Transverse target spin asymmetry in inclusive DIS with two-photon exchange

TL;DR

This paper investigates the transverse target spin asymmetry in inclusive deep inelastic scattering caused by two-photon exchange, providing a theoretical calculation that predicts a small but measurable asymmetry relevant for upcoming experiments.

Contribution

It introduces a novel calculation of the transverse spin-dependent two-photon exchange cross section in DIS, highlighting a parton-like mechanism involving quark transversity and helicity flip processes.

Findings

Asymmetry estimated at 10^{-4} for Jefferson Lab kinematics.

Asymmetry has opposite signs for proton and neutron.

The process is free of infrared and collinear divergences.

Abstract

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_chiral^2 >> Lambda_QCD^2. 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.