Positron Working Group Workshop
B1270
Science and Engineering Hall
LOCATION: George Washington University, Science and Engineering Hall (SEH), 800 22nd St. NW, Washington, DC, 20052
ROOM: B1270
DC METRO: Foggy Bottom Station (blue, orange, silver lines, see Metro Map)
HOTELS: In walking distance:
There are many many more hotels that are metro accessible.
PARKING: https://business-services.gwu.edu/visitor-parking
ZOOM LINK: https://gwu-edu.zoom.us/j/96090067523?pwd=dnlLQkhOV0kxcHFER080TkNiMHNrQT09
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9:00 AM
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12:30 PM
PWG - 1Convener: Douglas Higinbotham (Jefferson Lab)
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9:00 AM
Welcome 5mSpeaker: Axel Schmidt (George Washington University)
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9:15 AM
Ce$^+$BAF R&D plan 30m
In this presentation I will review the baseline design for Ce$^+$BAF and explain the R&D Plan for the next few years.
Speaker: Joe Grames (JLab) -
9:45 AM
A high intensity polarized beam photogun for the Ce+BAF positron source 30m
A photogun to generate high intensity, high polarization electron beam with unprecedented kC lifetime is being developed at JLab. The proposed Ce+BAF polarized positron source will require > 1 milliampere CW with > 90% polarization electron beam at 120 MeV. To be practical for a user program, a photogun operating at 1 milliampere should deliver ~ 2 kC high polarization beam for a month without intervention. The limiting factor is ion-back bombardment of the delicate strained super lattice photocathode. The number of ion can be reduced by improving vacuum in the accelerating anode-cathode gap and biasing the anode. Additionally, the ion induced damage can be spread out by illuminating the photocathode with larger laser spot sizes. Earlier tests of this approach resulted in improved lifetime, but was limited by the photogun electrode size. The envisioned photogun design will incorporate large electrodes to accommodate unprecedented large laser spot sizes, and large conical insulators to operate at > 300 kV for beam capture into a SRF booster.
Speaker: Carlos Hernandez-Garcia (Thomas Jefferson National Accelerator Facility) -
10:15 AM
Coffee break 30m
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10:45 AM
Ce$^+$BAF target update 30m
I will give an update about the positron target effort at Jefferson Lab.
Speaker: Silviu Covrig Dusa (Jefferson Lab) -
11:15 AM
A high-power positron converter based on a recirculated liquid metal in-vacuum target 30m
The very large power in electron beams needed to generate positrons and the accompanying enormous power densities preclude solid targets unless some complicated means of heat removal is implemented (e.g., cooled high-speed target rotation). A promising alternative is to use a windowless target and a liquid metal converter. Two liquid metals that have desirable properties for such a converter are gallium-indium-tin (galinstan) and lead-bismuth eutectics. Galinstan is a liquid at room temperature, while lead-bismuth melts at 124°C. Both materials have very low vapor pressures at operating temperatures.
The objective of the Xelera project is to design, build, and verify the performance of a high-power (100 kW electron beam) positron converter based on a recirculated liquid metal in-vacuum target. To this end, a liquid metal target prototype using a free GaInSn jet has been constructed and tested. The key component of the target is the nozzle that generates the jet. The properties of a nozzle that generates a 3 mm thick GaInSn jet (optimized for positron production by a 10 MeV electron beam) were investigated using the open-source computational fluid dynamics software OpenFOAM. A fifth-order polynomial stainless steel nozzle was constructed and tested, first with water in air, and then with GaInSn in vacuum. The GaInSn liquid metal was recirculated by a Liquiflo H7F Mag-Drive gear pump, and the 10 m/s jet velocity was measured by a laser Doppler velocimeter.
Effects of the thermal load, mechanical pressure, and behavior of the jet in the high magnetic field of a solenoid required to collect the positrons are under investigation. A modified GaInSn positron converter target system will be tested at the Upgraded Injector Test Facility at Jefferson Lab.Speaker: Vaclav Kostroun (Xelera Research LLC) -
11:45 AM
Simulations of positron beam generation and capture at Ce+BAF 30m
The concept of generation and capture of polarized positron beams for the CEBAF upgrade is presented. In order to provide the highly polarized positron beam and the high current low polarized positron beam for nuclear physics experiments, the positron source requires a flexible capture system with an adjustable energy selection band. The simulation results of the positron beam generation in a 4 mm thick, water-cooled, rotated tungsten target by a 120 kW polarized electron beam (1 mA, 120 MeV, >90% polarization), the results of beam dynamics calculations in the positron capture section (focusing solenoid downstream of the target and normal conducting 1497 MHz cavities) and the distributions of power deposited by beams in the capture system are presented.
