[Tpe] abstract for fall APS meeting

[Larry Weinstein]

Dear TPEers,

Here is my proposed abstract for the Fall APS meeting. Please comment.

Resolving the Proton Form Factor Problem with Positron-Proton Scattering

Lawrence B. Weinstein for the CLAS Collaboration

The proton electromagnetic form factors are essential pieces of our 
knowledge of nucleon structure.  However, Rosenbluth separation 
measurements of the proton electric form factor, $G_E(Q^2)$, differ from 
polarization transfer measurements by a factor of three at $Q^2 = 5.6$ 
(GeV/$c$)$^2$. This discrepancy must be resolved.  One possible 
resolution is to include the contribution of hard two-photon exchange 
(TPE) contributions that are not included in the usual radiative 
corrections.  These contributions are very difficult to calculate due to 
the need to integrate over a large number of intermediate 
excited-nucleon states. However, we can directly determine the TPE 
effect by measuring the ratio of the positron-proton to electron-proton 
elastic scattering cross sections, $R=\sigma(e^+p)/\sigma(e^-p)$, 
because the TPE amplitude has the same sign as the $e^+p$ born amplitude 
and the opposite sign as the $e^-p$ born amplitude.

We have measured $R=\sigma(e^+p)/\sigma(e^-p)$ over a wide range of 
momentum transfer, $0.2 \le Q^2 \le 2$ (GeV/$c$)^$2$, and virtual photon 
polarization, $0.1 \le \epsilon \le 0.9$, using a mixed identical beam 
of electrons and positrons in Hall B at Jefferson Lab.  This talk will 
describe the experimental techniques used to produce this beam, the 
analysis techniques to identify elastic scattering events, and some 
preliminary results.

-- 
				Sincerely,
				Larry

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Lawrence Weinstein
University Professor
Physics Department
Old Dominion University
Norfolk, VA 23529
757 683 5803
757 683 3038 (fax)
weinstein at odu.edu
http://www.lions.odu.edu/~lweinste/