[Theory-seminars] Joint physics-theory seminar 11 a.m. F113 Monday Oct 22

[Alexei Prokudin]

Dear all,

we will have a joint physics-theory seminar on Monday 22 at 11 a.m. in 
F113 (NOTE TIME
AND ROOM)

Speaker: Larry Weinstein (ODU)

Title:   Two Photon Exchange and the Proton Form Factor Problem

Abstract: 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
difficult-to-calculate contribution of hard two-photon exchange (TPE)
contributions. This contribution is directly proportional to the difference
between positron-proton and electron-proton elastic scattering cross 
sections.

We directly determined the TPE effect by measuring the ratio of the
positron-proton to electron-proton elastic scattering cross sections,
$R=\sigma(e^+p)/\sigma(e^-p)$ over a wide range of momentum transfer, 
$Q^2$,
and virtual photon polarization, epsilon, using a mixed beam of electrons
and positrons in Hall B. This talk will describe the experimental 
techniques
used to produce this beam, the analysis techniques to identify elastic
scattering events, and some preliminary results.




A complete list of upcoming theory seminars and talks from previous
seminars is available from:
http://www.jlab.org/div_dept/theory/seminars/

Alexei, Alvaro and Carlos
JLab Theory Seminar Organizers