Comments to the calculation of transverse beam spin asymmetry for electron proton elastic scattering

The transverse beam spin induced asymmetry is calculated for the scattering of transversally polarized electrons on a proton target within a realistic model. Such asymmetry is due to the interference between the Born amplitude and the imaginary part of two photon exchange amplitude. In particular, the contribution of non-excited hadron state (elastic) to the two photon amplitude is calculated. The elastic contribution requires infrared divergences regularization and can be expressed in terms of numerical integrals of the target form factor. The inelastic channel corresponding to the one pion hadronic state contribution is enhanced by squared logarithmic terms. We show that the ratio of elastic over inelastic channel is of the order of 0.3 and cannot be ignored. Enhancement eects due to the decreasing of form factors bring the transverse beam asymmetry to values as large as 10 4 for particular kinematical conditions.