Eur. Phys. J. A 24, s2.39-s2.40 (2005)
DOI: 10.1140/epjad/s2005-04-008-0

Normal beam spin asymmetries during the G⁰ forward angle measurement

P.M. King<sup>1, 2

1</sup>  University of Illinois at Urbana-Champaign, Urbana, IL, USA
²  Experiment E-00-006, Jefferson Laboratory, Newport News, VA, USA

(Received 01 November 2004 / Published online 08 February 2005)

Abstract

The vector analyzing power measured in elastic scattering of transversely polarized electrons from an unpolarized nucleon is directly proportional to the imaginary part of the two photon exchange amplitude. There has been recent interest in explorations of the two photon exchange amplitude, as the real part has been proposed as a possible resolution of the discrepancy between Rosenbluth separation and polarization observable measurements of the ratio of the electric to magnetic proton form factor. The vector analyzing power appears in the experiment as an azimuthal asymmetry. It has been measured previously in the SAMPLE and A4 experiment with different kinematics than those achievable with the G⁰ apparatus. As part of the systematic checks for the G⁰ forward angle measurement at TJNAF, the normal beam spin asymmetry in the G⁰ detector array was measured with a 3 GeV beam incident upon a liquid hydrogen target. The experimental configuration was identical to the standard G⁰ forward angle running except that the beam was transversely polarized in the plane of the accelerator. The data collected cover a range in center of mass angle from 19° to 37°, with an eight-fold azimuthal symmetry. About 30 hours of data were taken in this configuration, resulting in an extracted vector analyzing power with a precision of a few ppm, which may already be able to provide some constraint on model predictions.

PACS: 13.60.-r Photon and charged-lepton interactions with hadrons - 14.20.Dh Protons and neutrons - 29.27.Hj Polarized beams in accelerators

The Author, for the G⁰ Collaboration. The G⁰ Experiment is supported by the U.S. National Science Foundation (NSF) under grant PHY94-10768, the U.S. Department of Energy (DOE), the Natural Sciences and Engineering Research Council (NSERC) of Canada, and the Centre National de la Recherche Scientifique (CNRS) of France through the Institut National de Physique Nucléaire et de Physique des Particules (IN2P3).

© Società Italiana di Fisica, Springer-Verlag 2005