantum fields, from scalars to SU(3) Yang-Mills, on qubit degrees of freedom and for controllably performing subsequent dynamical evolutions.
The theoretical description of jet substructure observables involves the study of their radiation pattern in all corners of phase space. In this talk, I will
focus on the properties of the hardest splitting in a QCD jet, as defined by the Dynamical Grooming method, both in vacuum and in heavy-ion collisions. After presenting some interesting properties from the resummation point of view, I will present the first theory-to-data comparison of dynamically groomed observables in proton-proton collisions. On the heavy-ion part of the talk I will show that the opening angle and transverse momentum of this splitting are potential candidates to experimentally measure the resolution length and the quasi-particle nature of the Quark-Gluon Plasma, respectively.
It is customary to distinguish two main regimes of perturbative QCD processes: the Bjorken (moderate x) limit and the Regge (small x) limit. The main difficulty when addressing the continuity between these two regimes was solved recently. Indeed it was proven that semi-classical descriptions of observables in the saturated or unsaturated Regge limit, such as the Color Glass Condensate, can be entirely rewritten in terms of more familiar parton distributions encountered in the Bjorken regime, the Transverse Momentum Dependent (TMD) distributions. I will discuss this equivalence, its limits, its consequences on our understanding of the structure of hadrons. In particular, I will show how to distinguish kinematic and genuine saturation effects and how to probe them more precisely.
Since the discovery of the European Muon Collaboration (EMC) effect, where it was observed that the quark longitudinal momentum distributions in a nucleus are different from those of free nucleons, there has been a long standing question in nuclear physics as to how the structure of a free nucleon might change when bound in a nucleus or embedded in a nuclear medium. One of the cleanest signatures of in-medium modification can be achieved by testing the Coulomb Sum Rule through quasi-elastic electron scattering; where one counts the expected total nuclear charge by integrating the longitudinal response function of the nucleus and comparing it to the incoherent sum of electric form factors of the constituent nucleons. Standard nuclear effects that quench the charge response of the nucleus are well understood, leaving any additional quenching to be interpreted as a signature of modification of these form factors in-medium. Experiment E05-110 at Jefferson Lab collected inclusive electron scattering data on nuclear targets over a |q| range of 500 to 1000 MeV/c. Results from recent analyses of Carbon and Iron data will be presented and discussed.
co-hosts: Phiala Shanahan and Ian Cloët
co-host: Matt Sievert
High energy heavy ion collisions enable the production of a state of strongly interacting deconfined nuclear matter called the Quark Gluon Plasma (QGP). Jets are a powerful probe of this nuclear medium as the partons inside the jet are expected to lose energy as they interact with the medium causing the phenomenon known as jet quenching. Through studying a variety of jet properties at both the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) insight can be gained on the mechanisms of jet energy loss. This talk will discuss recent jet measurements of jet production and correlations in heavy ion collisions from both RHIC and the LHC.
Whether quark- and gluon-initiated jets are modified differently by the quark-gluon plasma produced in heavy-ion collisions is a long-standing question that has thus far eluded a definitive experimental answer. A crucial complication for quark-gluon discrimination in both proton-proton and heavy-ion collisions is that all measurements necessarily average over the (unknown) quark-gluon composition of a jet sample. In the heavy-ion context, the simultaneous modification of both the fractions and substructure of quark and gluon jets by the quark-gluon plasma further obscures the interpretation. Here, we demonstrate a fully data-driven method for separating quark and gluon contributions to jet observables using a statistical technique called topic modeling. Assuming that jet distributions are a mixture of underlying "quark-like" and "gluon-like" distributions, we show how to extract quark and gluon jet fractions and constituent multiplicity distributions as a function of the jet transverse momentum. This proof-of-concept study is based on proton-proton and heavy-ion collision events from the Monte Carlo event generator Jewel with statistics accessible in Run 4 of the Large Hadron Collider. These results suggest the potential for an experimental determination of quark and gluon jet modifications.
Jet substructure, defined by observables constructed from the distribution of constituents within a jet, provides the versatility to tailor observables to specific regions of QCD radiation phase space. This flexibility allows us to test not only our understanding of perturbative QCD but also the nature of nonperturbative effects including hadronization — and has resulted in jet substructure becoming an essential tool to study rare event topologies in searches for new physics. In this talk, I will highlight recent jet substructure measurements at the LHC and RHIC. I will discuss measurements in proton-proton collisions, which enable differential tests of our understanding of perturbative QCD, and measurements in heavy-ion collisions, which provide exciting new opportunities to reveal the nature of the quark-gluon plasma.
In this talk we will outline the connections among the inclusive jet correlator, the single-hadron fragmentation correlator, and the quark propagator, which open the way to studies of fundamental mechanisms in QCD -such as the dynamical generation of mass- by looking at the fully inclusive hadronization of a quark. In particular, we will focus on observables in semi-inclusive DIS and $e^+e^-$ annihilation at the twist three level.
