Session LA: Spin, Size and Structure of the Nucleon

Precision studies of the DVCS process at JLab

Generalized Parton Distribution (GPDs) describe the correlation be- tween the spatial distribution of the quarks and its longitudinal momentum fraction. Their definition in the mid 1990’s has revolutionized our approach to the description of the internal structure of the nucleon. The study of the GPDs together with the study of similar quantities are at the fore-front of today hadronic physics enterprise. Deeply Virtual Compton Scattering (DVCS) off the nucleon (\gamma $^{\ast}$N \rightarrow \gamma $N$)$ is the simplest process which is sensitive to the GPDs. It has been measured at Hera, Compass and JLab. In this talk, I will review the recent results of the Hall A @ JLab scheme, including results of the experiment performed with the upgraded JLab 12 GeV beam.   

Sea quark polarization results from STAR

Polarized proton-proton collision experiments at RHIC provide unique opportunities to study the spin structure of nucleon. One of the primary motivations of RHIC spin program is to probe sea quark spin-flavor structure via the $W$ boson production at $\sqrt{s}=$ 500 GeV proton-proton collisions. The $W$ longitudinal single-spin asymmetry, $A_L$, measurements with STAR have provided significant constraints on the polarized Parton Distribution Functions and especially the first experimental indication of a flavor asymmetry of polarized sea. In this talk, the final $W$ $A_L$ results from STAR and their impacts on the sea quark polarization will be presented.   

Measurement of the Flavor Asymmetry in the Proton’s Sea Quarks

For over 40 years we have understood protons to be bound states of quarks and gluons interacting through the strong force, described by Quantum Chromodynamics (QCD). At large energy scales, perturbative QCD successfully describes the strong interaction, yet our understanding of the low energy dynamics that form a physical proton from quarks and gluons is, at best, poor. Both experiment and theory fail to explain basic properties including the proton spin, mass, and the flavor composition of the antiquark sea. Contrary to naïve assumptions, a remarkable asymmetry between the anti-down ($\bar{d}$) and anti-up ($\bar{u}$) quarks has been observed. This large of an symmetry cannot be generated through perturbative QCD and demonstrates that at any energy scale, there is a fundamental anti-quark component in the proton. The Drell-Yan reaction is uniquely sensitive to antiquark distributions of the interacting hadrons because the reaction requires an anti-quark in one of the initial state hadrons. With the kinematics of the SeaQuest spectrometer, this reaction is particularly sensitive to the anti-quarks distributions in the target nuclei. The E906/SeaQuest collaboration has measured the ratio of deuterium to hydrogen Drell-Yan cross sections. From these data, we have extracted the ratio of $\bar{d}/ \bar{u}$. These data extend the range of previous measurements to larger x$_{Bj}$ and improve the statistical significance for a range of lower x$_{Bj}$. This talk will present these measurements and the extracted ratio. It will also highlight other continuing analyses and future Drell-Yan measurements at Fermilab.   

The Result on the Proton Charge Radius from the PRad Experiment

Motivated by the desire to resolve the proton charge radius puzzle that started in 2010, the PRad experiment (E12-11-106) was performed in 2016 in Hall B at Jefferson Lab, with 1.1 GeV and 2.2 GeV unpolarized electron beams to measure the e-p elastic scattering cross sections at very low values of four-momentum transfer squared (Q$^{2}$), covering a range of $2\times10$$^{-4}$$ - 6\times10$$^{-2}$(GeV/c)$^{2}$, with a sub-percent precision. The proton electric form factor is then extracted from the measured cross section in order to extract the proton charge radius. The experiment utilized a calorimetric method with a high-resolution calorimeter (HyCal) that is magnet free, and two large- area, high-spatial resolution Gas Electron Multiplier (GEM) detectors. The experiment also used a windowless target flowing cryogenically cooled H$_{2}$ gas to remove typical backgrounds from target cell windows. The systematic uncertainties of the absolute cross section measurement from e-p elastic scattering are also controlled by the well-known M\"{o}ller scattering process, which was measured simultaneously within similar kinematics and detector acceptances in this experiment. In this talk, I will discuss the experiment, the data analysis, and present the result on the proton charge radius from PRad. \\ \\ For the PRad Collaboration