Session K12: Hadronic Physics I

Live

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. \\[-0.3mm] \linebreak 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. \\[-0.3mm] \linebreak This talk will summarize the results of the experiment and the status of ancillary analyses such as $1s/1p$ shell dependence on transparency. \\[-0.3mm]   

Live

The Generalized Polarizabilities (GPs) are fundamental properties of the nucleon. They characterize the nucleon's response to an applied electromagnetic field, offering access to the polarization densities inside the nucleon, and as such they represent an essential part for a complete understanding of the nucleon structure and dynamics. The GPs can be explored through the measurement of the Virtual Compton Scattering reaction. The VCS experiment (E12-15-001) at JLab was recently carried out in Hall C using the new Super High Momentum Spectrometer (SHMS) for electron detection and the High Momentum Spectrometer (HMS) for proton detection, with the virtual photon identified by missing mass. The experiment focuses on extracting the electric and the magnetic GPs of the proton in the intermediate four-momentum transfer squared region, namely from Q$^{\mathrm{2}}=$0.3 (GeV$^{\mathrm{2}})$ to Q$^{\mathrm{2}}=$0.7 (GeV$^{\mathrm{2}})$. The status of the experiment analysis, preliminary results and future prospects will be discussed in this talk.   

Live

The proton charge radius puzzle is the discrepancy between the smaller values from the muonic hydrogen spectroscopy and the larger radius previously established by electron-proton scattering and hydrogen spectroscopy. This puzzle has motivated new experiments of all three methods and re-analysis of previous experiments' data during the past couple of years. Regardless of those new results, the puzzle has not been fully settled yet on a consensus in the community. In late 2019, the PRad experiment observed a smaller radius than all previous electron-proton scattering experiments and is compatible with the one from muonic hydrogen spectroscopy. Besides, it also raised a discrepancy of the proton electric form factor in $Q^2$ range from 0.02 (GeV/c)$^2$ to 0.06 (GeV/c)$^2$ from previous scattering experiments. I will present an on-going electron-proton scattering experiment of the A1 collaboration at the Mainz Microtron, which had data-taking runs in early 2020. This experiment uses a windowless gas jet target and circularly movable spectrometers, aiming to measure proton electric form factor in 0.006 (GeV/c)$^2 \leq Q^2 \leq$ 0.07 (GeV/c)$^2$, covering the region with the discrepancy. This work is partially supported by DOE grant DE-FG02-94ER40818 and NSF grant 2012114.   

Live

Understanding the structure of hadronic resonant states has proven difficult due to their short lifetimes as well as the non-perturbative nature of QCD. However, such information can be extracted from a class of scattering amplitudes that couple two-body states via a single external current. In this talk, I will present an exact analytic representation for the on-shell amplitudes in terms of generalized form-factors and known kinematic functions. From these amplitudes, I will then show how to define the elastic form-factors of resonant states. The finite-volume formalism presented in [Phys. Rev. D 100, 034511] allowed for the extraction of these amplitudes from lattice QCD. Combined with this formalism, this work provides a comprehensive foundation for calculating resonant from-factors using lattice techniques.   

Live

Charge symmetry is generally assumed to be valid in global fits of data to extract parton distribution functions. At the quark level, the violation of this symmetry arises due to the small mass difference between the up and the down quarks as well as the electromagnetic interaction between the quarks. Although the charge symmetry violation (CSV) is expected to be very small, the precision of the existing data can only constrain it to be $\ <$ $\pm$ 10 \%. Jefferson Lab Hall-C experiment E12-09-002 was the first experiment that aimed to place constraints on the degree of CSV in the valence quark distributions in the nucleon via semi-inclusive deep inelastic scattering (SIDIS). In this experiment, a 10.6 GeV electron beam was incident on a liquid deuterium target with the scattered electrons and charged pions detected in coincidence in the HMS and SHMS spectrometers respectively. We measured the ratios of charged pion cross-sections with high precision to extract and place limits on the charge symmetry violating parton distribution functions. Some preliminary cross-section ratios of charged pions and the current progress in data analysis will be discussed in this talk.   

Not Participating

The data collected by the CLAS12 - the large acceptance detector located in the Hall B of the Jefferson Laboratory - allow to obtain a fully multidimensional description of Semi Inclusive Deep Inelastic Scattering (SIDIS) in a wide kinematic range. In this talk, we focus on the study of transverse momentum distribution from pion electroproduction data collected by CLAS12 collaboration on hydrogen target with a 10.6 GeV energy electron beam. The multiplicity of pions will be shown, and phase space effects will be discussed with respect the extraction of the intrinsic partonic transverse momentum.   

Live

Generalized transverse momentum-dependent parton distributions (GTMDs) provide a comprehensive framework for imaging the internal structure of the proton. In particular, by encoding the simultaneous distribution of quark transverse positions and momenta, they allow one to directly access longitudinal quark orbital angular momentum (OAM), and, moreover, to correlate it with the quark helicity. The relevant GTMD is evaluated through a lattice calculation of a proton matrix element of a quark bilocal operator (the separation in which is Fourier conjugate to the quark momentum) featuring a momentum transfer (which is Fourier conjugate to the quark position), as well as the Dirac structure appropriate for capturing the quark helicity. The weighting by quark transverse position requires a derivative with respect to momentum transfer, which is obtained in unbiased fashion using a direct derivative method. The lattice calculation is performed directly at the physical pion mass, using domain wall fermions to mitigate operator mixing effects. Both the Jaffe-Manohar as well as the Ji quark spin-orbit correlations are extracted.   

Live

The $k_{T}$ moment of the generalized transverse momentum distribution (GTMD) describing the orbital motion of quarks in the nucleon, $F_{14}$, was shown in Refs. [1,2] to be related to a combination of the twist-2 generalized parton distributions (GPDs) H and E, and the twist-3 GPD $\widetilde{E}_{2T}$. We compare the behavior of the GTMD $F_{14}$ and the twist-3 GPD $\widetilde{E}_{2T}$ using an overlap model with and without the contribution of the gauge link. This tests the validity of the Wandzura-Wilczek relation for the twist-3 GPD $\widetilde{E}_{2T}$. [1] A. Rajan, M. Engelhardt and S. Liuti, Phys. Rev. D 98, 074022 (2018). [2] A. Rajan, A. Courtoy, M. Engelhardt, and S. Liuti, Phys. Rev.D94, 034041 (2016), 1601.06117.   

Live

The CLAS12 physics program is involved in the studies of the nucleon resonance spectrum and structure in electroproduction of a large variety of final states. With new CLAS12 data, we can investigate the electroproduction of the $\eta$ and $\eta$' mesons, which will provide a complementary tool to study nucleon resonances N*. Due to the fact that both $\eta$ and $\eta$' have isospin I = 0 this provides an “isospin filter” that allows us to access nucleon resonances where I = 1/2. One of the primary steps for this work is the identification of $\eta$ and $\eta$' in the 6.5 GeV data collected by Run Group K (center of mass energy W $\sim$ 1-3 GeV, photon virtuality Q$^2$ $\sim$ 0-6 GeV$^2$, and luminosity $\sim$ 54 fb$^{-1}$) via their decays into $\gamma$'s and $\pi$'s. This will require the use of Monte Carlo simulations and the development of programs that filter the data for the relevant final states. Some channels pose unique issues of particle identification and complex combinatorics. To verify the detection of $\eta$ and $\eta$', mass, momentum, angle and other plots will need to be produced and compared to MC simulations. This work is in contrast to the bulk of the current world database, which focuses on $\pi$N final states.