Bulletin of the American Physical Society
2010 Fall Meeting of the APS Division of Nuclear Physics
Volume 55, Number 14
Tuesday–Saturday, November 2–6, 2010; Santa Fe, New Mexico
Session ND: The Proton's Electroweak Form Factors |
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Chair: Brian Raue, Florida International University Room: Sweeny D |
Saturday, November 6, 2010 10:30AM - 10:42AM |
ND.00001: Proton Form Factor Ratio $G_E/G_M$ from Double Spin Asymmetry with Polarized Beam and Target Anusha Liyanage Along with experiment E07-003 (SANE, Spin Asymmetries of the Nucleon Experiment), which has been performed in Hall C at Jefferson Lab with a polarized ammonia target to study the proton's spin structure, elastic scattering was carried out simultaneously, with electrons detected in the Big Electron Telescope Array (BETA) in coincidence with recoiling protons reconstructed by the High Momentum Spectrometer (HMS). The elastic double spin asymmetry allows to extract the proton electric to magnetic form factor ratio $G_E/G_M$ at high momentum transfers, $Q^2$ = 5.25 (GeV/c)$^2$ and $Q^2$ = 6.25 (GeV/c)$^2$. In addition to the coincidence data, inclusively scattered electrons from polarized ammonia were detected by HMS, which allows to measure the beam-target asymmetry in the elastic region and to extract the $G_E/G_M$ at $Q^2$ = 2.2 (GeV/c)$^2$. This alternative measurement of $G_E/G_M$ will verify the dramatic discrepancy at high $Q^2$ between the Rosenbluth and the recoil polarization transfer method with a different technique. The current status of the analysis and some preliminary results will be presented. [Preview Abstract] |
Saturday, November 6, 2010 10:42AM - 10:54AM |
ND.00002: Experimental Status of the Proton Electric Form Factor at Large $Q^2$ Andrew Puckett Jefferson Lab experiments E99-007 in Hall A and the recently published E04-108 in Hall C extended the recoil polarization data for the proton electric form factor $G_E^p$ from $Q^2 = 3.5GeV^2$ to $Q^2 = 8.5\ GeV^2$. A common feature of both experiments was the use of large solid-angle lead-glass electromagnetic (EM) calorimeters to detect elastically scattered electrons in coincidence with scattered polarized protons. Since lead-glass calorimeters have relatively poor energy resolution and do not distinguish between electrons and photons of similar energies, both experiments relied on two-body angular correlations to suppress non-elastic backgrounds. While the recent E04-108 results for the ratio $R=\mu_p G_E^p/G_M^p$ are statistically compatible with the E99-007 results where they overlap, they hint at a possible systematic difference in the $Q^2$ dependence of $R$ between the two experiments. In light of the improved understanding of photon backgrounds from $\pi^0$ photoproduction and Compton scattering gained from the analysis of the recent E04-108 data, the data from E99-007 have been reanalyzed, resulting in improved consistency between the two experiments. The new and improved $G_E^p$ data for $4.0\le Q^2 \le 8.5\ GeV^2$ will be presented and discussed. [Preview Abstract] |
Saturday, November 6, 2010 10:54AM - 11:06AM |
ND.00003: A search of an $\epsilon$ dependence of the proton form factor ratio using recoil polarization technique Mehdi Meziane Intensive theoretical and experimental efforts have been made over the past decade aiming at explaining the discrepancy between the data for the proton form factor ratio, $G_{Ep}/G_{Mp}$, obtained at Jefferson Lab using polarization transfer technique, and the world data obtained by the Rosenbluth method based on cross section measurements. One possible explanation for this difference is a two-photon exchange contribution, where both photons share the momentum transfer about equally. In the Born approximation for a fixed Q$^{2}$, the form factors do not depend upon the energy of the incident electron. We will report the results of the Jlab Hall-C $G_{Ep}-2\gamma$ experiment which was designed to measure a possible kinematical variation of the ratio $G_{Ep}/G_{Mp}$ with statistical uncertainties of $\pm$0.01 at $Q^{2}=2.5 GeV^{2}$, using the recoil polarization technique. Three kinematics were chosen, corresponding to values of the kinematic factor $\epsilon$=0.15, 0.63 and 0.77. We will describe the new detectors built for both $G_{Ep}-2\gamma$ and $G_{Ep}-III$ experiments, the electromagnetic calorimeter BigCal which detected the scattered electron, and the focal plane polarimeter (FPP) which measured the polarization of the recoil proton. [Preview Abstract] |
Saturday, November 6, 2010 11:06AM - 11:18AM |
ND.00004: Target Single-Spin Asymmetry Measurements in Quasi-Elastic $^{3}$He$\uparrow$(e, e$'$) Bo Zhao The target single-spin asymmetry for the neutron, $A_{y}$, was measured using the inclusive quasi-elastic $^{3}$He$\uparrow$(e, e$'$) reaction in Hall A at Jefferson Lab with a vertically polarized $^{3}$He target for $Q^{2}$ = 0.13, 0.46 and 0.97 GeV$^{2}$. Since the target single-spin asymmetry is expected to be zero in the one-photon exchange approximation, the non-zero results from this experiment clearly demonstrate the effects due to two-photon exchange. They establish the two-photon exchange process as a powerful tool to probe hadron structure, such as information on Generalized Parton Distributions. The preliminary results of this experiment will be presented. [Preview Abstract] |
Saturday, November 6, 2010 11:18AM - 11:30AM |
ND.00005: A Status Report of the $Q_{weak}$ Experiment at Jefferson Lab Katherine Myers The $Q_{weak}$ Collaboration at Jefferson Lab will perform the first direct measurement of the proton's weak charge, $Q^p_W$, to a precision of 4\% by measuring the parity-violating asymmetry in elastic electron-proton scattering. This asymmetry is expected to be small, $\sim$ 250 ppb and is proportional to the proton's weak charge. At tree level, the weak mixing angle is related to the weak charge of the proton by $Q^p_W$=1-4$\sin^2\theta_W$, leading to a 0.3\% measurement of $\sin^2\theta_W$ at low energy - making this the best low energy measurement to date. This measurement will test the Standard Model prediction for the running of the weak mixing angle and be sensitive to new parity-violating physics at the TeV scale. The experiment was recently installed and commissioned in Hall C. The status of the experiment and commissioning will be discussed in this talk. Preliminary results may be presented. [Preview Abstract] |
Saturday, November 6, 2010 11:30AM - 11:42AM |
ND.00006: Diamond multi-strip electron detector and readout for Compton polarimetry Amrendra Narayan The Q$_{weak}$ experiment will use a new Compton polarimeter for the non-invasive, continuous measurement of the electron beam polarization. The Compton polarimeter will use four planes of multi-strip diamond detectors to detect the Compton scattered electrons and a CsI crystal to detect the back-scattered photons. The diamond detectors will be read out using custom-built electronic modules that chain together a preamplifier, a shaper and a discriminator for each micro-strip. The digitized signal will be processed by a general purpose logic module based on field programmable gate arrays. We have assembled a complete electronic readout chain and data acquisition system for the diamond micro-strip detector. The logic modules have been programmed to collect data in both single event mode and accumulation mode. The diamond micro-strip detectors along with the complete data acquisition system is being tested with various electron sources and cosmic rays. We will show the results from these tests and the preliminary data collected with the Compton polarimeter. [Preview Abstract] |
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