Session T12: Electromagnetic Interactions II

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The CREX experiment has recently completed the high precision measurement of the parity-violating (PV) asymmetry of $^{48}Ca$ elastic electron scattering. The CREX experiment at Jefferson Lab measured the electroweak PV asymmetry to access neutron distributions of neutron-rich $^{48}Ca$ nucleus and obtain the RMS neutron radius of $^{48}Ca$. Such precise measurement will have a significant impact on nuclear theory, providing a unique experimental input to help bridge ab-initio theoretical approaches used for light nuclei (including 3-neutron forces) and density functional theories appropriate for heavy nuclei. This talk will highlight the recent progress made by the CREX collaboration in required data analysis to achieve this precise measurement.   

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The parity-violating electroweak asymmetry in the elastic scattering of longitudinally polarized electrons from nuclei provides a clean measurement of the RMS radius of neutrons within the nucleus. A precise measurement of this radius can provide meaningful constraints to the density dependence of the symmetry energy in neutron rich nuclear matter, a parameter of the nuclear equation of state. In the summer of 2019, the PREX-II collaboration successfully measured this asymmetry using an electron beam from the CEBAF accelerator at Jefferson Lab incident on an isotopic lead-208 target. The analysis and results of the experiment will be presented, along with a brief discussion of the experimental techniques and challenges required to achieve this precise measurement.   

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Parity-violating electron scattering provides a clean probe of neutron densities that is model independent and free from most of the strong interaction uncertainties. The PREX-2 and CREX experiments that ran at Jefferson lab aimed to measure the nucleon skin thickness in 208Pb and 48Ca via parity violating electroweak asymmetry in the elastic scattering of longitudinally polarized electrons. One of the crucial systematic uncertainty that both of these experiments were sensitive to is the non-parity violating asymmetries that resulted from the helicity-correlated false asymmetries in the polarized electron beam.This talk will describe the polarized source setup and helicity-correlated beam asymmetry results for PREX-2 and CREX.   

Live

The Transverse Asymmetry (AT), also known as the beam-normal single-spin asymmetry, as measured by PREX-II and CREX is defined as the asymmetry of electron scattering cross section between opposite transverse polarities against an unpolarized target. It arises from the higher order EM interaction (the interference between One Photon Exchange (OPE) and Two Photon Exchange (TPE)), giving it a small magnitude. The PREX-II and CREX experiments provide us an opportunity to investigate the underlying physics of TPE. Moreover, the measurements in PREX-II and CREX bring precision beyond HAPPEX and PREX-I, leading to possible new understanding of relationship between AT and A/Z-scaling of different nuclei. Lastly, AT itself is a systematic uncertainty in the PREX-II/CREX data; by measuring it precisely, we will improve the PREX-II and CREX results. In this presentation, I will report the analysis and result of the new AT data collected during 2019-2020 operation of PREX-II/CREX and their implications.   

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Charge symmetry in the parton distributions assumes the distribution of quarks in the proton are related to those in the neutron. Indirect experimental evidence constrains Charge Symmetry Violation (CSV) to be less than 9\%. In Quantum Chromo-Dynamics (QCD), charge symmetry is broken by the mass difference between up and down quarks. CSV in the valence region can be extracted from precision measurements of the cross section ratio of charged pion production in semi-inclusive deep inelastic scattering (SIDIS) from deuterium. The experiment E12-09-002 was conducted at Jefferson Lab in Hall C from fall 2018 to spring 2019 with an upgraded 10.6 GeV electron beam. The experiment detected charged pions in coincidence with scattered electrons covering $Q^2$ from 4 to 5.5 GeV, $x$ for 0.35-0.65, and $z$ from 0.4-0.7. Some preliminary results and the current progress in data analysis will be discussed in this talk.   

Live

A central goal of the Jefferson Lab 12 GeV upgrade is the imaging of the 3-dimensional internal structure of the nucleon at high resolution. The exclusive phi electroproduction process off of a hydrogen target probes the proton’s gluon GPD in the valence region, providing insight into the spatial distribution of the gluons within the proton. An update is given on the exclusive phi meson electroproduction cross section measurement at 10.6 GeV by Jefferson Lab’s CLAS12 detector.   

Live

Understanding the relation between QCD evolution in the Bjorken limit and the Regge limit is crucial to achieve a complete and smooth picture of proton and nuclear structure. The hope in the small $x$ regime, where gluon density is expected to reach saturation and the naif partonic breaks down, was that by computing higher order corrections to small x evolution (BK equations) one would capture more and more of the physics at moderate $x$. However, this research program has encountered some challenges. At NLO large collinear logarithms are present and need to be resummed spoiling the renormalization group structure established at LO. In order to overcome these formal difficulties, we revisit the shock wave approach for high energy scattering. A new gauge invariant operator definition of the unintegrated gluon distribution that accounts systematically for the collinear limit of structure functions emerges naturally in our framework. I will discuss in particular inclusive DIS as a first application.