Contribution of hard photon emission to the charge asymmetry in elastic lepton- and antilepton-proton scattering

The influence of the hard photon emission on the charge asymmetry in the lepton- and antilepton-proton elastic scattering was evaluated for the first time beyond the ultrarelativistic limit, while retaining the lepton mass at all steps of the calculation. This contribution-responsible for the charge asymmetry-is induced by interference between real photon emission from the lepton and proton. During the calculation any excited states of the intermediated proton were not considered, allowing us to use the standard fermionic propagator for this particle. The infrared divergence extracted using Lorenz-invariant approach of Bardin-Shumeiko is canceled by the corresponding soft part of the two-photon exchange contribution. Numerical analysis was performed within kinematic conditions of Jefferson Lab measurements and MUSE experiment in PSI.