Meson exchange effects in elasticepscattering at loop level and the electromagnetic form factors of the proton

Abstract

A new form of two-photon exchange (TPE) effect is studied to explain the discrepancy between unpolarized and polarized experimental data in elastic $ep$ scattering. The mechanism is based on a simple idea that apart from the usual TPE effects from box and crossed-box diagrams, the mesons may also be exchanged in elastic $ep$ scattering by two-photon coupling at loop level. The detailed study shows such contributions to reduced unpolarized cross section (${\ensuremath{\sigma}}_{\mathrm{un}}$) and polarized observables (${P}_{t},{P}_{l}$) at fixed ${Q}^{2}$ are only dependent on proton's electromagnetic form factors ${G}_{E,M}$ and a new unknown universal parameter $g$. After combining this contribution with the usual TPE contributions from box and crossed-box diagrams, the ratio ${\ensuremath{\mu}}_{p}{G}_{E}/{G}_{M}$ extracted from the recent precise unpolarized and polarized experimental data can be described consistently.