Suppression of hard two-photon-exchange contributions to Re+e- elastic scattering cross-section ratios: A phenomenological approach

We present a new extraction of the positron-proton and electron-proton elastic-scattering cross sections ratio Re+e- based on a new phenomenological parametrization of the two-photon-exchange (TPE) corrections to electron-proton (ep) elastic-scattering cross section σR. We compare our results to several previous phenomenological extractions, TPE hadronic calculations, and world data on the ratio Re+e-, including the recent direct measurements from the CLAS, VEPP-3, and OLYMPUS experiments. The ratio Re+e- as extracted from this work is consistent with unity, and shows little sensitivity to ϵ and essentially no Q2 dependence, suggesting a smaller hard-TPE contribution to Re+e- than that predicted by several previous phenomenological extractions and TPE hadronic calculations. In this work, we show that, under certain assumptions and constraints imposed on the TPE amplitudes YE and Y3, the proton magnetic form factor squared (GMp)2, and the proton form factors ratio μpRp=μpGEp/GMp, the TPE corrections to σR can be suppressed, yielding a small hard-TPE contribution to Re+e-. Our results are in agreement with the recent analysis by Bytev and Tomasi-Gustafsson [Phys. Rev. C 99, 025205 (2019)2469-998510.1103/PhysRevC.99.025205]. We believe that the assumption that hard-TPE corrections could account for the discrepancy in the ratio μpRp is still an open question.