The sPHENIX detector at the BNL Relativistic Heavy Ion Collider (RHIC) benefits from the extensive advances of the Large Hadron Collider (LHC) and Electron-Ion Collider (EIC) detector R&D. The combination of electromagnetic calorimetry, hermetic hadronic calorimetry, precision tracking, and the ability to record data at high rates without trigger bias enables pioneering measurements of jets, jet substructure, and jet correlations. Jet observables are a particularly useful probe of the Quark Gluon Plasma (QGP) formed in heavy-ion collisions since the hard scattered partons that fragment into final state jets are strongly quenched through interactions with the medium they traverse. These measurements will have a kinematic reach that not only overlaps those performed at the LHC, but extends them into a new, low-pT regime where quenching effects are large. Thus the sPHENIX physics program, starting in 2023, can answer fundamental questions about the parton energy loss process, and the underlying nature of the QGP. This talk will give an overview of the status of jet reconstruction and performance within sPHENIX, and the envisioned jet physics program.
co-hosts: Lamiaa El Fassi and Dave Gaskell
A technique has been recently proposed to address the main limitations of past neutrino scattering experiments. In particular, it allows precise measurements of high statistics samples of (anti)neutrino-hydrogen interactions together with various nuclear targets. The planned high intensity LBNF beams give access to a broad mixture of measurements of electroweak parameters, QCD and hadron structure of nucleons and nuclei, nuclear physics, form factors, structure functions and cross-sections, as well as searches for new physics or verification of existing outstanding inconsistencies. A few example related to isospin physics and nuclear parton distributions will be discussed.
We present a new determination of nuclear Parton Distributions Functions (PDFs) from a number of new processes that constrain the nuclear gluons: single jet and dijet cross-sections from ATLAS and CMS, direct photon production from ATLAS, and charm production by LHC. We use NNLO perturbative QCD calculations for all processes included in the fit. The proton baseline, which nNNPDF3.0 reduces to in the limit A=1, is a variant of the upcoming global NNPDF4.0 determination of proton PDFs, which includes several processes constraining the gluon PDF. We also present an impact study of the upcoming Electron-Ion collider pseudo-data on unpolarized proton and nuclear PDFs determined by a sequential global analysis of inclusive cross sections for lepton-proton and lepton-nucleus Deep-Inelastic Scattering (DIS) pseudo-data.
As we strive for higher precision QCD predictions, a detailed knowledge of the nuclear PDFs is critical. The nCTEQ collaboration has recently examined LHC W/Z production data in heavy ion collisions, as well as JLab DIS data on a range of nuclear targets. Both these sets strongly influence the resulting nPDFs, and these advances are reflected in the recent nCTEQ PDF releases. This data yields improved PDF constraints, and may provide insights as to the optimal means to organize the QCD expansion in an expanded kinematic range.
The CJ15 global next-to-leading-order analysis of PDFs utilized, amongst others, inclusive and tagged DIS structure function data from Jefferson Lab (JLab) and high-precision charged lepton and W-boson asymmetry data from Fermilab. As an extension to the CJ15 analysis, new JLab Hall A and C data from experiments jl00106F2, e06009d, e99118, jlcee96 and e03103, and Hall B data from the BONuS and CLAS6 experiments are included to determine their impact on the extraction of PDFs at intermediate and high x, as well as on the nuclear corrections in the deuteron. Preliminary results will be presented in this talk.
In the atomic nucleus there exist a strong nucleon-nucleon (NN) interaction at short distance. When a two-nucleon pair are separated by $r_{1,2} \leq 1.5$ fm, they will interact strongly and and the pair obtains large back-to-back relative momenta. The probability of these short distance interactions is enhanced by the tensor force in isospin=0 neutron-proton ($np$) pairs. Under certain kinematic conditions, 4-momentum transfer $Q^2>1.3$ GeV$^2$, Bjorken $x>1.4$, Quasi-elastic electron scattering (QES) isolates those events producing nucleons with high initial momentum allowing the study this isospin dominance.
In 2018, the experiment E12-11-112 at Jefferson Lab measured inclusive electron cross sections of deuterium, tritium, and helium-3 at $0.4 \leq Q^2 \leq 2.2 $GeV$^2$. In the extracted $^3$H/$^2$H ($^3$He/$^2$H) cross section ratios, we observed plateaus in the region of the NN short-range correlation (NN-SRC) dominant kinematics. The predicted scaling behavior of NN-SRC pairs in A=3 nuclei and in the deuteron is confirmed as is the isospin dominance of NN-SRC pairs. By comparing the mirror nuclei of $^3$H and $^3$He, we extracted the relative of of $np$ to $pp$($nn$) SRC pairs in the A=3 system.
co-host: Ian Cloët
Future quantum computers may serve as a tool to access non-perturbative real-time correlation functions. In this talk, we discuss the prospects of using these to study Compton scattering for arbitrary kinematics. In particular, the need to restrict the size of the spacetime in quantum computers prohibits a naive determination of such amplitudes. However, we present a practical solution to this challenge that may allow for future determinations of deeply virtual Compton scattering amplitudes, as well as many other reactions that are presently outside the scope of standard lattice QCD calculations.
