Bulletin of the American Physical Society
2012 Fall Meeting of the APS Division of Nuclear Physics
Volume 57, Number 9
Wednesday–Saturday, October 24–27, 2012; Newport Beach, California
Session DA: Novel Electroweak Probes of Nucleon Structure and the Standard Model |
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Chair: Wally Melnitchouk, Thomas Jefferson National Accelerator Facility Room: Plaza I |
Thursday, October 25, 2012 10:30AM - 11:06AM |
DA.00001: Resolving the Proton Form Factor Problem with Positron-Proton Scattering Invited Speaker: Lawrence Weinstein The proton electromagnetic form factors are essential pieces of our knowledge of nucleon structure. However, Rosenbluth separation measurements of the proton electric form factor, $G_E(Q^2)$, differ from polarization transfer measurements by a factor of three at $Q^2 = 5.6$ (GeV/$c$)$^2$. This discrepancy must be resolved. One possible resolution is to include the contribution of hard two-photon exchange (TPE) contributions. These contributions are very difficult to calculate. However, we can directly determine the TPE effect by measuring the ratio of the positron-proton to electron-proton elastic scattering cross sections, $R=\sigma(e^+p)/\sigma(e^-p)$, because the TPE amplitude has the same sign as the $e^+p$ born amplitude and the opposite sign as the $e^-p$ born amplitude. We have measured $R$ over a wide range of momentum transfer, $0.2 \le Q^2 \le 2$ GeV$^2$, and virtual photon polarization, $0.1 \le \epsilon \le 0.9$, using a mixed identical beam of electrons and positrons in Hall B at Jefferson Lab. This talk will describe the experimental techniques used to produce this beam, the analysis techniques to identify elastic scattering events, and some preliminary results. [Preview Abstract] |
Thursday, October 25, 2012 11:06AM - 11:42AM |
DA.00002: First results from the Qweak measurement of the proton's weak charge Invited Speaker: Mark Dalton The Qweak experiment has made the first direct measurement of the weak charge of the proton, $Q^p_W$, through precision measurement of the parity-violating asymmetry in elastic scattering of longitudinally polarized electrons from unpolarized protons. The data were taken in Hall C at Jefferson Laboratory, during 12 months of running over a 2 year period, ending in May 2012. A beam current of up to 180 $\mu$A on a 35 cm long liquid-hydrogen target produced a scattered electron flux of $\sim$6.5 GHz incident on the detectors with Q$^2\sim0.026\ (\mathrm{GeV}/c)^2$. It is expected that the weak charge of the proton will be extracted with a 4.1\% combined statistical and systematic uncertainty. The proton weak charge is suppressed in the Standard Model so that these data may be used to determine the weak mixing angle, $\sin^2\theta_W$, with a relative uncertainty of 0.3\%, providing a competitive measurement of the running of $\sin^2\theta_W$ to low $Q^2$. In addition, these data will impose a strong constraint on a combination of the vector weak charges of the $u$ and $d$ quarks, $C_{1u}$ and $C_{1d}$, which is distinct from and complimentary to the combination from measurements of parity violation in atomic systems, and thereby provide an independent constraint on physics beyond the Standard Model. A first look at a subset of the data will be presented. [Preview Abstract] |
Thursday, October 25, 2012 11:42AM - 12:18PM |
DA.00003: Measurement of Parity Violation Asymmetry in Deep Inelastic Scattering at JLab Invited Speaker: Xiaochao Zheng The parity violation asymmetry in electron-deuteron deep inelastic scattering is sensitive to the electron-quark neutral weak couplings. During the JLab 6 GeV PVDIS experiment in 2009, this asymmetry was measured at two four momentum transfer squared values of 1.1 and 1.9 GeV squared, to statistical precisions of 3 and 4 percent, respectively. In this talk I will review the experiment and present preliminary results on the asymmetries as well as the quark weak axial charge. [Preview Abstract] |
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