[Jlab-seminars] Theory Center Seminar

[Mary Fox]

Theory Center Seminar
Wed., March 20, 2013
2:00 p.m. (coffee at 1:45 p.m.)
CEBAF Center, Room L102


Nikolay Kivel
Holmholtz Institute, Mainz

*Two-Photon Exchange Corrections to Elastic Electron-Proton Scattering
at Large Momentum Transfer Within the SCET Approach

*We calculate the two-photon exchange (TPE) correction in the region 
where the kinematical variables describing the elastic $ep$ scattering 
are moderately large $s\sim-t\sim -u\gg \Lambda^{2}$. For this 
kinematics we use the QCD factorization  approach formulated in the 
framework of the soft-collinear effective theory (SCET). Such technique 
allows us to develop a description for the soft spectator scattering 
contribution which is found to be important in the region of moderately 
large scales.

Together with the hard spectator contribution we present the complete 
factorization formulas for the TPE amplitudes at the leading power and 
leading logarithmic accuracy. The momentum region where both photons are 
hard is described by only one new nonperturbative SCET form factor. The 
same form factor also arises for wide-angle Compton scattering which is 
also described in the framework of the SCET approach. This allows us to 
estimate the soft spectator contribution associated with the hard 
photons in a model independent way.

The main unknown in our description of theTPE contribution is related 
with the configuration where one photon is soft. The  nonperturbative 
dynamics in this case is described by two unknown SCET amplitudes. We 
use a simple model in order to estimate their contribution. The 
formalism is then applied to a phenomenological analysis of existing 
data for the reduced cross section as well as for the transverse and 
longitudinal polarization observables. *
*
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