Nuclear Physics Seminars

ics Nuclear
Tuesday, April 15, 2014
3:30 PM
Physics Building, Room 204
Dipak Rimal [Host: Xiaochao Zheng]
Florida International University
"Proton Form Factor Puzzle and the CLAS Two-Photon Exchange Experiment"
 Slideshow (PDF)
The electromagnetic form factors of the proton are extracted by analyzing unpolarized and polarized electron scattering data assuming the Born approximation. The extracted ratios of electric to magnetic form factors (GE/GM) by the two methods show a significant discrepancy that increases with Q2. One possible explanation for the observed discrepancy is the contribution from two-photon exchange (TPE) eff ects, which are not generally accounted for in the standard treatment of radiative corrections. Theoretical calculations estimating the magnitude of the TPE effect are highly model-dependent and limited experimental evidence for such e ffect exists. The ratio of positron-proton to electron-proton elastic scattering cross sections (R = σ(e+ p)/σ(e- p)) provides a model-independent measurement of the TPE e ffect. One such measurement of R was performed by using a mixed electron-positron beam at Jeff erson Lab Hall B. Both electrons and positrons were elastically scattered from a 30-cm-long liquid hydrogen target. The resulting scattered particles were detected in CLAS. The elastic events were then identified by using elastic scattering kinematics. The measurement covers a range of Q2 < 2 GeV2 and 0:1 < ε < 1. In the kinematics of the experiment, our results are consistent with the hadronic calculation of the TPE e ffect by Blunden, Melnitchouk, and Tjon. The details of the CLAS TPE experiment and the results will be discussed.

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