Transverse target spin asymmetry in inclusive DIS with two-photon exchange
A. Afanasev, M. Strikman, and C. Weiss

We study the transverse target spin dependence of the cross section for inclusive electron--nucleon scattering with unpolarized beam. Such dependence is absent in the one--photon exchange approximation (Christ--Lee theorem) and arises only in higher orders of the QED expansion, from the interference of one--photon and absorptive two--photon exchange amplitudes as well as from real photon emission (bremsstrahlung). We demonstrate that the transverse spin--dependent two--photon exchange cross section is free of QED infrared and collinear divergences. We argue that in DIS kinematics the transverse spin dependence should be governed by a ``parton--like'' mechanism in which the two--photon exchange couples mainly to a single quark. We calculate the normal spin asymmetry in an approximation where the dominant contribution arises from quark helicity flip due to interactions with non--perturbative vacuum fields (constituent quark picture) and is proportional to the quark transversity distribution in the nucleon. Such helicity--flip processes are not significantly Sudakov--suppressed if the infrared scale for gluon emission in the photon--quark subprocess is of the order of the chiral symmetry breaking scale, $\mu^2_{\text{chiral}} \gg \Lambda^2_{\textrm{QCD}}$. We estimate the asymmetry in the kinematics of the planned Jefferson Lab Hall A experiment to be of the order $10^{-4}$, with different sign for proton and neutron. We also comment on the spin dependence in the limit of soft high--energy scattering.

© 2008 The American Physical Society.