Why nucleon resonance excitations beyond the photopoint can play a unique role in leading the path towards a comprehensive QCD theory will be laid out and illustrated by recent exclusive meson electroproduction cross sections off free and bound nucleons, new anticipated results, as well as potential upcoming opportunities.
In the early days of QCD phenomenology, resonances were thought to arise from single-particle constituent-quark-model-like states dressed by their decay channels to become short-lived resonances. The lowest lying resonances were of primary importance and sophisticated models were developed to accommodate them. Here the N*(1440) Roper resonance, its partner the Δ(1600) resonance and...
In this talk, we'll start by motivating the study of two meson photoproduction in association with a nucleon target. Then we'll review the formalism to describe these reactions before showing the latest results from the CLAS and GlueX experimental collaborations at the Jefferson Lab.
The progress on the so-called proton radius puzzle has been impressive in recent years. Corresponding advances for unstable baryons such as hyperons are more challenging to achieve, but are eagerly awaited: they would reveal the role of flavour in the strong interaction dynamics governing the femtometer structure of hadrons. Due to the short hyperon life-time, most methods that have been...
The High Acceptance Di-Electron Spectrometer (HADES) [1], installed at GSI/FAIR Helmholtzzentrum in Darmstadt, was designed for spectroscopy of positron-electron pairs in heavy-ion reactions in the SIS-18 energy range (1-2 GeV/nucleon). The HADES collaboration has measured inclusive e+e- production in proton-nucleus and nucleus-nucleus systems at various...
Generalized Parton Distributions (GPDs) are a well-established tool for the exploration of the 3D nucleon structure. While extensive studies have been performed to unravel the 3D structure of the ground-state nucleon, little is known about the 3D structure of baryon resonances. The nucleon-to-resonance transition GPDs provide a unique tool for exploring the 3D structure and mechanical...
Visible matter is characterised by a single mass scale; namely, the proton mass. The proton's existence and structure are supposed to be described by quantum chromodynamics (QCD); yet, absent Higgs boson couplings, chromodynamics is scale invariant. Thus, if the Standard Model is truly a part of the theory of Nature, then the distinct qualities of the proton and all its excitations are...
The initial scientific case for upgrading the Continuous Electron Beam Accelerator Facility (CEBAF) at
Jefferson Lab (JLab) to 22 GeV will be presented. The proposed physics program encompasses a large
and diverse range of transforamtive investigations centered around the non-perturbative dynamics
inherent in hadron structure and the exploration of strongly interacting systems. It builds...
At high energies, the (transition) form factors of hadrons are most sensitive to the respective minimal quark content. At low energies, however, the form factors are dominated by universal features related to pion physics. Only these Goldstone bosons can carry information over large distances. Dispersion theory is used to obtain a model-independent representation of vector-isovector baryon...
The existence of exotic multi-quark states beyond the conventional valence three quark and quark-antiquark systems has been unambiguously confirmed in the heavy quark sectors. Such states could manifest as single colour bound objects, or evolve from meson-baryon and meson-meson interactions, creating molecular like systems and re-scattering effects near production thresholds. Equivalent...
The quest of unraveling the nature of excited hadrons necessarily involves determination of universal parameters of these states. Such determinations require input, either from experiment or theory. The challenge in answering these questions from theory arises from the very structure of the theory of strong interaction (QCD).
Lattice gauge theory is the only tool available to us to tackle...
I will give an overview of the recent activities of the Joint Physics Analysis Center and the plans for the future.
The quark model predicts exotic hadrons beyond the conventional quark-antiquark mesons and three quark baryons. Exotic candidates have since been observed in the early 2000's. Since then several exotic states have been discovered. LHCb has reported on tetraquark candidates such as the X(3872), the discovery of pentaquark resonances in 2015, and the first double charmed tetraquark. Many...
Modern experimental facilities, new theoretical techniques for the continuum bound-state problem and progress with lattice-regularized QCD may have provided indications that soft quark+quark (diquark) correlations play a crucial role in hadron physics. For example, theory indicates that the appearance of such correlations is a necessary consequence of dynamical chiral symmetry breaking, viz. a...
Electroproduction reactions reveal the structure of light baryon resonances. Recent results of a simultaneous analysis of $\pi N$, $\eta N$, and $K\Lambda$ electroproduction data with the Juelich-Bonn-Washington (JBW) approach are presented. The extraction of multipoles and their uncertainties is discussed, and preliminary results for transition form factors at the resonance poles are shown.
Diquarks are often used as QCD effective degrees of freedom to describe nucleons and other baryons as well as exotic hadrons. However, even though they are successful in describing many of these low lying QCD states they and their properties have been difficult to pin down robustly. Here we present progress in studying diquarks in a gauge-invariant setup through embedding them in a parent...
The SPring-8 LEPS2 project explores the nature of hadrons through photoproduction processes by using a photon beam which is linear-polarized uniquely up to 2.4 GeV. One of the main subjects of LEPS2 is a light baryon spectroscopy in the s-channel of various meson photoproduction reactions. For this purpose, the BGOegg experiment was carried out with a detector setup where a liquid hydrogen...
Abstract
For the first time in the history of particle physics high intensity beam
of neutral long-lived kaons will be used at JLab to study strange hadron
spectroscopy. In this talk I will discuss the possibility to observe
dozens of missing hyperons predicted by LQCD and CQM, but not
yet observed. It will also allow to observe and measure with high accuracy
positions and widths...
The GlueX Experiment uses an intense photon beam of with energies up to 12 GeV and a large acceptance spectrometer to study many issues in hadron physics. These characteristics allow in particular for the study of comparatively rare photoproduction processes. The study of doubly strange baryons is particularly interesting for understanding the baryon spectrum, due to many states which are...
The term two-pole structure refers to the fact that particular single states in the hadron spec- trum as listed in the PDG tables are often two states. In this talk, I will review the status of this emerging field and its consequences for our understanding of bound states in QCD.
