Speaker: Mikhail Gorshteyn  (California Institute of Technology)

Title:
Dispersion analysis of the two-photon exchange effects in elastic
ep-scattering
			
Abstract:

Two- or multi-photon exchange is widely believed to be the origin of the
discrepancy between the Rosenbluth and polarisation transfer data for the
ratio of the elastic form factors $G_E/G_M$. I use dispersion relations to
investigate the two-photon exchange contributions to elastic ep-scattering
amplitude. The imaginary part of the nucleon box graph is related to the
on-shell nucleon form factors, and thus can be calculated using the
phenomenological form factors without introducing any additional model
dependence. The real part of the amplitude is related to the imaginary
part by a dispersion integral. I show that for one of the three invariant
amplitudes, the high energy behaviour of the amplitude requires a
subtraction. Therefore, I argue that additionally to the values of the
elastic form factors $G_{M,E}(Q2)$, the third unknown function of $Q2$
appears. On the experiment, there are three ways to measure the form
factors and $2\gamma$-effects: Rosenbluth separation, polarisation
transfer, and $e^+p/e^-p$ cross section ratio. It follows that all the
three measurements are necessary in order to disantangle these three
unknown functions and extract the true values of the elastic form factors
from the experiment.