Bulletin of the American Physical Society
APS April Meeting 2016
Volume 61, Number 6
Saturday–Tuesday, April 16–19, 2016; Salt Lake City, Utah
Session R10: Hadronic Physics |
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Sponsoring Units: DNP GHP Chair: Tanja Horn, Catholic University of America Room: 250B |
Monday, April 18, 2016 10:45AM - 10:57AM |
R10.00001: Measurement of $\bar{d}/\bar{u}(x)$? at Seaquest Bryan Kerns SeaQuest is a running fixed target experiment using Fermilab's 120 GeV Main Injector proton beam to study the structure of the proton. The comparison of the Drell-Yan cross section ratio for liquid hydrogen and liquid deuterium targets enables an extraction of the ratio of $\bar{d}/\bar{u}$?? at Bjorken x ranging from around 0.1 to 0.45, measuring this quantity at much higher x than previous experiments. A preliminary analysis of this data with the purpose of extracting $\bar{d}/\bar{u}(x)$?? will be presented. [Preview Abstract] |
Monday, April 18, 2016 10:57AM - 11:09AM |
R10.00002: Effect of Machine Learning Techniques on SeaQuest Physics Analyses Daniel Morton Fermilab E906, SeaQuest, implements a 120 GeV proton beam from the Main Injector incident on liquid Deuterium and Hydrogen targets and solid Tungsten, Carbon and Iron targets to produce leptons through the Drell-Yan process. Produced particles impinge on an iron beam dump, which absorbs all but muons and neutrinos. Muon pairs are divided and refocused with two dipole magnets. The primary objective is the extraction of the $\bar{d}/ \bar{u}$ ratio from the muon production cross section ratio $\sigma(d+p)/\sigma(p+p)$. The SeaQuest spectrometer is optimized to search for coincident dimuons, utilizing four detector stations containing scintillators, drift chambers and proportional tubes. The experiment relies on hodoscope coincidence to determine whether to accept the event. The goal of implementing machine learning algorithms (MLAs) is to improve trigger purity and event classification accuracy on both trigger and reconstruction levels, and thus improve statistical precision in all physics analyses and provide insight into spectrometer acceptance bias as well as potentially providing essential trigger optimization for the search of a dark Higgs candidate. We will report on the present status and plans to implement MLAs into the various triggers and its effect on physics analyses. [Preview Abstract] |
Monday, April 18, 2016 11:09AM - 11:21AM |
R10.00003: The Spring 2016 Deeply Virtual Compton Scattering (DVCS) Run (E12-06-114) in Hall A of TJNAF Mongi Dlamini The DVCS experiment in Hall A of the Thomas Jefferson National Accelerator Facility will provide high precision data necessary to map out the 3D structure of the nucleon in the Generalized Parton Distribution (GPD) framework. GPDs are a new class of light-cone matrix elements which encode the transverse momentum and spatial distribution of partons as a function of the longitudinal momentum of the nucleon. DVCS off the nucleon ($\gamma^*$N $\to$ $\gamma$N) is the cleanest process that can access GPDs. The Hall A DVCS experiment is set to run in the Spring of 2016 using a polarized electron beam with energy ranging from 8 to 11 GeV, allowing an extended kinematic range coverage. This experiment will measure both polarized and unpolarized cross sections of the H(e,e'$\gamma$)p and H(e,e'$\pi^0$)p reactions and hence probe proton GPDs. The run will complete Q$^2$ scans at $x_{Bj}$ ranging from 0.36 to 0.60. The Q$^{2}$-dependent cross sections allow the separation of the leading-twist GPD amplitude from the higher-twist scaling-violating terms. In this talk, the status of the experiment will be presented. [Preview Abstract] |
Monday, April 18, 2016 11:21AM - 11:33AM |
R10.00004: Initial State Helicity Correlation in Wide Angle Compton Scattering Donal Day, Dustin Keller, Jixie Zhang Wide-angle Compton scattering (WACS) belongs to the family of exclusive processes, with large values of $s$, $-t$, and $-u$, that can reveal nucleon structure. In the pQCD version of WACS, three active quarks and two hard gluons are required to share the momentum. pQCD predictions for the WACS disagree with the cross sections currently available. In contrast, handbag mechanism calculations involving a single quark coupled to the spectator through GPDs, are compatible with the cross sections. Measurements of the longitudinal polarization transfer parameter $K_{LL}$ have been found to be inconsistent with the predictions of pQCD yet consistent with calculations within the handbag mechanism, at least at very large angles. There are handbag calculations, including quark and hadron helicity flip, which contradicts pQCD by finding that $K_{LL} \neq A_{LL}$. A measurement of $A_{LL}$ has been approved to run at Jefferson Lab and which has the potential to clarify the nature of the reaction mechanism in WACS and illuminate the role of quark orbital angular momentum. It will utilize a pure untagged bremsstrahlung photon beam and a longitudinally polarized proton target. After an introduction, the experiment will be described and the expected results presented. [Preview Abstract] |
Monday, April 18, 2016 11:33AM - 11:45AM |