Quantum computers may become powerful tools to simulate various problems in quantum field theory, yet at present are restricted to lower dimensions and small volumes. Common digitization strategies are based on local Hilbert-space decomposition, which may not be optimal for systems with large volumes but few (or not so few) particles. Examples are (non-relativistic) quantum chemistry or low energy nuclear physics, but also relativistic systems in high energy scattering experiments. Using a relativistic scalar ϕ4 theory as a simple example, we propose a novel `single-particle' digitization strategy for relativistic quantum field theories and discuss quantum simulating S-matrix scattering experiments. For such problems our strategy uses significantly less resources than other approaches. We discuss renormalization and Lorentz covariance, and application in low energy nuclear physics.
Based on "Single-particle digitization strategy for quantum computation of a $ϕ^4$ scalar field theory, J. Barata, NM, A. Tarasov, R. Venugopalan, e-Print: 2012.00020 [hep-th]
We apply a quantum teleportation protocol based on the Hayden-Preskill thought experiment to quantify how scrambling a given quantum evolution is. It has an advantage over the direct measurement of out-of-time ordered correlators when used to diagnose the information scrambling in the presence of decoherence effects stemming from a noisy quantum device. We demonstrate the protocol by applying it to two physical systems: Ising spin chain and SU(2) lattice Yang-Mills theory. To this end, we numerically simulate the time evolution of the two theories in the Hamiltonian formalism. The lattice Yang-Mills theory is implemented with a suitable truncation of Hilbert space on the basis of the Kogut-Susskind formalism. On a two-leg ladder geometry and with the lowest nontrivial spin representations, it can be mapped to a spin chain, which we call Yang-Mills-Ising model and is also directly applicable to future digital quantum simulations. We find that the Yang-Mills-Ising model shows the signal of information scrambling at late times.
Quantum computers have the potential to outperform classical computers in a variety of tasks ranging from combinatorial optimization to machine learning to intrinsically evading the sign problem. However, current intermediate-scale quantum devices still suffer from a considerable level of noise. In this talk, we present a novel technique to mitigate measurement noise, which is based on classical bit-flip correction. The method can be extended to other errors sources beyond measurement noise, with overhead costs that scale polynomially in the number of qubits for local Hamiltonians. We demonstrate the experimental realization of the method on IBM quantum hardware, reducing the final measurement error by up to one order of magnitude.
We analyze how maximal entanglement is generated at the fundamental level in QED by studying correlations between helicity states in tree-level scattering processes at high energy. We demonstrate that two mechanisms for the generation of maximal entanglement are at work: i) s-channel processes where the virtual photon carries equal overlaps of the helicities of the final state particles, and ii) the indistinguishable superposition between t- and u-channels. We then study whether requiring maximal entanglement constrains the coupling structure of QED and the weak interactions. In the case of photon-electron interactions unconstrained by gauge symmetry, we show how this requirement allows reproducing QED. For Z-mediated weak scattering, the maximal entanglement principle leads to non-trivial predictions for the value of the weak mixing angle θW. Our results are a first step towards understanding the connections between maximal entanglement and the fundamental symmetries of high-energy physics.
co-host: Fatma Aslan
We made simultaneous extraction of spin averaged and spin dependent PDFs within multistep MC procedures, with combined analysis of inclusive unpolarized and polarized Jet from RHIC to Tevatron energies. By analyzing the preliminary results we had, we were able to further constrain on $\Delta g$.
There have been various attempts, both experimentally and theoretically, to understand the origin of the unexpectedly large transverse single-spin asymmetries ($A_N$) for inclusive hadron production at forward rapidity in p$^\uparrow$ + p collisions that persist at high center-of-mass energies. Two proposed potential sources are the twist-3 contributions in the collinear factorization and the transverse-momentum-dependent contributions from either the initial-state quark and gluon Sivers functions or the final-state Collins fragmentation function. In 2015 and 2017, RHIC collected data from transversely polarized pp collisions, which are ideal to further characterize $A_N$ and explore its potential sources. The STAR Forward Meson Spectrometer (FMS) and Endcap Electromagnetic Calorimeter (EEMC), having pseudo-rapidity ($\eta$) coverages of 2.6 - 4.2 and 1.1 - 2.0 respectively, can be used to detect photons, neutral pions, and eta mesons. We present an analysis update for $A_N$ of electromagnetic jets in FMS and EEMC using p$^\uparrow$ + p collisions at $\sqrt s = $ 200 GeV. In this analysis, we explore the dependences of $A_N$ on photon multiplicity inside the jet, jet transverse momentum, and jet energy.