Speaker: Andriy Ushakov (JLab)
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9:00 AM
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12:30 PM
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2:00 PM
Lunch 1h 30m
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2:00 PM
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5:30 PM
PWG - 2Convener: Eric Voutier (Université Paris-Saclay - CNRS/IN2P3/IJCLab)
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2:00 PM
Degraded electron beams for Ce$^+$BAF 30m
Degraded electron beams are electron beams with enlarged emittance due to multiple scattering in thin targets. This talk describes the effort to generate degraded electron beams in CEBAF to measure the machine acceptance and better understand the transport of large phase space beams, with first measurements expected later this year. We will also discuss the potential use of degraded electron beams in Ce$^+$BAF.
Speaker: Amy Sy (Jefferson Lab) -
2:30 PM
Simulation studies of beam-related background: what we have learned from CLAS12 30m
Simulations of beam particle interaction in the target have been used to estimate the background affecting the experiment, optimize the detector configuration, and determine the maximum acceptable luminosity. Lessons learned from CLAS12 background simulation studies will be discussed.
Speaker: Raffaella De Vita (Jefferson Lab) -
3:00 PM
DVCS identification with $e\gamma$ detection @ CLAS12 30m
Deeply Virtual Compton Scattering (DVCS) is a privileged channel to study the structure of the nucleon as their experimental observables let us access information about Generalized Parton Distributions (GPDs). In general, the identification of DVCS events relies on the detection of only two final state particles as the kinematics of the third one can be reconstructed from conservation laws. Nevertheless, the detection of all final states ensures the exclusivity of the process and restrict the background contributions to the $e \gamma N$ final state as shown in recent CLAS12 analysis. Considering a polarized electron beam directed to a liquid hydrogen target, we present the results of a new approach for DVCS event selection from experimental data taken by the CLAS12 detector at Jefferson Lab. The event selection relying on $e \gamma$ detection, together with Machine Learning techniques to restrict the background contribution from SIDIS processes, shows that ignoring the information of the recoil nucleon boost statistics, which reinforces the reported Beam Spin Asymmetry measurements, and give access to a larger phase space, mostly in the small t region of specific interest for GPD studies.
Speaker: Juan Sebastian Alvarado (IJCLab) -
3:30 PM
Tea break 30m
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4:00 PM
Amplitude-level effects in Bhabha scattering from dark boson exchange 30m
In Bhabha scattering, the virtual exchange of a dark boson would interfere with photon exchange to produce amplitude level effects in the yield and polarized-beam asymmetries. I will overview such resonant effects in half a dozen Bhabha scattering observables, then summarize which few may be appropriate for proposals that would still be competitive a decade from now.
Speaker: David Mack (JLab) -
4:30 PM
Challenges in the extraction of physics beyond SM from electron scatterings 30m
We investigate the corrections to the beam asymmetries in parity-violating electron scatterings arising from charge symmetry violation, strange quark, and charm quark distributions. Based on the parton distributions from the NNPDF Collaboration, these corrections could lead to $(1$-$2)\%$ uncertainties in the extraction of the weak couplings $g^{eq}_{AV}$ and $g^{eq}_{VA}$, and as large as $4\%$ uncertainty for $g^{eq}_{AA}$ at a typical scale of $Q^2$=10 GeV$^2$.
