The GEp-2$\gamma$ Experiment at Jefferson Lab Hall-C
ORAL
Abstract
$\\$Intensive theoretical and experimental efforts have been made over the past decade aiming at explaining the discrepancy between the data for the proton form factor ratio, G$_{Ep}$/G$_{Mp}$, obtained at Jefferson Lab using polarization transfer technique, and the world data obtained by the Rosenbluth method based on cross section measurements. One possible explanation for this difference is a two-photon exchange contribution, where both photons share the momentum transfer about equally. In the Born approximation for a fixed Q$^{2}$, the form factors do not depend upon the energy of the incident electron as they are relativistic invariants. We will report the results of the Jlab Hall-C GEp-2$\gamma$ experiment which was designed to measure a possible kinematical variation of the ratio G$_{Ep}$/G$_{Mp}$ with statistical uncertainties of $\pm$0.01 at Q$^{2}$=2.5 GeV$^{2}$, using the recoil polarization technique. Three kinematics were chosen, corresponding to values of the kinematic factor $\epsilon$=0.15, 0.63 and 0.77. We will describe the new detectors built for both GEp-2$\gamma$ and GEp-III experiments, the electromagnetic calorimeter BigCal which detected the scattered electron, and the focal plane polarimeter (FPP) which measured the polarization of the recoil proton.
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Authors
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Mehdi Meziane
The College of William $\&$ Mary