Bulletin of the American Physical Society
APS April Meeting 2011
Volume 56, Number 4
Saturday–Tuesday, April 30–May 3 2011; Anaheim, California
Session J10: Mini-symposium on Parity Violation II |
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Sponsoring Units: DNP Chair: Robert Michaels, Thomas Jefferson National Accelerator Facility Room: Garden 1 |
Sunday, May 1, 2011 1:30PM - 2:06PM |
J10.00001: Recent Results in Parity-Violating Electron Scattering at Jefferson Lab: PREX and HAPPEX-III Invited Speaker: The parity-violating asymmetry $A_{PV}$ in electron scattering from the $^{208}$Pb nucleus is cleanly sensitive to the neutron radius $R_n$. A precision measurement of $R_n$ would have important implications for the understanding of nuclear structure, and be a powerful constaint on the symmetry energy $S_{\nu}(n)$ of neutron-rich nuclear matter, including neutron stars. The PREX collaboration has completed a first run, measuring $R_{n}$ to a precision of $\sim2.5$\%. The measurement of $A_{PV}$ in electron-proton scattering is sensitive to vector form-factors associated with an intrinsic strange quark content of the nucleon. While at one time such contributions were considered to be potentially large, a world-wide program of parity-violation measurements has constrained these form-factors to be smaller than a few percent of the electric and magnetic form-factors of the proton at low momentum-transfer. HAPPEX-III has recently completed a measurement to improve the precision of this constaint at $Q^2\sim0.6\mbox{GeV}^2$, a region in which previous experiments had indicated the possibility of intriguingly large strange contributions. Results from each experiment, and prospects for more precise $R_n$ measurements, will be discussed. [Preview Abstract] |
Sunday, May 1, 2011 2:06PM - 2:18PM |
J10.00002: Recent Results and Future Plans from the A4 Experiment Wouter Deconinck In the A4 experiment at the MAMI facility in Mainz, Germany, we use the parity-violating asymmetry present in the scattering of longitudinally polarized electrons from unpolarized protons or deuterons to measure the strangeness contribution to the electromagnetic form factors of the nucleon. The A4 experiment uses a PbF$_2$ calorimeter that can be positioned in the forward or backward direction to measure the electrons scattered in a liquid hydrogen or deuterium target. Recent results for the proton at a momentum transfer $Q^2 = 0.23$\,GeV$^2/c^2$ and the ongoing analysis of the data at $Q^2 = 0.61$\,GeV$^2/c^2$ will be discussed. Future plans include the measurement of the strangeness form factor at $Q^2 = 0.1$\,GeV$^2/c^2$ with the current detector to a twice higher precision than the currently available data, and a high precision measurement at an even lower $Q^2$ with an upgraded polarimeter and detector. [Preview Abstract] |
Sunday, May 1, 2011 2:18PM - 2:30PM |
J10.00003: HAPPEX III Instrumentation and Systematic Errors Rupesh Silwal HAPPEX III measures the parity-violating asymmetry in the elastic scattering of electrons from protons at an average $Q^2$ of $0.62GeV^2$. A linear combination of strange-quark vector form factors $G^S_E + 0.53G^S_M$ is extracted from this measurement. The corrections to this measurement and associated bounds on systematic errors are discussed in detail. Corrections include rescattering inside the spectrometers, aluminum contamination, detector non-linearities, and radiative and finite acceptance corrections in $Q^2$ measurements. [Preview Abstract] |
Sunday, May 1, 2011 2:30PM - 2:42PM |
J10.00004: Precision Measurement of Spectrometer Angles for Hall A Parity Violating Experiments Kiadtisak Saenboonruang Jefferson Lab Hall A High Resolution Spectrometer (HRS) pair provides high momentum resolution at the $10^{-4}$ level and high angular precision at the level of 0.1-0.2 mrad level. However, in order to meet the experimental requirements, the spectrometer central angle ($\theta_{0}$) must also be well known. Although $\theta_{0}$ can be reasonably measured by performing a spectrometer survey, the survey uncertainty can be as large as 0.8 mrad. This uncertainty is unacceptable for very forward angle experiments such as PREX (87.3 mrad). We used an alternate experimental method to determine $\theta_{0}$ with higher accuracy. This method uses electrons elastically scattered off nuclei with different masses; it is based on the dependence of angle and energy of elastic scattered electrons on the nuclear mass. In the case of HAPPEX-III and PREX experiments, we used elastic scattering off Hydrogen ($^{1}H$) and Oxygen ($^{16}O$) nuclei in a water-cell target. The energy loss corrections cancel in the difference. Multiple measurements of the $^{1}H$ and $^{16}O$ energy difference were made using different central momenta of the spectrometer to improve the accuracy and to establish the systematic uncertainty. We used our method to determine $\theta_{0}$ for the PREX (HAPPEX-III) experiment with an accuracy of 0.4 (0.5) mrad. The resulting uncertainty in $Q^{2}$ is 0.9\% (0.4\%) for PREX (HAPPEX-III), within the 1\% accuracy required. [Preview Abstract] |