Speaker: Xuangong Wang (University of Adelaide) -
5:00 PM
The Nuclear Radius Extraction Collaboration (NREC) 30m
The extraction of form factors, radii and related quantities have been
an important focus of research in the last decades. The research
intensity has increased with the proton radius puzzle about a decade ago, and more recently with results on the gravitational form factors. But the field is much wider, including also weak form factors, other radii and a multitude of particles beyond the proton, including mesons, hyperons, light and heavy nuclei and so on. However, many techniques are common and translatable between these subfields. The founding members of NREC hope that NREC can provide a forum for information exchange to find synergies between these groups. Of utmost importance for reliable extractions are radiative corrections, including two photon exchange, and a positron beam at JLAB will be able to provide crucial input. In the talk, I will provide an overview of NREC, our goals and the next steps. I will also discuss how I see the goals of PWG and NREC will overlap and what synergy can be found.Speaker: Jan Bernauer (Stony Brook University / RBRC)
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2:00 PM
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9:00 AM
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12:30 PM
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12:30 PM
PWG - 3: TBDConvener: Jan Bernauer (Stony Brook University / RBRC)
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9:00 AM
Polarization transfer in $e^+p$ elastic scattering using the Super BigBite Spectrometer 30m
One of the principal motivations for accelerating positrons in CEBAF is to perform direct measurements of positron-proton elastic scattering observables, toward a conclusive resolution of the long-standing discrepancy between extractions of the proton electromagnetic form factor ratio based on cross section measurements and double-polarization observables, especially polarization transfer. Hard two-photon exchange (TPE) contributions ignored by standard radiative correction procedures are widely thought to be responsible for this discrepancy. However, conclusive empirical verification of this explanation remains elusive more than two decades after the discovery of the discrepancy, owing largely to the difficulty of precision measurements of positron-proton elastic scattering at large momentum transfers. Two complementary proposals to measure positron-proton/electron-proton cross section ratios and to perform Rosenbluth separations of positron-proton elastic cross sections (in Halls B and C respectively) have already been (conditionally) approved by the CEBAF PAC. A precise measurement of polarization transfer using positron beams would provide complementary sensitivity to TPE amplitudes as compared to cross section measurements. This observable has never been measured before and would provide precise constraints and consistency checks toward a model-independent extraction of TPE amplitudes in elastic lepton-nucleon scattering. The large-acceptance Super BigBite Spectrometer apparatus would enable such measurements with competitive precision in the regime where the discrepancy is most significant. In this talk, I will discuss a proposed series of measurements of positron-proton polarization transfer, the detailed motivation for such measurements, and the status of PAC proposal development.
Speaker: Andrew Puckett (University of Connecticut) -
9:30 AM
Single-spin asymmetries in $e^-p$ scattering with intermediate state resonances 30m
In this talk I will discuss beam and target normal single-spin asymmetries in electron--proton elastic scattering. Our calculation of the imaginary part of two-photon exchange amplitudes considers resonance intermediate states of spin-parity 1/2± and 3/2± and mass W<1.8 GeV. The latest CLAS exclusive meson electroproduction data are used as input for the transition amplitudes from the proton to the excited resonance states. Of particular interest for future measurements are possibilities for target normal SSA at low energies that could be measured at JLab.
Speaker: Peter Blunden (University of Manitoba) -
10:00 AM
Spin effects in $e^{\pm} p$ scattering in the resonance and DIS regions 30m
Two-photon exchange gives rise to distinctive spin effects in electron/positron-nucleon scattering, which can be observed in inclusive or exclusive (elastic/inelastic) measurements. We report about recent progress in computing the transverse target single-spin asymmetry in ep scattering the resonance region, using systematic methods based on the 1/Nc expansion of QCD [1, 2]. We discuss the phenomena and questions arising in the transition between the resonance and DIS regions, and how they could be studied experimentally. We comment on the prospects of future measurements of the transverse target single-spin asymmetry at JLab.
[1] J. Goity, C. Weiss, C. Willemyns, Phys.Lett. B 835 (2022) 137580
[2] J. Goity, C. Weiss, C. Willemyns, Phys. Rev. D 107 (2023) 9, 094026Speaker: Christian Weiss (Jefferson Lab) -
10:30 AM
Coffee break 30m
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11:00 AM
Impact of a positron beam at JLab on the extraction of GPDs 30m
The connection of experimental data, notably in the DVCS channel, to generalized parton distributions (GPDs) faces a notoriously difficult deconvolution problem. I will present an impact study of the plausible impact of the positron beam on the extraction of Compton form factors, and discuss how this is a very useful data to consolidate the extraction of GPDs.
Speaker: Hervé Dutrieux (College of William & Mary) -
11:30 AM
Application of novel computational techniques for the determination of the proton’s gravitational form factors and mechanical properties 30m
For the past 25 years, many JLab experiments have been dedicated to determining the Generalized Parton Distributions (GPDs), especially by measuring Deeply Virtual Compton Scattering (DVCS) observables. The GPDs describe the correlation between longitudinal momentum fraction and transverse position of partons in the nucleons. GPDs also give access to the Gravitational Form Factors (GFFs) describing the interaction of the gravitational field with the nucleons. This has opened a new avenue of experimental hadronic physics, allowing to extract as-yet unknown quantities such as the pressure distributions inside the nucleon. In a recent Jefferson Lab LDRD project, we propose to extract the GFFs using the DVCS data already taken at Jefferson Lab, by developing a data analysis and fitting procedure based on Neural Networks (NNs). To better perform this extraction, we will rely on input provided by Lattice QCD (LQCD). Finally, we will assess the potential of future experiments aimed at measuring GPDs and the extraction of the GFFs by using simulated pseudo-data.