While significant steps toward the formal definition of quark TMDs and their extraction from experimental data through global fits has been made in the last years, the gluon-TMD field represents a largely unexplored territory. Pursuing the goal of extending our knowledge of this sector, we present analytic expressions for unpolarized and polarized gluon TMDs at twist-2, calculated in a spectator model for the parent nucleon. At variance with respect to previous works, our approach encodes a flexible parametrization for the spectator-mass spectral density, allowing us to improve the description in the small-x region. We build a common framework where valence, sea quark and gluon densities are concurrently generated. Our results can be used to predict the behavior of observables sensitive to TMD dynamics.
We calculate the single transverse spin asymmetry in polarized proton-proton and polarized proton-nucleus collisions ($A_N$) generated by a partonic lensing mechanism. The polarized proton is considered in the quark-diquark model while its interaction with the unpolarized target is calculated using the small-$x$/saturation approach. The phase required for the asymmetry is caused by a final-state gluon exchange between the quark and diquark, as is standard in the lensing mechanism of Brodsky, Hwang and Schmidt. The expression we obtain for the asymmetry $A_N$ of the produced quarks has the following properties:(i) The asymmetry is generated by the dominant elastic scattering contribution and $1/N^2_c$ suppressed inelastic contribution;(ii) The asymmetry grows or oscillates with the produced quark's transverse momentum $p_T$ until the momentum reaches the saturation scale $Q_s$, and then only falls off as $1/p_T$ for larger momenta;(iii) The asymmetry decreases with increasing atomic number $A$ of the target for $p_T$ below or near $Q_s$, but is independent of $A$ for $p_T$ significantly above $Q_s$. We discuss how these properties may be qualitatively consistent with data published by the PHENIX collaboration and with preliminary data reported reported by the STAR collaboration.
One of the main goals of the RHIC spin program is the determination of the transverse-spin structure of the proton, which can in turn provide some insight into the angular-momentum component of partons. The transverse single spin asymmetry ($A_{N}$) of pion production at mid-rapidity in pp collision provides good access to initial and final state transverse spin effects of quark and gluons. In contrast, electron single spin asymmetries from heavy flavor decays are mostly sensitive to gluon related initial state effects. Charged pion and electron measurements at mid-rapidity are preformed with the central arm detectors which consist of an Electro-Magnetic Calorimeter, a Ring-imaging Cherenkov Detector, Drift and Pad Chambers, and so on. The status of the pion and electron measurements from the 2015 running period with transversely polarized proton collisions will be presented.
co-hosts: Dave Gaskell, Garth Huber, and Ramona Vogt
co-host: Garth Huber
Heavy quark production processes, for both open and bound heavy quark pair production, provide promising probes of transverse momentum dependent gluon distributions (gluon TMDs). In this talk the prospects for gluon TMD extractions using heavy quark production processes will be discussed, focussing on the distributions of unpolarized and linearly polarized gluons inside unpolarized protons, and on gluon distributions inside transversely polarized protons, such as the gluon Sivers TMD. Besides allowing extraction of gluon TMDs, quarkonium production TMD processes present opportunities to learn more about the quarkonium production process itself, by offering new probes of certain poorly known NRQCD long-distance matrix elements and their transverse momentum dependent generalization in the form of shape functions.
The Compton scattering process in different kinematical regimes provides a variety of information on the proton structure. In the near threshold regime, real and virtual Compton scattering give access, respectively, to the static polarizabilities and the generalized polarizabilities of the proton. In recent years, a series of new measurements provided new extractions of the polarizabilities. The analysis of the experimental data has been performed using different theoretical frameworks. We will focus on the use of dispersion relation techniques, summarising the latest advances and applications. In the deep inelastic region, the virtual Compton scattering gives access to generalized parton distributions (GPDs). GPDs encode new key information on the partonic structure of the nucleon, among which the form factors of the energy momentum tensors. We will discuss in particular the so-called D form factor and present its dispersive representation in comparison with available model predictions and phenomenological extractions.
Lattice QCD has emerged as an essential framework for understanding the properties of hadrons from first principles. Many features of strong-coupling QCD, such as the excited state spectrum and the internal structure of hadrons characterized by light-cone-separated matrix elements, that were once thought inaccessible to ab initio computation, are now explored in detail. In this talk, I focus on the internal structure of the nucleon and pion, from the one-dimensional distributions encapsulated in the parton distribution functions and electromagnetic form factors, through the three-dimensional measures of GPDs and TMDs. I conclude with the opportunities emerging in the exascale era, and how the amalgam of lattice computations and experiment will be key to the faithful imaging of hadrons.
