Bulletin of the American Physical Society
APS April Meeting 2018
Volume 63, Number 4
Saturday–Tuesday, April 14–17, 2018; Columbus, Ohio
Session D12: Hadronic Physics II |
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Sponsoring Units: GHP DNP Chair: Holly Szumila-Vance, Jefferson Lab Room: A222-223 |
Saturday, April 14, 2018 3:30PM - 3:42PM |
D12.00001: Abstract Withdrawn
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Saturday, April 14, 2018 3:42PM - 3:54PM |
D12.00002: Data Analysis and Preliminary Results of the Proton Charge Radius Experiment (PRad) at JLab Weizhi Xiong, Chao Peng In order to investigate the proton radius puzzle, the PRad experiment (E12-11-106\footnote{Spokespersons: A. Gasparian (contact), H. Gao, M. Khandaker, D. Dutta}) was performed in 2016 in Hall B at Jefferson Lab, with both 1.1 and 2.2 GeV unpolarized electron beams. The experiment aims to measure the $e-p$ elastic scattering cross section in an unprecedented low values of momentum transfer squared region ($\rm{Q}^2 = 2\times10^{-4} - 0.06~\rm{(GeV/c)}^2$), with a sub-percent precision. The PRad experiment utilizes a non-magnetic calorimetric method with a large acceptance and high resolution calorimeter (HyCal), and two large area, high spatial resolution Gas Electron Multiplier (GEM) detectors. To have a better control over the systematic uncertainties, the absolute $e-p$ elastic scattering cross section is normalized to that of the well-known M$\o$ller scattering process, which is measured simultaneously within similar kinematics and geometrical acceptances. The windowless H$_2$-gas-flow target utilized in the experiment largely removes a typical background source, the target cell windows. In this talk, we will discuss details of the data analysis and present preliminary results from both beam energy settings. [Preview Abstract] |
Saturday, April 14, 2018 3:54PM - 4:06PM |
D12.00003: Method for robust extraction of proton charge radius from electron-proton-scattering experiments Xuefei Yan A lot of efforts have been devoted to the measurements of the radius of the proton charge distribution ($R$), but results from different experiments and/or analyses have sizable differences. In high-precision muonic hydrogen Lamb shift experiments, $R$ was measured to be $0.8409 \pm 0.0004$ fm, while typical global data analyses of the electron-proton ($ep$) scattering experiments yielded $R = 0.879 \pm 0.009$ fm. This difference has been known as the proton radius puzzle. Some recent global $ep$-scattering-data analyses found $R \approx 0.84$ fm, but the fitting methods used by them were criticized by others in the literature. We formulated a robust fitting method to find the ``real" $R$ in upcoming $ep$-scattering experiments. Comprehensive tests have been carried out using the binning and statistical uncertainties of the PRad experiment (E12-11-106\footnote{Spokespersons: A. Gasparian (contact), H. Gao, M. Khandaker, D. Dutta}). In this talk, we will discuss the details of this method and present the test results. [Preview Abstract] |
Saturday, April 14, 2018 4:06PM - 4:18PM |
D12.00004: Beam tracking for the MUon Scattering Experiment (MUSE) at PSI Tanvi Patel, Michael Kohl, Anusha Liyanage The proton is not an elementary particle but has a substructure governed by the interaction of quarks and gluons. The size of the proton is manifest in the spatial distributions of the electric charge and magnetization, which determine the response to electromagnetic interaction.The proton radius puzzle is known as the large discrepancy between measurements using electrons and muons, respectively. The MUon Scattering Experiment (MUSE) in preparation at the Paul-Scherrer Institute (PSI) has the potential to resolve the puzzle by measuring the proton charge radius with electron and muon scattering simultaneously and with high precision, including any possible difference between the two, and with both beam charges. In the presentation the status of the installation and performance of the beam tracking detectors based on Gas Electron Multipliers will be reported. [Preview Abstract] |
Saturday, April 14, 2018 4:18PM - 4:30PM |
D12.00005: Gluon Helicity Distribution from Longitudinally Polarized Proton Collisions at STAR Suvarna Ramachandran The spin program at STAR has been exploring a wide range of measurements with longitudinally polarized protons to determine the gluon helicity distribution inside the proton and understand its contribution to the spin of the proton. The inclusive jets and pions in the kinematic range accessed by RHIC are dominantly produced from quark-gluon and gluon-gluon scattering processes. The longitudinal double-spin asymmetry ($A_{LL}$) is sensitive to polarized parton distributions and can be used to extract information about the gluon helicity contribution ($\Delta G$) to the spin of the proton. Previous $A_{LL}$ measurements have shown the first evidence of polarized gluons for gluon momentum fractions above 0.05. The data collected at $\sqrt s = 510$ GeV will extend the current constraints on $\Delta G$ to lower gluon momentum fractions, and the measurement of dijet $A_{LL}$ will allow for the reconstruction of the partonic kinematics at leading order. This presentation will focus on the recent results from the $A_{LL}$ measurements at STAR, and how they extend the sensitivity to the gluons at lower momentum fractions. [Preview Abstract] |
Saturday, April 14, 2018 4:30PM - 4:42PM |
D12.00006: Small-$x$ Asymptotics of the Gluon Helicity Distribution Yuri Kovchegov, Daniel Pitonyak, Matthew Sievert We determine the small-$x$ asymptotics of the gluon helicity distribution in a proton at leading order in perturbative QCD at large $N_c$. To achieve this, we begin by evaluating the dipole gluon helicity TMD at small $x$. We then construct and solve novel small-$x$ large-$N_c$ evolution equations for the operator related to the dipole gluon helicity TMD. Our main result is the small-$x$ asymptotics for the gluon helicity distribution: $\Delta G \sim \left( \tfrac{1}{x} \right)^{\alpha_h^G}$ with $\alpha_h^G = \tfrac{13}{4 \sqrt{3}} \, \sqrt{\tfrac{\as \, N_c}{2 \pi}} \approx 1.88 \, \sqrt{\tfrac{\as \, N_c}{2 \pi}}$. We note that the power $\alpha_h^G$ is approximately 20$\%$ lower than the corresponding power $\alpha_h^q$ for the small-$x$ asymptotics of the quark helicity distribution defined by $\Delta q \sim \left( \tfrac{1}{x} \right)^{\alpha_h^q}$ with $\alpha_h^q = \tfrac{4}{\sqrt{3}} \, \sqrt{\tfrac{\as \, N_c}{2 \pi}} \approx 2.31 \, \sqrt{\tfrac{\as \, N_c}{2 \pi}}$ found in our earlier work. [Preview Abstract] |
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