R10.00005: A Measurement of the Proton Spin Dependent Structure Function, $g_2$, at Low Q$^2$ Toby Badman Over the past several decades Jefferson Lab has proven to be extremely successful in its endeavor to study the polarized structure of nucleons. Measurements of these nucleon structure functions have proven to be powerful tools in testing and understanding a number of effective theories of QCD. The neutron spin structure functions, $g_{1,2}^n$, and the proton spin structure function, $g^{p}_{1}$, have been measured to very high precision over a wide kinematic range. However, the second proton structure function, $g^{p}_{2}$, remains largely unmeasured. The primary goal of Jefferson Lab Hall A experiment E08-027 was to perform an inclusive measurement of the proton $g_2$ structure function for the first time in the low momentum transfer range of $0.02 <\ $Q$^2 < 0.2\ $GeV$^2$. The experiment will allow us to test the Burkhardt-Cottingham Sum Rule at low Q$^2$ as well as extract the longitudinal-transverse generalized spin polarizability and compare it to predictions from Chiral Perturbation Theory. The experiment acquired data at Jefferson Lab in Hall A during March - May of 2012. The details of the experiment and preliminary results will be presented. [Preview Abstract] |
Monday, April 18, 2016 11:45AM - 11:57AM |
R10.00006: Two-photon exchange corrections in elastic lepton-proton scattering at small momentum transfer Oleksandr Tomalak, Marc Vanderhaeghen In recent years, elastic electron-proton scattering experiments, with and without polarized protons, gave strikingly different results for the electric over magnetic proton form factor ratio. A mysterious discrepancy ("the proton radius puzzle") has been observed in the measurement of the proton charge radius in muon spectroscopy experiments versus electron spectroscopy and electron scattering. Two-photon exchange (TPE) contributions are the largest source of the hadronic uncertainty in these experiments. We compare the existing models of the elastic contribution to TPE correction in lepton-proton scattering. A subtracted dispersion relation formalism for the TPE in electron-proton scattering has been developed and tested. Its relative effect on cross section is in the $ 1-2~\% $ range for a low value of the momentum transfer. An alternative dispersive evaluation of the TPE correction to the hydrogen hyperfine splitting was found and applied. For the inelastic TPE contribution, the low momentum transfer expansion was studied. In addition with the elastic TPE it describes the experimental TPE fit to electron data quite well. For a forthcoming muon-proton scattering experiment (MUSE) the resulting TPE was found to be in the $ 0.5 - 1~\% $ range, which is the planned accuracy goal. [Preview Abstract] |
Monday, April 18, 2016 11:57AM - 12:09PM |
R10.00007: Applying Statistical Methods To The Proton Radius Puzzle Douglas Higinbotham In recent nuclear physics publications, one can find many examples where chi2 and reduced chi2 are the only tools used for the selection of models even though a chi2 difference test is only meaningful for nested models. With this in mind, we reanalyze electron scattering data, being careful to clearly define our selection criteria as well as using a co-variance matrix and confidence levels as per the statistics section of the particle data book. We will show that when applying such techniques to hydrogen elastic scattering data, the nested models often require fewer parameters than typically used and that non-nested models are often rejected inappropriately. [Preview Abstract] |
Monday, April 18, 2016 12:09PM - 12:21PM |
R10.00008: A measurement of two-photon exchange in unpolarized elastic electron-proton scattering Mikhail Yurov Jefferson Lab experiment E05-017 was designed to study 2-photon exchange contributions to elastic electron-proton scattering over a wide kinematic range. By detecting the scattered proton instead of the electron these measurements will be very sensitive to the $\epsilon$ dependence of the cross section and consequently the ratio $\frac{G_E}{G_M}$. The goals of the experiment, the experimental technique and the kinematic range will be presented. The analysis sequence and results of the early steps will be outlined. [Preview Abstract] |
Monday, April 18, 2016 12:21PM - 12:33PM |
R10.00009: Status of the OLYMPUS Experiment Axel Schmidt The OLYMPUS Experiment finished collecting data at DESY, Hamburg, in 2013, and the analysis effort is well underway. The goal of the experiment is to measure the ratio of electron-proton to positron-proton elastic scattering cross sections with 1 percent uncertainty. Deviation in this ratio from unity is evidence of hard two-photon exchange, an effect which may be responsible for the current proton form-factor discrepancy. At OLYMPUS, alternating beams of positrons and electrons were directed through a windowless hydrogen gas target, and the scattered lepton and recoiling proton were detected in coincidence in a large-acceptance magnetic spectrometer. Forward tracking telescopes, and M\o ller/Bhabha calorimeters independently monitored the relative luminosity of the electron and positron running modes. Over 4~fb$^{-1}$ of integrated luminosity were collected, giving OLYMPUS excellent statistical precision. The analysis procedure as well as the current status will be presented. [Preview Abstract] |
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