Deep inelastic scattering with a tagged photon: QED corrections for the Sigma method

After a brief review of the kinematics of deep inelastic scattering (DIS) w ithin the so-called Sigma method. we derive the necessary formulae for the treatment of QED radiative corrections to DIS originating from hard photon radiation. The results are applied to a calculation of the corrections to D IS with a tagged photon with next-to-leading logarithmic accuracy under HER A conditions. It turns out that the next-to-leading logarithmic corrections are quite important for the Sigma method. We also discuss the dependence o f the corrections on the longitudinal structure function of the proton, F-L , in the region of low Q(2) and moderate x.