Bulletin of the American Physical Society
APS April Meeting 2013
Volume 58, Number 4
Saturday–Tuesday, April 13–16, 2013; Denver, Colorado
Session C15: Form Factors and Two-photon Exchange |
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Sponsoring Units: DNP GHP Chair: Brad Sawatzky, Thomas Jefferson National Accelerator Facility Room: Plaza Court 4 |
Saturday, April 13, 2013 1:30PM - 1:42PM |
C15.00001: $\pi^-/\pi^+$ Exclusive Pion Electroproduction Results from Jefferson Lab Garth Huber Forward exclusive meson production is a particularly useful tool in the study of hadronic structure. Measurements at different squared four-momenta of the exchanged virtual photon, $Q^2$, and at different four-momentum transfer, $t$, allow one to probe QCD's transition from meson-nucleon degrees of freedom at long distances to quark-gluon degrees of freedom at short scales. Furthermore, by forming ratios of separated response functions in $\pi^-$ and $\pi^+$ electroproduction, there may be a partial cancellation of nonperturbative QCD contributions, allowing this transition to be more readily apparent. These strong theoretical motivations make worthwhile the significant experimental challenges needed to accurately separate the four response functions in forward, exclusive $\pi^-/\pi^+$ electroproduction off deuterium. We report the results of our study from Jefferson Lab Hall C, where for the first time ratios of separated response functions were extracted at several kinematic settings: $Q^2$ = 0.6, 1.0, 1.6 GeV$^2$ with $W$=1.95 GeV, and $Q^2$=2.45 GeV$^2$ with $W$=2.22 GeV, respectively. [Preview Abstract] |
Saturday, April 13, 2013 1:42PM - 1:54PM |
C15.00002: Studying the Proton Radius Puzzle with $\mu$p Elastic Scattering Katherine Myers The Proton Radius Puzzle refers to the disagreement between the proton charge radius as determined from muonic hydrogen and the radius determined from atomic hydrogen level transitions and $ep$ elastic scattering form factor data. The discrepancy of 7$\sigma$ is not yet explained, and though numerous resolutions have been proposed there is no generally accepted resolution to the puzzle. The MUon proton Scattering Experiment (MUSE) Collaboration plans to simultaneously measure $ep$ and $\mu p$ scattering at the Paul Scherrer Institute, testing the interesting possibility that $ep$ and $\mu p$ scattering are different. The experiment will also measure scattering with both polarities of $e$ and $\mu$, directly measuring two-photon exchange effects. In scattering experiments, the proton radius comes from an extrapolation of the form factor slope at low $Q^2$ as $Q^2\rightarrow 0$. We plan to measure in the kinematic region of $Q^2 = 0.002 - 0.07$ GeV$^2$, and determine the relative cross sections to a few tenths of a percent. This should allow the proton radius to be extracted at the level of $\sim$0.01 fm, similar to previous $ep$ measurements. An overview and potential impact of this proposed experiment will be presented. [Preview Abstract] |
Saturday, April 13, 2013 1:54PM - 2:06PM |
C15.00003: Strangeness Vector and Axial-Vector Form Factors of the Nucleon Dennis Trujillo, Stephen Pate A revised global fit of electroweak $ep$ and $\nu p$ elastic scattering data has been
performed, with the goal of determining the strange quark contribution to the vector and axial-vector form factors of the nucleon in the momentum-transfer range $0 |
Saturday, April 13, 2013 2:06PM - 2:18PM |
C15.00004: The CLAS Two Photon Exchange Experiment Dasuni Adikaram, Robert Bennett, Larry Weinstein, Dipak Rimal, Puneet Khetarpal, Brian Raue There is a large discrepancy between the proton electron form factor (${G_E}^p$($Q^2$)) measured using the Rosenbluth separation and polarization transfer methods. The most likely explanation of this discrepancy is the inclusion of two-photon exchange (TPE) amplitude contributions to the elastic electron-proton cross section. The TPE contribution can be extracted in a model-independent way from the measured ratio of the cross sections of positron-proton and electron-proton elastic scattering. This ratio was measured in Hall B at Jefferson Lab using a simultaneous mixed tertiary beam of electrons and positrons incident on a liquid hydrogen target in the center of the CLAS detector in 2010-2011. In this talk, the experimental techniques to produce $e^+$/$e^-$ beam, the analysis techniques to identify the elastic scattering events, and some preliminary results will be presented. [Preview Abstract] |
