Session J16: Nucleon Structure and Nucleon Spin

Live

The Gerasimov-Drell-Hearn (GDH) sum rule links the anomalous magnetic moment of any target particle to the helicity-dependence of its total photo-production cross section. We plan to measure the integrand of the GDH sum rule for protons and neutrons, for the first time using photons of energy between 3 and 12 GeV. Hall D at Jefferson Lab, with its high-luminosity photon tagger and its large solid angle detector is well-suited for a high precision experiment using existing standard equipment and a new polarized target. A failure of the sum rule would reveal new nucleon structural processes. Using 3 weeks of beam time this experiment would be able to check the convergence of the sum rule for both nucleons, significantly improve the statistics and systematics on the intercept of the $a_1$ Regge trajectory, yield the first non-zero polarized deuteron asymmetry in the diffractive regime, significantly improve the uncertainty in the polarizability contribution to the proton-structure correction in muonic hydrogen hyperfine splitting, provide a real-photon baseline for studies of the transition between polarized DIS and diffractive regimes, and constrain quark compositeness or size. In this talk we will introduce the measurement and discuss some of these implications.   

Live

Nuclear physics seeks to create a model of how quarks and gluons interact. For decades, electron scattering has been used to study the distribution of charge and magnetization in the nucleon. Even though electric and magnetic form factors have been measured, gluonic form factors in the proton have not. J/$\psi $ photoproduction near threshold is sensitive to the color charge distribution of the proton. Available data on J/$\psi $ photoproduction at high energies are described by a diffractive, two-gluon exchange mechanism. However, J/$\psi $ photoproduction at low energies may deviate from the two-gluon model in favor of the three-gluon model, which has been supported by the recent data from GlueX in Hall D at Jefferson Lab. With beam energies up to 11 GeV from CEBAF, the CLAS12 detector is capable of studying J/$\psi $ photoproduction near threshold. The reaction is studied in the untagged photoproduction regime when the incoming electron scatters at approximately 0 degrees. In this talk, we present an analysis of data from the Fall 2018 run of CLAS12 with a 10.6 GeV beam impinging on a liquid Hydrogen target. Details of analysis will be discussed, including particle identification and event selection, using the experimental data and Monte-Carlo simulations.   

Live

The third Deeply Virtual Compton Scattering experiment (DVCS-3) at the Thomas Jefferson National Accelerator Facility's (TJNAF) Hall A measured both helicity-dependent and helicity-independent cross sections of the H(e,e',$\gamma )$p Deeply Virtual Compton Scattering (DVCS) in a wide Q$^{\mathrm{2}}$ range (from 3 to 9 GeV$^{\mathrm{2}})$, made possible by the recent 12 GeV upgrade, at different values of Bjorken-x (x$_{\mathrm{B}}$ from 0.36 to 0.60). A measurement over this range in Q$^{\mathrm{2}}$ provides a strong test of the Generalized Parton Distribution (GPD) formalism in explaining the proton structure. DVCS is the cleanest way to study GPDs. The key to extracting GPDs from experiments are the Quantum Chromodynamics (QCD) factorization theorems. While DVCS data have given hints of the factorization regime being attained, such hints have yet to be confirmed for Deeply Virtual Meson Production (DVMP) data. Exclusive $\pi^{\mathrm{0}}$ electroproduction has been measured by the DVCS-3 experiment in order to test factorization in DVMP processes. In this talk, I will discuss the experimental setup and preliminary results of the neutral pion electroproduction cross sections for x$_{\mathrm{B}}$\textgreater 0.3 from this experiment.   

Live

There has been significant progress in the theoretical description of the nucleon structure in terms of QCD degrees of freedom, in particular through generalized Parton distributions (GPDS). The flavor degrees of freedom of the produced meson selectively probe aspects of the reaction mechanism and the transition from hadronic to partonic degrees of freedom. Kaon data at larger virtual photon mass allows one to search for the onset of the partonic picture. In this regime, hard and soft physics have been shown to factorize. The lack of necessary experimental facilities has left a gap in l-t separated data for exclusive k+ production from the proton above the resonance region. The newly upgraded 12 GeV beam energy at Jlab has provided an opportunity to expand kaon data. The l-t separated cross section extraction procedure, along with early analysis results, will be discussed following the run of e12-09-011, the Jlab 12 GeV kaon experiment.   

Live

The proton radius puzzle was established in 2010 when the results of a reduced size of the proton obtained with muonic hydrogen spectroscopy were first released, in contradiction with previous measurements based on electron scattering and regular hydrogen spectroscopy. More recently, several new measurements with various techniques seem to prefer a smaller radius, yet without ruling out the older data. The primary motivation of the Muon proton Scattering Experiment (MUSE), which is presently carried out at the Paul Scherrer Institute (PSI), is to measure the proton charge radius with muons, and to specifically test lepton universality and two-photon exchange with definitive precision, i.e. whether there is any difference between muon and electron elastic scattering, or between scattering of leptons with opposite charges from the proton. The motivation, present status and timeline for completion of MUSE will be reviewed.   

Live

The Muon proton Scattering Experiment (MUSE) at the PiM1 beam line of the Paul Scherrer Institute is aiming to simultaneously measure elastic scattering of electrons and muons of either charge from a liquid hydrogen target to extract the charge radius of the proton. By comparing the four scattering cross sections, the experiment will provide more data for the proton radius puzzle and determine if the radius is the same when using different particle types in obtaining the proton radius, and whether there are any sizable two-photon exchange effects. During 2019 the experiment commissioned detectors, studied beam properties, and performed a first scattering measurement with the full experimental system, recording about 500 M events. Initial results for these measurements will be presented.   

Not Participating

The success of all experiments in the Super BigBite Spectrometer (SBS) program depend on large area gas electron multipliers (GEMs) which can handle extremely high particle rates. Over the last decade, our research group at the University of Virginia (UVa) has developed and constructed over 50 large GEMs for the SBS. We are developing new “U-V” GEM trackers that will ensure the success of GEn-RP and other SBS experiments by supplementing "X-Y" trackers already present. The new detectors serve as an additional (front) tracking layer which coordinates using a "U-V" basis (a modified X-Y type basis rotated by 45$^{\circ}$). These new GEMs (150 x 40 $cm^2$ active area each) will be the largest in the world. The design of these new GEM chambers is completed, and production of their components is in process. The GEM chambers will be assembled, tested, and characterized at UVa. Upon completion of this initial construction and verification phase at UVa, the GEMs will then be transported to Jefferson Lab where they will be installed and commissioned onto the SBS apparatus to ensure the success of GEn-RP and other SBS experiments.