Bulletin of the American Physical Society
5th Joint Meeting of the APS Division of Nuclear Physics and the Physical Society of Japan
Volume 63, Number 12
Tuesday–Saturday, October 23–27, 2018; Waikoloa, Hawaii
Session LE: Mini-symposium: Photoproduction and Electroproduction of Hadrons III |
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Chair: Julie Roche, Ohio University Room: Hilton King's 1 |
Saturday, October 27, 2018 9:00AM - 9:15AM |
LE.00001: Searching for the onset of color transparency in Hall C at Jefferson Lab Holly Szumila-Vance
Color transparency (CT) is a fundamental phenomenon of QCD postulating that at high momentum transfer, one can preferentially measure hadrons that fluctuate to a small color neutral transverse size in the nucleus, and final state interactions within the nuclear medium are suppressed. CT is observed experimentally as a rise in the measured nuclear transparency as a function of the momentum transferred. While CT has been observed for mesons, it remains unconfirmed in baryons. An enhancement in the nuclear transparency was observed in A(p,2p) reactions at Brookhaven. This experiment seeks to confirm the measurement of proton transparency as well as to measure the onset. During the spring of 2018, this experiment was the first to run in Hall C at Jefferson Lab using the recently upgraded 12 GeV electron beam and obtained four kinematic points at momentum transfer Q2 from 8-14.3 GeV2, overlapping the same region where Brookhaven previously observed an enhancement. This experiment used the High Momentum Spectrometer (HMS) and Super High Momentum Spectrometer (SHMS) in coincidence to measure A(e,e'p) on a carbon target. This talk will summarize the status of the experiment since the completion of data taking as well as some preliminary results. |
Saturday, October 27, 2018 9:15AM - 9:30AM |
LE.00002: Down Quark Polarization Measurements with CLAS12 Tony Forest Perturbative Quantum Chromodynamics (pQCD) predicts that the spin of a valence quark will align with the net spin of the nucleon if the quark carries most of the nucleon’s momentum. Previous measurements have yet to clearly illustrate this prediction for the down quark in the proton. A comprehensive program to measure deep inelastic electron scattering using a polarized electron beam and polarized proton and deuteron targets will be performed using the recently upgraded facilities at Jefferson Lab. This program will facilitate the extraction of the down quark polarization within a proton to a kinematic regime where the struck quark may carry up to 80% of the proton's momentum. The measurement will use a combination of the accelerator's 12 GeV polarized electron beam, the upgraded detector system, and a new longitudinally polarized target in Jefferson Lab's Hall B. The anticipated precision of these measurements and their potential as a test of the pQCD prediction above will be presented. |
Saturday, October 27, 2018 9:30AM - 9:45AM |
LE.00003: F2 structure function measurements at large x in Hall C at Jefferson Lab Sanghwa Park We present new measurements of the F2 structure function on proton and deuteron at large x, up to 0.99. These measurements were performed at a four-momentum transfer Q2 range up to about 17 GeV2 in Hall C at Jefferson Lab. This experiment was one of the first to be run in Hall C after the completion of the JLab 12 GeV upgrade. We have collected data using the High Momentum Spectrometer and the newly installed Super High Momentum Spectrometer. These new data will be used for the study of the quark-hadron duality and will contribute to a precise determination of the parton distributions at large x. In this talk, the experimental overview and status of the analysis will be discussed. |
Saturday, October 27, 2018 9:45AM - 10:00AM |
LE.00004: New $^3$He Elastic Cross Section Measurements and Global Fits Scott K Barcus Mining data from Jefferson Lab Hall A experiment E08-014, a new measurement of the $^3$He elastic cross section at Q$^2 \approx $ 35 fm$^{-2}$ was extracted. This new data point falls approximately halfway between the first and second diffractive minima of the $^3$He charge form factor. This point's location, along with other recent $^3$He elastic cross section measurements, fill out a previously understudied kinematic region. The new data make an improved fit of the $^3$He elastic cross section world data possible. This presentation will discuss the new cross section measurement and fits in comparison to previous fit results and current theoretical predictions. |
Saturday, October 27, 2018 10:00AM - 10:15AM |
LE.00005: Decay correction for tritium target experiments at Jefferson Lab Tyler T Kutz