Saturday, April 13, 2013 2:18PM - 2:30PM |
C15.00005: The OLYMPUS Experiment Simulation Axel Schmidt The OLYMPUS Experiment aims to measure the ratio of electron-proton to positron-proton elastic scattering cross-sections to better than 1\% systematic uncertainty. Achieving this goal requires a precise understanding of a wide range of systematic effects, such as the radiative corrections internal to the reaction, the varying acceptance of the detector aparatus, and efficiency of the tracking algorithms. A detailed Geant4 simulation of the OLYMPUS experiment has been developed to study these effects, and using the Monte Carlo method, properly account for their convolution. Radiative corrections are applied by the event generator, whose events are propagated through the simulation. Simulated detector signals are produced with identical format to the raw OLYMPUS data, so that simulated data can be processed using the same analysis software. The simulation, therefore, serves as a benchmark for comparison with the final OLYMPUS results. A discussion of the radiative corrections procedure and an overview of the simulation will be presented. This work is supported by DOE Grant DE-FG02-94ER40818. [Preview Abstract] |
Saturday, April 13, 2013 2:30PM - 2:42PM |
C15.00006: An Update on the OLYMPUS Two-Photon Exchange Experiment at DESY Colton O'Connor The OLYMPUS experiment seeks to provide a high-precision measurement ($<$1\% statistical error) of the positron-proton versus electron-proton elastic scattering cross-section ratio. Divergence from unity is attributed to two-photon exchange, predicted to be a few-percent effect in the kinematic region studied, $(0.6 |
Saturday, April 13, 2013 2:42PM - 2:54PM |
C15.00007: 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] |
Saturday, April 13, 2013 2:54PM - 3:06PM |
C15.00008: Measurement of the Target Single-Spin Asymmetry in Quasi-Elastic $^3$He$^{\uparrow}$(e,e') Yawei Zhang Jefferson Lab Hall A E05-015 (A$_y$) experiment measures target single-spin asymmetries, A$_y$, through electron scattering from a vertically polarized Helium-3 target in the quasi-elastic $^3$He(e, e') at Q$^2$ values of 0.13, 0.46 and 0.96 GeV$^{2}$. Single spin asymmetry is forbidden under one photon exchange assumption and parity conservation. But with two photon exchange, the amplitudes interference term gives rise to an imaginary piece, such that single spin asymmetry is allowed. This experiment is to be the first to firmly establish a non-vanishing A$_y$, providing new constraints on Generalized Parton Distribution (GPD) models and new information on the dynamics of the two-photon exchange process. The experiment will be introduced and the results from this experiment will be presented in this talk. [Preview Abstract] |
Saturday, April 13, 2013 3:06PM - 3:18PM |
C15.00009: Multiple Compton Scattering as a Function of Detector Size Tareq Alrefae As photons penetrate detector matter, various kinds of interactions take place. One important kind is Compton scattering, a process by which an incoming photon inelastically collides with an electron thus resulting in a photon with a lower energy. This less energetic photon may escape the detector, or may continue its propagation through the detector matter and undergo more Compton scattering events before escaping. In the output spectrum, the region of multiple scattered photons (MSC) is distinct from those of single scattered photons (Compton continuum) and photoelectric absorption (photopeak). Such distinction is qualitatively discussed in the literature without quantitative treatment. Hence, the goal of this study is to gain quantitative insight on MSC. Specifically, this study aims to investigate the behavior of MSC as a function of detector size. A simulation code is prepared using the Geant toolkit. Results of the simulation reveal a decrease in the spectral MCS region with increase of detector size. Moreover, the results show an increase in the average number of pre-escape multiple Compton collisions undergone by photons with the increase of detector size. These findings agree with theory. [Preview Abstract] |
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