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Theory Center Seminar<br>
Wed., March 20, 2013<br>
2:00 p.m. (coffee at 1:45 p.m.)<br>
CEBAF Center, Room L102<br>
<br>
<br>
Nikolay Kivel<br>
Holmholtz Institute, Mainz<br>
<br>
<b>Two-Photon Exchange Corrections to Elastic Electron-Proton
Scattering<br>
at Large Momentum Transfer Within the SCET Approach<br>
<br>
</b>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. <br>
<br>
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. <br>
<br>
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. <b><br>
</b>
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