The MARATHON experiment ran at Jefferson Lab this spring. MARATHON will determine the neutron to proton structure function ratio $F_2^n/F_2^p$ by exploiting the mirror symmetry of tritium ($^3$H) and helium ($^3$He), as well as measure the magnitude of the EMC effect in these A = 3 nuclei. Alongside MARATHON, several other tritium target experiments have or will run in 2018. In addition to requiring special safety precautions, tritium's radioactive nature presents a unique problem: the composition of the target continually changes due to $^3$H $\rightarrow$ $^3$He decay. Given the half-life of tritium and the length of the experimental run period, the resulting contamination in the tritium target is non-negligible and must be corrected for. This talk will discuss the magnitude of the effect and the time-dependent correction used in the data analysis for MARATHON and other tritium target experiments. |
Saturday, October 27, 2018 10:15AM - 10:30AM |
LE.00006: Exclusive Coherent Electroproduction of the Neutral Pion Off Helium-4 and The Case for Kinematic Fitting Frank Thanh Cao We discuss the results from the exclusive coherent neutral pion electroproduction off a helium-4 target in the CLAS EG6 experiment. With event selection playing a crucial role, the beam-spin asymmetry was extracted and in agreement with a recently published general formulation of pseudoscalar (0−+) electroproduction off scalar targets. The standard use of a series of exclusivity cuts for event selection reduces much of the background but becomes inadequate for low statistics processes where background contamination becomes increasingly important. Kinematic fitting uses full detector information and 4-momenta conservation of exclusivity to select events much more cleanly. It is the events selected from kinematic fitting that are in agreement with the recent formulation. We will explore the discrepancies between the event selection methods and show that kinematic fitting is the clear choice even in the absence of recent formulation. |
Saturday, October 27, 2018 10:30AM - 10:45AM |
LE.00007: Higher Order (Dispersive) Corrections To The Born Approximation In Elastic Electron-Nucleas Scattering In The Intermediate Energy Regime Paul Gueye, Al Amin Kabir Two-photon exchange contributions have become a necessary ingredient in theoretical calculations trying to precisely calculate hydrogen elastic scattering cross sections. This correction typically modifies the cross section at the few percent level. In contrast, dispersive effects can cause significantly larger changes from the Born approximation. The LEDEX Collaboration at Jefferson Lab measures the 12C elastic cross section in Hall A at beam energies of 362 MeV and 685 MeV around the 1stdiffractive minimum, where the Born term contributions to the cross section are small to maximize the sensitivity to dispersive effects. The results are in very good agreement with previous world data and the average deviation from a static charge distribution expected from linear and quadratic fits indicate a 38.8% contribution of dispersive effects to the cross section at 1 GeV. Their magnitude has been confirmed to be large with a strong energy dependence and could account for a large fraction of the observed quenching of the longitudinal nuclear response. They could also be important for nucleon radii extracted from parity violating asymmetries measured near a diffractive minimum. |
Saturday, October 27, 2018 10:45AM - 11:00AM |
LE.00008: Electric Form Factor of the Neutron from Asymmetry Measurements Richard F Obrecht The electromagnetic form factors are fundamental quantities containing information on the spatial and momentum distributions of charge and current within the nucleon. While nucleon form factors may be experimentally extracted via purely electromagnetic interactions, the functions encode information regarding the most basic constituents of matter, namely the strongly interacting quarks and gluons, that collectively give rise to the observed properties of the nucleon, e.g. the radius and mass. The form factor ratio of the neutron has been extracted at a negative momentum transfer squared of $Q^2=1.16$ GeV$^2$ via a beam-target helicity asymmetry measurement using the semi-exclusive reaction $^3\vec{\textrm{He}}(\vec{e},e'n)pp$. The Jefferson Lab Hall A experiment E02-013 ran in 2006 utilizing the 6 GeV CEBAF for its high-duty, longitudinally polarized electron beam. The double-arm coincidence experiment detected the quasielastically scattered electrons in a large angular and momentum acceptance spectrometer. The recoiling nucleons were detected and momentum analyzed in a large scintillator-iron based neutron detector. Presented will be the analysis and the results of a new $G_E^n/G_M^n$ extraction at $Q^2$=1.16 GeV$^2$. |
Saturday, October 27, 2018 11:00AM - 11:15AM |
LE.00009: Skyrme model study of baryon properties in a strong magnetic field Bing-Ran He The baryon properties in a uniform magnetic field are investigated. |
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