In 2010, a new method using muonic hydrogen spectroscopy led to a proton charge radius ($r_p$) result that was nearly ten times more precise but significantly smaller than results obtained using the two traditional methods, namely e-p scattering and ordinary Hydrogen spectroscopy. This
discrepancy triggered the so-called "proton charge radius puzzle". To investigate this discrepancy, the PRad collaboration performed a new experiment in 2016 in Hall B at the Thomas Jefferson National Accelerator Facility. With both 1.1 and 2.2 GeV electron beams, the experiment measured the e-p elastic scattering cross sections in an unprecedentedly low values of momentum transfer squared region ($Q^2 = 2.1 \times 10^{-4} - 0:06$ (GeV/c)$^2$),
with a sub-percent precision. The PRad experiment utilized a magnetic-spectrometer-free setup, which was based on a large acceptance and high resolution calorimeter (HyCal), a plane of two large-area Gas Electron Multiplier (GEM) detectors, and a windowless H2 gas-flow target. In this talk, I
will discuss details of the data analysis and present the results of this experiment. I will also discuss briefy the PRad-II experiment, which will improve the uncertainty of $r_p$ by a factor of approx 4 compared to that of PRad.
co-hosts: Garth Huber and Dave Gaskell
co-host: Dave Gaskell
The discovery of many unexpected new resonance candidates such as the XYZ states and pentaquarks has been challenging the quark model. While high-energy collisions of hadrons and nuclei are a good tool for investigating these topics, they are prone to kinematical effects from hadron decays into the final states. In turn, lepton colliders have been limited by statistics so far: a high-luminosity Electron Ion Collider (EIC) at high energies is called for. In view of the upcoming EIC and JLab experiments, we present sensitivity and feasibility studies for XYZ photoproduction. We also study polarization observables in pentaquark photoproduction as a means of improving detection sensitivity and tackling the quantum numbers and couplings of these states.
In this talk, I present a lattice-QCD calculation of the maximal-isospin, three-pion scattering amplitude (3π+ to 3π+). The calculation combines finite-volume energies with a relativistic field-theoretic formalism, required to interpret the results. I will describe the full work-flow required to reach the final amplitude, implemented here for the first time, and discuss the complicated singularities appearing in the latter. I will further discuss how the approach promises to be a powerful tool for resonant three-particle systems.
based up material appearing in
Phys.Rev.Lett. 126 (2021) 012001, arXiv: 2009.04931
M. T. Hansen, R. A. Briceño, R. G. Edwards, C. E. Thomas, D. J. Wilson
for the Hadron Spectrum Collaboration
Lattice QCD offers a systematic pathway to numerically compute the resonant hadronic spectrum from first principles. A set of integral equations connects the short-distance dynamics computed from lattice QCD to on-shell infinite-volume scattering amplitudes. In this talk, I will discuss our recent study on systematically improvable methods for the numerical solution of integral equations of a relativistic three-hadron scattering system. I focus on the case where two hadrons form a bound state and compare our results with an alternative approach using the finite-volume framework. Our study, along with lattice QCD results, forms a complete program to obtain model-independent determinations of three-body interactions from QCD.
Much of the resonant spectrum of QCD consists of states which decay strongly into two- and three-body final states. Lattice QCD calculations have matured to the stage where these states can be reliably resolved in first principles numerical calculations. While connecting these finite-volume results to infinite-volume scattering is now commonplace in the two-body sector, three-body physics presents more difficulties.
On the back of the significant progress made in connecting three-body scattering in infinite-volume to finite-volume states, the first determinations of three-body interactions from lattice QCD have recently begun to appear. Building on success in the two-pion sector, I will present our recent lattice QCD calculations of three-meson systems in maximal isospin (3$\pi^+$, 3$K^-$), with a focus on a recent extraction of the 3$\pi^+$ three-body force, and a comparison to other determinations.
A rich variety of phenomena in the Standard Model and its extensions manifest in long-range processes involving hadrons. These are processes where intermediate hadronic states propagate over a long distance, between electroweak interactions (or new-physics), e.g. deeply virtual Compton scattering. Such processes are at the cusp of what can be systematically studied thanks to significant progress in overcoming two challenges. First, these reactions involve hadrons, and as a result, one must use a non-perturbative tool like lattice QCD. Second, lattice QCD is defined in a finite, Euclidean spacetime. In this talk, I explain how these issues can all be resolved systematically for a large kinematic region.
co-host: Michael Strickland
We establish the existence of a far-from-equilibrium attractor in weakly-coupled gauge theory undergoing one-dimensional Bjorken expansion. We demonstrate that the resulting far-from-equilibrium evolution is insensitive to certain features of the initial condition, including both the initial momentum-space anisotropy and initial occupancy. We find that this insensitivity extends beyond the energy-momentum tensor to the detailed form of the one-particle distribution function. Based on our results, we assess different procedures for reconstructing the full one-particle distribution function from the energy-momentum tensor along the attractor and discuss implications for the freeze-out procedure used in the phenomenological analysis of ultra-relativistic nuclear collisions.
The equilibration and hydrodynamization of pre-equilibrium quark-gluon plasma in ultrarelativistic heavy-ion collisions are of interest. We established a numerical implementation of the QCD effective kinetic theory based on Arnold, Moore, Yaffe framework at leading order, including both gluon and light quark/antiquark degrees of freedom. A universal hydrodynamic attractor is present even at finite baryon density, where we also show the pre-equilibrium trajectory of QGP in the T-mu diagram and how that connects to hydrodynamics. To apply the universal attractor, we constrain the initial distribution in heavy-ion collisions from pre-equilibrium entropy production with a TMD approach.
