Bulletin of the American Physical Society
2008 APS April Meeting and HEDP/HEDLA Meeting
Volume 53, Number 5
Friday–Tuesday, April 11–15, 2008; St. Louis, Missouri
Session X13: Electromagnetic Interactions II |
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Sponsoring Units: DNP Chair: Jen-Chieh Peng, University of Illinois at Urbana-Champaign Room: Hyatt Regency St. Louis Riverfront (formerly Adam's Mark Hotel), St. Louis F |
Tuesday, April 15, 2008 1:30PM - 1:42PM |
X13.00001: Proton Compton Scattering in the Wide-angle Regime David Hamilton, Ron Gilman, Alan Nathan, Bogdan Wojtsekhowski Compton scattering on the proton in the wide-angle regime (WACS), in which the kinematic variables s, -t and -u are large on the hadronic scale, is a powerful and, until quite recently, under-utilized probe of nucleon structure. Results from recent Jefferson Lab measurements on both the spin-averaged cross section over a broad kinematic range and polarization observables at a single kinematic point will be presented. These results confirm the powerful role of WACS in determining dominant reaction mechanisms in exclusive hard reactions in the few GeV domain. Unambiguous disagreement with predictions based on perturbative QCD approaches and agreement with predictions based on the handbag mechanism has been established. The results also provide access to non-perturbative proton structure information. The overall WACS program at Jlab will be presented, including plans to extend measurements into a higher kinematic regime following the 12 GeV upgrade. [Preview Abstract] |
Tuesday, April 15, 2008 1:42PM - 1:54PM |
X13.00002: New proton form factor ratio measurements in Jefferson Lab Hall C Andrew Puckett Experiments E04-108 and E04-019 in Hall C at Jefferson Lab measure the ratio of the proton's electric ($G_E$) and magnetic ($G_M$) form factors using the recoil polarization technique. E04-108 will extend the $Q^2$ reach of this technique to approximately 8.5 GeV$^2$, while E04-019 will measure the form factor ratio with high precision at a fixed $Q^2$ of 2.5 GeV$^2$ and three different values of $\epsilon$ in the range $0.1 \leq \epsilon \leq 0.8$. Measuring the $\epsilon$ dependence of the form factor ratio from polarization transfer at fixed $Q^2$ will establish or significantly constrain the size of two-photon effects in elastic electron-proton scattering. Such effects have been proposed to explain the different values of $G_E/G_M$ obtained from polarization transfer and cross section (Rosenbluth separation) measurements at Q$^2 > 1$ GeV$^2$. E04-019 was completed in January 2008, and E04-108 will be completed in April-June of 2008. Form factor ratios extracted from a preliminary analysis of the data taken so far will be reported and discussed. [Preview Abstract] |
Tuesday, April 15, 2008 1:54PM - 2:06PM |
X13.00003: Barely Off-shell Nucleon Structure. Svyatoslav Tkachenko We know much less about the neutron than the proton due to the absence of free neutron targets. Neutron information has to be extracted from data on nuclear targets like deuterium. This requires corrections for off-shell and binding effects which are not known from first principles and therefore are model-dependent. As a consequence, the same data can be interpreted in different ways, leading to different conclusions about important questions such as d/u quark ratio at large momentum fraction $x$. The Barely Off-shell NUcleon Structure (BONUS) experiment at Jefferson Lab addressed this problem by tagging spectator protons in coincidence with inelastic electron scattering from deuterium. A novel compact radial time projection chamber was built to detect low-momentum, backward moving protons, ensuring that the scattering took place on a loosely bound neutron. The scattered electron was detected with Jefferson Lab's CLAS spectrometer. Data were taken at beam energies of 2, 4 and 5 GeV. We will present our experimental method and preliminary results on the extracted structure function $F_2^n$ of the neutron, both in the resonance and deep inelastic regions. [Preview Abstract] |
Tuesday, April 15, 2008 2:06PM - 2:18PM |
X13.00004: The Effect of Pion Exchange in a Relativistic Quark Model of Baryons T. Goldman, R. R. Silbar We examine the effect of adding pion exchange between quarks and pion self-energy corrections to the Los Alamos Relativistic Quark model with a short-distance cutoff of the Bethe form. The contributions to the nucleon and the $\Delta$-baryon are small. We conclude that the model is stable under this change in the sense that significant changes to the model parameters are not required. [Preview Abstract] |
Tuesday, April 15, 2008 2:18PM - 2:30PM |
X13.00005: Measurement of the dressed spin effects of $^3$He Pinghan Chu, Andrea Esler, Douglas Beck, Jen-Chieh Peng, Steve Williamson, Jacob Yoder A new experiment to search for neutron electric dipole moment (EDM) will use ultracold neutrons produced in superfluid helium. Polarized $^3$He will be utilized as a comagnetometer to detect the precession frequency of polarized ultracold neutron. An RF magnetic field will be applied to modify the effective magnetic moments of neutron and $^3$He. This dressed- spin technique, proposed by Golub and Lamoreaux, aims at a reduction of the systematic uncertainty of the EDM measurement. Using a polarized $^3$He cell prepared with the meta-stability exchange technique, we have studied the dressed spin effects for $^3$He for a variety of dressing field configurations. Results from this measurement will be presented and compared with theoretical calculations. Implications of this study on the neutron EDM experiment will also be discussed. [Preview Abstract] |
Tuesday, April 15, 2008 2:30PM - 2:42PM |
X13.00006: ABSTRACT WITHDRAWN |
Tuesday, April 15, 2008 2:42PM - 2:54PM |
X13.00007: Qweak Main Detector Status David Mack The Qweak experiment at Jefferson Laboratory will make the first measurement of the weak charge of the proton. Because this electroweak observable is suppressed in the Standard Model, even the modest projected 4\% uncertainty will allow us to constrain new electron-quark interactions at the multi TeV-scale. The weak charge is accessed through the small ($<$ 1 ppm), parity-violating asymmetry in the elastic scattering of longitudinally polarized electrons from unpolarized protons. Scattered electrons which pass through fused silica bars emit Cerenkov light which is collected and converted to current by phototubes. Most large procurements and R\&D are complete. The status of the construction effort will be summarized. We remain on-track for installation in Spring 2009. [Preview Abstract] |
Tuesday, April 15, 2008 2:54PM - 3:06PM |
X13.00008: A Prototype Diamond Multistrip Detector for the JLab Hall C Compton Polarimeter Amrendra Narayan A Compton Polarimeter is essential for any precision polarization experiment. The QWeak experiment at JLab will use parity violating electron scattering from the proton to perform a precision measurement of the weak charge of the proton (Q$^{P}_{\small{{\mbox{Weak}}}}$). This experiment requires the knowledge of the electron beam polarization at a level of $\sim$1\%. To achieve this, a Compton Polarimeter is under construction in JLab Hall C. The Polarimeter includes a recoil electron detector. The QWeak experiment plans to use a 180 $\mu$A polarized electron beam, in order to get the highest luminosity possible at JLab. At these luminosities, the typically used silicon detectors are rendered unsuitable due to rapid radiation damage. We have hence proposed a Chemical Vapor Deposition (CVD) diamond multi-strip detector for this Polarimeter. CVD Diamond detectors are well known for their radiation hardness. A prototype diamond multi-strip detector is being characterized at Mississippi State University. We will present preliminary spectra obtained from this detector. Based on the results of this study, a full scale diamond multi-strip detector is being constructed. [Preview Abstract] |
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