Bulletin of the American Physical Society
APS April Meeting 2011
Volume 56, Number 4
Saturday–Tuesday, April 30–May 3 2011; Anaheim, California
Session H10: Mini-symposium on Parity Violation I |
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Sponsoring Units: DNP Chair: Charles Horowitz, Indiana University Room: Garden 1 |
Sunday, May 1, 2011 10:45AM - 11:21AM |
H10.00001: MeV to TeV Physics with Parity-Violating Electron Scattering: An Overview Invited Speaker: The technique of parity-violating electron scattering, involving measurements of the asymmetry in the scattering of longitudinally polarized electrons off fixed targets, has become increasingly precise and broad in its scope over the past three decades. Such asymmetries are sensitive to weak neutral current interactions between electrons and quarks, mediated by the $Z^0$ boson, and are being used to investigate the strangeness content of the nucleon, the neutron distribution in heavy nuclei and to probe for the limits of the validity of the electroweak theory in a manner complementary to direct searches for new physics at high energy scales at colliders. Steady improvements to the experimental techniques now allow for asymmetries to be measured with statistical and systematic errors better than 1 part per billion. Further precise measurements have now become feasible in parity-violating elastic and deep inelastic scattering off hadronic targets, and in electron-electron (M\o ller) scattering. In this talk, we introduce the experimental technique, provide an overview of recent experimental progress and physics results, and set the context for the development of the future program. [Preview Abstract] |
Sunday, May 1, 2011 11:21AM - 11:33AM |
H10.00002: Setup of polarized electron source for parity-violation experiments Mark Dalton Measurement of parity-violation in electron scattering has evolved to measuring sub- part-per-million asymmetries with a precision better than 10 parts-per-billion. This places strict requirements on the electron beam properties, which can only be met through careful configuration of the polarized electron source. In order to control systematics and keep corrections small, the cumulative helicity-correlated position differences of the beam on target are controlled at the level of 1 nm and helicity- correlated angle differences to the 1 nrad level. For the recent Qweak measurement, utilizing the highest power cryotarget ever constructed, a high helicity reversal rate of 960 Hz is required to reduce noise due to density changes in the liquid target. The need for fast reversal presents new challenges for the optimization of the reversal apparatus. In this talk, the setup of the Jefferson Lab polarized electron source for recent parity-violation experiments is discussed along with expected future developments. [Preview Abstract] |
Sunday, May 1, 2011 11:33AM - 11:45AM |
H10.00003: The Proton's Weak Charge D.S. Armstrong The Qweak collaboration has embarked on a measurement, at Jefferson Lab, of the proton's vector neutral-current coupling, the weak charge. Electroweak interference leads to a parity-violating helicity-dependence (asymmetry) in the scattering of longitudinally polarized electrons from an unpolarized hydrogen target. The weak charge can be extracted from a measurement of this asymmetry at sufficiently low momentum transfer. This is the first experiment dedicated to a measurement of the weak charge, and will serve as sensitive test of the Standard Model prediction for this quantity. The anticipated result will have sensitivity to certain classes of new physics at the few TeV mass scale. An overview of the experimental approach will be presented, along with a status report on the data-taking and performance of the apparatus. [Preview Abstract] |
Sunday, May 1, 2011 11:45AM - 11:57AM |
H10.00004: The status of the liquid hydrogen target for the Qweak experiment Adesh Subedi The Qweak experiment in Hall C at Jefferson lab aims to make the first direct measurement of the weak charge of the proton with about 4{\%} overall uncertainty by measuring the parity violating asymmetry in elastic electron-proton scattering. One of the crucial elements of the experimental setup is the 35 cm long liquid hydrogen target. This cryogenic target was designed using computational fluid dynamics to operate with 2500W of beam power at 180 microamps. Not only is this the highest power cryogenic target ever built but it also has very tight constraints on the target related uncertainties. An update on the commissioning and the measured performance of the Qweak experiment liquid hydrogen target will be presented. [Preview Abstract] |
Sunday, May 1, 2011 11:57AM - 12:09PM |
H10.00005: Results from the Particle Tracking System of the Qweak Experiment Siyuan Yang The goal of the Qweak experiment is to measure the value of the weak charge of the proton, $Q^{p}_{weak}=1-4\sin^2\theta_{w}$, to 4\% precision. Because the Standard Model makes a firm prediction of $Q^{p}_{weak}$, the measurement of $Q^{p}_{weak}$ will either confirm the prediction or will provide clear evidence for new physics. In this experiment, we study elastic electron proton scattering at four momentum transfer $Q^2$=0.03GeV$^2/$c$^2$ by scattering a high current polarized electron beam on a liquid hydrogen target. A crucial element to the precise measurement of $Q^{p}_{weak}$ is to determine the four momentum transfer $Q^2$ to a high precision because the parity violation asymmetry is directly proportional to $Q^2$: $A^p_{LR}=A_0[Q^p_{weak}Q^2+B_4(Q^2)Q^4+...]$. Therefore, the experiment is also designed to run in a low current mode in which the $Q^2$ is determined by using the tracking detectors. The tracking system is aimed at reconstructing the track of a particle to get the $Q^2$ with an accuracy of 1\%. The experiment is now in the phase I data production run, which will end in May 2011. I will describe the procedure of the track reconstruction and the first results coming from it. [Preview Abstract] |
Sunday, May 1, 2011 12:09PM - 12:21PM |
H10.00006: The QWeak Experiment's Main Detector System Scott MacEwan The $\mbox{Q}_{Weak}$ experiment will determine the weak charge of the proton $Q_w^p$ to 4\% accuracy by measuring the parity violating electron scattering asymmetry from a liquid Hydrogen target at a fixed $Q^2$. The Standard Model makes a precise prediction of the value of $Q_w^p$, and as such makes it sensitive to new physics. $\mbox{Q}_{Weak}$'s main detector subsystem is comprised of eight identical 2 m long quartz bar \^{C}erenkov detectors that can operate in both integrating and counting configurations, depending on the task at hand. Where integrated detector yields are required to measure asymmetries, counting mode data acquisition is required in order to properly characterize detector performance as well as accurately measure momentum $Q^2$. A summary and status review of the main detectors will be discussed for each running mode. [Preview Abstract] |
Sunday, May 1, 2011 12:21PM - 12:33PM |
H10.00007: Dispersion $\gamma Z$-box correction to parity violating electron scattering Mikhail Gorshteyn, Charles Horowitz, Michael Ramsey-Musolf We review the status of the dispersion $\gamma Z$ correction to parity violating elastic electron scattering. This correction is of particular importance for the measurement of the weak charge of the proton within the Q$_{\rm WEAK}$ experiment. Recently, it was shown that the dispersion $\gamma Z$ correction is substantially larger than it was assumed in earlier analyzes. We review the sources of the uncertainty when calculating this correction, and carefully examine possible model dependence of theoretical calculations. Our conclusion is that currently, the theoretical uncertainty associated with this correction exceeds the projected full theory uncertainty of the Q$_{\rm WEAK}$ experiment and needs further study. [Preview Abstract] |
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