Motivated by recent interest in collectivity in small systems, we calculate harmonic flow response to initial geometry deformations within weakly coupled QCD kinetic theory using the first correction to the free-streaming background. We derive a parametric scaling formula that relates hadronic flow in systems of different sizes and different generic initial gluon distributions. We comment on similarities and differences between the full QCD effective kinetic theory and the toy models used previously.
Motivated by the early-time dynamics of the quark-gluon plasma in high-energy heavy-ion collisions, we study spectral excitations of overoccupied gauge theories far from equilibrium using classical-statistical lattice simulations. In 3+1 dimensions we find that the spectral function exhibits quasiparticle excitations at all momenta that are mostly consistent with perturbative hard-thermal loop predictions, while partially showing nonperturbative deviations. In contrast, the structure of excitations in 2+1 dimensions is nontrivial and nonperturbative. These nonperturbative interactions lead to broad excitation peaks in the spectral function. Their width is comparable to the frequency of soft excitations, demonstrating the absence of soft quasiparticles in these theories. Our results thus suggest that effective kinetic theory descriptions of such 2+1D systems require collision kernels that must be nonperturbatively determined.
Penetrating probes in heavy-ion collisions, like jets and photons, are sensitive to the transport coefficients of the produced quark-gluon plasma. Quantifying this sensitivity requires a detailed understanding of photon emission and jet-medium interaction in a non-equilibrium plasma during the hydrodynamic stages of heavy-ion collisions. Up to now, such an understanding has been hindered by plasma instabilities. These instabilities arise out of equilibrium and lead to spurious divergences when evaluating the rate of interaction of hard probes with the plasma. In this talk, we show that taking into account the time evolution of an unstable plasma cures these divergences. Specifically, we calculate the time evolution of gluon two-point correlators in a setup with a small initial momentum anisotropy and show that it factorizes into a term describing a fluctuating cloud of soft gluons and a finite, time-dependent term that describes the instabilities. Finally, we discuss phenomenological implications for probes of non-equilibrium plasma.
co-host: David Richards
We present a Monte-Carlo-based analysis in the Jefferson Lab Angular Momentum (JAM) Collaboration framework of the combined world polarized deep-inelastic scattering (DIS) data at moderately small values of the Bjorken $x$ variable using spin-dependent quark dipoles. We demonstrate that the world data on the double-spin asymmetries $A_{||}$ and $A_1$ at $x<0.1$ can be successfully described in this framework with Born-level initial conditions and in the limit of a large number of quark colors. This is the first ever phenomenological demonstration that global polarized DIS data at small $x$ can be fit solely by evolving spin-dependent quark dipoles. In addition, we assess the impact of the data to be collected at the Electron-Ion Collider on our knowledge of these observables, including the potential role of parity-violating DIS.
We discuss the role of the chiral “triangle” anomaly in deeply inelastic scattering (DIS) of electrons off polarized protons employing a powerful worldline formalism which allows for the efficient computation of perturbative multi-leg Feynman amplitudes. We demonstrate how the triangle anomaly appears at high energies in the DIS "box diagram" for the polarized structure function $g_1(x_B, Q^2)$ in both the Bjorken limit of large $Q^2$ and in the Regge limit of small $x_B$. We show for the first time that the off-forward infrared pole of the anomaly appears in both limits. We motivate a small x effective action, consistent with anomalous chiral Ward identities, that shows how non-perturbative effects cancel the infrared pole, leading to an effective axion-like dynamics at small x. There are two non-perturbative scales that control this dynamics: one is the saturation scale and the other is the pure Yang-Mills topological susceptibility; we discuss how their dynamical inter play can be uncovered in polarized DIS at the Electron-Ion Collider.
We discuss evolution of the small-x gluon helicity operator in the rapidity factorization approach. The operator is constructed from a polarized Wilson line with one non-eikonal local operator insertion and gives rise to gluon helicity TMDs at small x. To obtain the evolution equation for this operator we employ the background field method and derive the form of the leading sub-eikonal correction to the gluon propagator in the background-Feynman gauge. We also discuss relation to the large-x polarized DGLAP evolution.
The small-$x$ quark helicity evolution equations at double-logarithmic order, with the kernel $\sim \alpha_s\ln^2 (1/x)$, had been derived previously, and the equations were solved analytically at large $N_c$ and numerically at large $N_c$ and $N_f$. (Here, $N_c$ and $N_f$ are the numbers of quark colors and flavors, respectively.) In this work, we derive the single-logarithmic corrections to the double-logarithmic equations derived previously, that is, we find the correction to order $\alpha_s\ln (1/x)$ of the evolution kernel. The new equations include the effects of the running coupling and the unpolarized small-$x$ evolution, both of which are parametrically significant at single-logarithmic order. The large-$N_c$ and large-$N_c \& N_f$ approximations to the equation are computed. Their solution will provide a more precise estimate of the quark helicity distribution at small $x$, contributing to the resolution of the proton spin puzzle.
