Speaker: Christian Weiss (Jefferson Lab)


Title:
Probing quark helicity flip in DIS with two-photon exchange

Abstract:
Inclusive eN scattering with unpolarized beam exhibits a subtle 
dependence on the transverse target spin, arising from the interference 
of one-photon and two-photon exchange amplitudes in the cross section. 
This observable is free of QED infrared and collinear divergences and 
represents an interesting new tool for studying nucleon structure. 
We argue that in DIS kinematics it can be used to probe the quark 
helicity-flip amplitudes induced by the non-perturbative vacuum structure 
of QCD (spontaneous chiral symmetry breaking). This conjecture is based on 
(a) the absence of significant Sudakov suppression of the helicity-flip 
process, (b) the expectation that the quark helicity-conserving twist-3 
contribution is small. We estimate the transverse target spin asymmetry 
to be of the order 10^{-4} in the kinematics of the planned JLab Hall A 
experiment (E-07-013), and give an outlook on future theoretical studies
in this field (arXiv:0709.0901 [hep-ph]).