Abstract
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.
Highlights
The elastic form factors of the proton play an essential role in our understanding the nucleon electromagnetic structure
Direct measurements of the two-photon effect on the unpolarized scattering cross section [7,8,9,10] are based on the observation that this contribution changes its sign with a sign of the scattering lepton, it can be evaluated via measurements of the charge asymmetry
The dashed lines correspond to the contribution of only the soft photon approximation calculated in [14], while the solid lines include the hard photon emission presented in this paper
Summary
The elastic form factors of the proton play an essential role in our understanding the nucleon electromagnetic structure. Q2-dependence of the ratio of the electric to magnetic proton form factors, GEðQ2Þ=GMðQ2Þ, obtained from the unpolarized [1,2] and polarized [3,4] electron elastic scattering data disagreed with each other, and the disagreement was attributed to two-photon exchange effects. The challenge in computing two-photon exchange contribution is in the need for modeling nucleon’s structure. It motivated a dedicated program of both direct and indirect experimental measurements of two-photon effects in electron scattering, along with a significant theory effort; the recent status of the problem is reviewed in, e.g., Ref.
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