Sunday, May 1, 2011 2:42PM - 2:54PM |
J10.00005: $e-^2$H Parity Violating Deep Inelastic Scattering (PVDIS) at CEBAF 6 GeV Kai Pan The parity violating (PV) asymmetry $A_{d}$ in $e-^2$H deep inelastic scattering (DIS) was measured in Hall A at Jefferson Lab at $Q^2$ = 1.11 and 1.90 $(\mathrm{GeV}/c)^2$ at $x \approx 0.3$ to a statistical precision of $3\%$ and $4\%$, respectively. The combination of the two measurements will provide the first significant constraint on higher-twist (HT) effects in PVDIS. With HT effects thus measured, this experiment will constrain the poorly known effective weak coupling constant combination $(2C_{2u} - C_{2d})$. The measurement will also allow the extraction of couplings $C_{3q}$ from high energy $\mu-C$ DIS data. Precision measurements of all these phenomenological couplings are essential to comprehensively search for possible physics beyond the Standard Model. The experiment DAQ system will be introduced. Current data analysis progress and preliminary results will be presented. [Preview Abstract] |
Sunday, May 1, 2011 2:54PM - 3:06PM |
J10.00006: One-loop Electroweak Radiative Corrections for Polarized M{\o}ller Scattering Svetlana Barkanova, Aleksandrs Aleksejevs, Alexander Ilyichev, Yury Kolomensky, Vladimir Zykunov M{\o}ller scattering measurements are a clean, powerful probe of new physics effects. However, before physics of interest can be extracted from the experimental data, radiative corrections must be taken into account very carefully. Using two different approaches, we perform updated and detailed calculations of the complete one-loop set of electroweak radiative corrections to parity violating electron-electron scattering asymmetry at low energies relevant for the ultra-precise 11~GeV MOLLER experiment planned at JLab. Although contributions from some of the self-energies and vertex diagrams calculated in the two approaches can differ significantly, our full gauge-invariant set still guarantees that the total relative weak corrections are in excellent agreement for the two methods of calculation. Our numerical results are presented for a range of experimental cuts and the relative importance of various contributions is analyzed. We also provide very compact expressions analytically free from non-physical parameters and show them to be valid for fast yet accurate estimations. [Preview Abstract] |
Sunday, May 1, 2011 3:06PM - 3:18PM |
J10.00007: Design of the 12 GeV MOLLER Spectrometer Juliette Mammei The MOLLER experiment is an ultra-precise low-energy measurement of the weak mixing angle, sin$^2\theta_W$, which will run in Hall A of Jefferson Lab after the 12 GeV upgrade. It will measure the parity-violating asymmetry in elastic electron-electron (M\o ller) scattering which arises due to the interference of scattering via a Z boson with that of single photon exchange. The expected precision of the measurement is comparable to that of the two highest-energy measurements, making it a low-energy standard model test complementary to measurements at the Large Hadron Collider. In order to achieve the design goals of high ($\geq$120 GHz) scattered electron rate and small ($\leq$10\%) contributions from backgrounds, it will employ two high-power resistive toroidal magnets. Their design is constrained by the small scattering angles (5.5 - 19 mrad) as well as the requirement that the coils fill less than half of the azimuth. In order to focus the M\o ller electrons 28m downstream of the 150cm long liquid hydrogen target, a novel hybrid toroid with multiple current returns was proposed. A more conventional magnet is placed upstream in order to provide azimuthal pre-focusing and also to provide some separation between the signal M\o ller electrons and the electrons which scatter elastically from the proton. I will present the results of TOSCA and GEANT4 simulations used to achieve a buildable magnet system which meets our design criteria. [Preview Abstract] |
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