Color charge correlations in the proton at moderately small x are extracted from its light-cone wave function. Charge fluctuations are far from Gaussian and the correlators exhibit non-trivial dependence on impact parameter as well as on the relative transverse momentum (or distance) of the gluon probes.
This analysis also provides initial conditions for small-x Balitsky-Kovchegov evolution of the dipole scattering amplitude which include impact parameter and r*b dependence, and with non-zero C-odd component due to three-gluon exchange.
The color charge correlators could be measured through various exclusive processes at the EIC. They also determine unintegrated gluon distributions of the proton relevant for the physics of p - p, and p - A collisions, e.g. for understanding the initial state.
co-host: Garth Huber
The gravitational structure of hadrons is encoded in the gravitational form factors (GFFs),which are the form factors of the energy-momentum tensor of QCD. Just like the energy-momentum tensor, GFFs can be split between the quark and gluon contributions, and are directly related to the frame dependent energy, pressure and pressure anisotropy densities within hadrons. We use lattice QCD techniques to calculate the gluon GFFs of spin 0, 1/2, 1 and 3/2 hadrons in the spacelike kinematic region of 0 < -t < 2 $\text{GeV}^2$ at an unphysical pion mass of 450 MeV and 2+1 fermion flavors. From the renormalized GFFs, we extract the energy, pressure and pressure anisotropy densities in different frames, as well as the mass and mechanical radii, and compare between the different hadrons and different frames.
The mass radius is a fundamental property of the proton that so far has not been determined from experiment. Basing on my recent paper arXiv:2102:00110, I will show that the mass radius of the proton can be rigorously defined through the formfactor of the trace of the energy-momentum tensor (EMT) of QCD in the weak gravitational field approximation, as appropriate for this problem. I will then demonstrate that the scale anomaly of QCD enables the extraction of the formfactor of the trace of the EMT from the data on threshold photoproduction of J/ψ and Υ quarkonia, and use the recent GlueX Collaboration data to extract the r.m.s. mass radius of the proton R_m = 0.55 ± 0.03 fm. The extracted mass radius is significantly smaller than the r.m.s. charge radius of the proton R_C = 0.8409 ± 0.0004 fm. I will discuss the possible origin of this difference, and outline future measurements at JLab, RHIC and EIC that should enable a more precise determination.
We briefly review the key aspect of the QCD instanton vacuum in relation to the quantum breaking of conformal symmetry and the trace anomaly. We use Ji's invariant mass decomposition of the energy momentum tensor together with the trace anomaly, to discuss the mass budget of the nucleon and pion in the QCD instanton vacuum. A measure of the gluon condensate in the nucleon, is a measure of the compressibility of the QCD instanton vacuum as a dilute topological liquid.
A number of different decompositions for the proton mass have been proposed in the literature. All of them can be expressed as matrix elements of operators related to the Energy-Momentum Tensor in quantum chromodynamics. We review and revisit these decompositions, by paying special attention to recent developments with regard to the renormalization of the energy-momentum tensor. We present numerical results for the various terms of the mass decompositions up to the third order in the strong coupling, discussing their scheme dependence.
The proton mass can be decomposed into the sum of various energies through the QCD Hamiltonian operator. I discuss what is natural and physically meaningful in this decomposition, clarifying misconceptions in the literature. I particularly emphasize the role of quantum anomalous energy which can be accessed through quarkonium production experiments at JLab 12 GeV and future EIC.
co-host: Vincent Cheung
Ultra-peripheral collisions are the energy frontier for photon-mediated interactions, reaching, at the Large Hadron Collider (LHC), γ−p center of mass energies five to ten times higher than at HERA and reaching γγ energies higher than at LEP. Photoproduction of vector mesons and dijets in AA collisions probes the gluon distribution in the nuclei at Bjorken-x values down to 10−5, far smaller than can be otherwise studied. Overview of the recent experimental results measured by LHC and RHIC collaborations as well as advances in theoretical studies of the gluon distributions in the nuclei is presented.
Diffractive vector meson production in ultra-peripheral p+A and A+A collisions provides insight into the gluon distribution and its fluctuations in protons and heavier nuclei. I will present calculations of coherent and incoherent differential J/Psi cross sections within a dipole model and the Color Glass Condensate effective theory. We compute the energy/rapidity dependence of the fluctuating gluon distributions using small x evolution equations, which leads to clear predictions for the ratio of the coherent to incoherent cross sections. We further present results for J/Psi production in ultra-peripheral d+Au collisions and compare to experimental data from the STAR collaboration.
The ultra-peripheral collisions (UPCs) of relativistic ion beams lead to a prevalent and diverse set of photon-nucleus interactions in the case of Pb+Pb beams and photon-proton interactions in the case of $p$+Pb beams. The measurements of particles and their interactions produced in photo-nucleon reactions can shed light on the QCD dynamics of EM-induced, extremely asymmetric colliding systems, with energies between those available at RHIC and the LHC. Understanding the hadronic fluctuation spectrum of the photon in this fashion is also critical for maximizing the precision of measurements at a future Electron Ion Collider facility.