Speaker: Pierre Chatagnon (Jefferson Lab) -
12:00 PM
Parton structure from electroweak processes with positrons 30m
We discuss opportunities with positron beams for the study of parton distribution functions of the nucleon in deep-inelastic scattering and related processes. Specific topics include the flavor separation of the unpolarized PDFs as well as the spin-dependent distributions.
Speaker: Wally Melnitchouk (Jefferson Lab)
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9:00 AM
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12:30 PM
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2:00 PM
Lunch 1h 30m
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2:00 PM
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6:00 PM
Excursion to the National Cherry Blossom Festival
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9:00 AM
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12:30 PM
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9:00 AM
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12:00 PM
PWG - 4Convener: Axel Schmidt (George Washington University)
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9:00 AM
This presentation is canceled 30m
Axial structure of the nucleon with positron capture at medium energies
A medium energy polarized positron beam would enable the extraction of the axial form factor $G_A(Q^2)$ of the nucleon and its four-momentum transfer square ($Q^2$) dependence, using the weak capture reaction in deuterium ($ \vec{e^+} + {^2H} \rightarrow 2p + \bar{\nu}_{e}$). A polarized positron beam with beam energies between 2.0 - 6.0 GeV can be used for a cross section measurement and a clean measurement of the background utilizing parity violation in the weak capture process. In addition to the poorly known axial form factor it will be possible to extract the axial charge radius ($r_A$), the axial coupling constants ($g_A$), and the axial dipole mass ($M_A$). We propose an experiment using a setup similar to the Tagged Deep Inelastic Scattering experiment (TDIS)-- a thin walled target cell inside a compact solenoidal magnet and a radial recoil detector to tag a pair of recoil protons. The proposed positron capture based measurement would have a completely different set of systematic uncertainties compared to all currently used methods and may help resolve several current discrepancies of the weak interaction parameters. We will discuss progress since the LOI was submitted to PAC 51.
Speaker: Dipangkar Dutta (Mississippi State University) -
9:30 AM
Exploring the partonic structure of $^4$He through exclusive processes with positrons 30mSpeaker: Whitney Armstrong (Argonne National Laboratory)
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10:00 AM
Two-Photon Exchange for semi-inclusive meson electroproduction 30m
Semi-inclusive meson production is used to access transverse momentum dependent distributions (TMD) or partons in a nucleon. We investigate a role of two-photon exchange (TPE) for this process. Such calculations require a model of nucleon structure. At the first step, we perform separation of hard and soft photon exchange. Soft photon exchange is demonstrated to be model-independent due to a low energy theorem. At the same time, hard two-photon exchange is calculated at a quark level, using a diquark model as input. The results demonstrate an effect of a few per cent for a cross-section that can be observed as electron-positron asymmetries.
Speaker: Stinson Lee (George Washington University) -
10:30 AM
Coffee break 30m
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11:00 AM
Use of 160 GeV polarized $\mu^+$ and $\mu^-$ beams to study exclusive reactions at COMPASS 30mSpeaker: Nicole d'Hose (IRFU, CEA, Université Paris-Saclay)
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11:30 AM
Status of the MUon Scattering Experiment (MUSE) at PSI 30m
Inspired by the proton radius puzzle, the MUon Scattering Experiment (MUSE) at Paul Scherrer Institute (PSI) in Villigen, Switzerland, was introduced to provide new information by simultaneously measuring elastic scattering of electrons and muons, as well as positrons and antimuons from a liquid hydrogen target. MUSE aims to provide precise cross sections with extractions of the electric form factor and charge radius for each of the four beam leptons, while addressing the issues of two-photon exchange and lepton universality through charge and species ratios, respectively. An overview of this experiment and the current status of the analysis will be presented. This material is based upon the work supported by the National Science Foundation (NSF) under award PHY-2113436. The MUSE experiment is supported by the Department of Energy (DOE), NSF, PSI, and the US-Israel Binational Science Foundation (BSF).
Speaker: Michael Kohl (Hampton University)
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9:00 AM
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12:00 PM
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2:00 PM
Lunch 2h
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2:00 PM
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5:30 PM
PWG - 5Convener: Joe Grames (JLab)
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2:00 PM
Status of Jefferson Lab upgrade initiatives 45mSpeaker: Cynthia Keppel (Jefferson Lab)
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2:45 PM
Discussion on the contribution of the JLab PWG to the Ce+BAF pre-CDR 45mSpeaker: Douglas Higinbotham (Jefferson Lab)
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3:30 PM
Tea break 30m
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4:00 PM
Discussion on the contribution of the JLab PWG to the Ce+BAF pre-CDR 45mSpeaker: Douglas Higinbotham (Jefferson Lab)
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2:00 PM
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9:00 AM
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12:00 PM