This talk presents measurements of two-particle long-range azimuthal correlations in photo-nuclear collisions using 1.73 nb$^{-1}$ of 5.02 TeV Pb+Pb data collected by the ATLAS experiment as well as long-range azimuthal correlations in photon-proton collisions using 68.8 nb$^{-1}$ of 8.16 TeV $p$Pb data collected by the CMS experiment. Candidate photo-nuclear and photo-proton events are selected using a combination of the zero-degree calorimeter and reconstructed pseudorapidity gaps.
Correlation functions are constructed using charged-particle tracks, separated in pseudorapidity, and binned in trigger-particle $p_{T}$ as well as charged-particle event multiplicity. The ATLAS experiment performs a template fitting procedure to subtract the non-flow contribution. After subtraction, significant elliptic and triangular flow-like correlations are observed in photo-nuclear collisions and are presented as a function of charged-particle multiplicity and $p_{T}$. The results are compared to flow coefficients obtained in proton-proton and proton-lead collisions in similar multiplicity ranges.
The electromagnetic moments of the tau lepton are highly sensitive to new physics but are challenging to measure due to the short tau lifetime. We propose a strategy using heavy ion collisions at the LHC as an intense source of photon collisions in order to surpass 15 year old lepton collider constraints on the tau anomalous magnetic moment. This exciting possibility could be achievable today using data which has already been recorded.
Based on joint work with J. Liu: Phys. Rev. D 102, 113008 (2020).
Ultra-relativistic heavy-ion collisions produce the strongest electromagnetic fields in the known Universe..
These highly-Lorentz contracted fields manifest themselves as linearly polarized quasi-real photons that can interact via the Breit-Wheeler process to produce lepton anti-lepton pairs. The energy and momentum distribution of the produced dileptons carry information about the strength and spatial distribution of the colliding fields. Recently it has been demonstrated that photons from these fields can interact even in heavy-ion collisions with hadronic overlap, providing a purely electromagnetic probe of the produced medium.
In this talk I will review the recent experimental progress in measuring photon-mediated processes both in ultra-peripheral and in heavy-ion collisions with nuclear overlap where these purely electromagnetic processes may provide a pristine probe of the produced medium. The theoretical description of these processes will be outlined to guide the discussion of the physics implications and future prospects related to photon-mediated processes.
Muon pairs produced via two-photon scattering processes in hadronic Pb+Pb collisions provide a potentially sensitive electromagnetic probe of the quark gluon plasma. First measurements by ATLAS and STAR of dileptons produced via two-photon scattering in non-ultra-peripheral (non-UPC) nucleus-nucleus collisions showed an unexpected centrality-dependent broadening of the angular correlation between the two leptons and/or of the two-lepton $p_{\mathrm{T}}$ distribution. Recent measurements by ATLAS with significantly increased statistics have allowed new features to be observed in the data, both in the yields of the pairs as well as in their angular distributions. This talk will summarize the differential measurements of the dependence of the pair-distribution on the transverse-momentum and rapidity of the two muons, as well as the dependence on the event centrality. The possible physics implications will also be discussed.
co-host: Vincent Cheung
Quarkonium in exclusive electro- and photoproduction provides for a unique probe to the gluonic structure of the nucleon. A new generation of experiments at Jefferson Lab in the 12 GeV era, and at the electron-ion collider (EIC) will use near-threshold quarkonium production to study topics related to the role of the quantum anomalous energy in the origin of the proton mass, the nature of the color Van der Waals force, and the existence of the LHCb hidden-charm pentaquark. I will discuss the current and future threshold quarkonium program at Jefferson Lab and the EIC.
The Relativistic Heavy-Ion Collider (RHIC) is a unique facility. It is the world's only polarized proton + proton collider capable of delivering highly polarized protons up to a center-of-mass energy of 510 GeV. Polarized proton + proton collisions allow one to study the proton's spin structure using strong interactions by measuring single and double spin asymmetries. Using longitudinally polarized protons, RHIC can probe the proton's longitudinal spin structure, providing insights into the proton's parton helicity distributions. Employing transversely polarized proton collisions, RHIC can study the transverse spin structure of the proton and properties of QCD through transverse spin effects. Presented here is a summary of recent RHIC results and how they play a crucial role in our understanding of the proton spin structure.
I describe recent work on the equilibration process in heavy ion collisions by outlining some old ideas from QCD kinetics, and their recent realizations in practical computer codes. These calculations connect hydrodynamic phenomenology in large nuclei with properties of the incoming wave-functions, placing reasonable constraints on the magnitude of the saturation scale in central heavy ion collisions. I will also review efforts to probe this current theoretical understanding of the equilibration processes using the measured of correlations in smaller nuclei.
co-host: Vincent Cheung and Ramona Vogt