Two photon exchange radiative correction to electron-proton elastic scattering

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Ahmed, Jaseer
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The extraction of hadron structure information from precision electron-proton elastic scattering experiments to date requires precise knowledge of the electromagnetic interaction via multi-photon exchange. The proton structure-dependent radiative correction two-photon exchange (TPE) is a promising source to resolve the existing discrepancy in the measurement of the proton electric to magnetic form factor ratio μ_p G_E/G_M, but contains significant uncertainty in its calculation. We use a recently developed dispersive approach to compute TPE corrections to elastic electron-proton scattering, including contributions from hadronic J^(P) = 1/2^(±) and 3/2^(±) resonant intermediate states below 1.8 GeV. For the transition amplitudes from the proton ground state to the resonant excited states we employ new exclusive meson electroproduction data from CLAS at Q^2 ≲ 5 GeV^2, and assess for the first time the effects of finite widths for the Breit-Wigner resonances. Among the resonant states, the N(1520)3/2^(−) becomes dominant for Q^2 ≳ 2 GeV^2, with an opposite sign compared to the ∆(1232)3/2^(+) correction, leading to an overall increase of the TPE correction to the cross section at higher Q^2 values. The results are in good agreement with recent e^(+)p to e^(−)p cross section ratio and polarization transfer measurements, and provide a partial resolution of the electric to magnetic form factor ratio discrepancy. This same model is also applied to the inelastic processes involving the imaginary part of TPE amplitudes, as seen in single-spin asymmetries, with either beam or target being polarized normal to the scattering plane.
Nuclear physics, Electron scattering, Radiative corrections, Proton structure
J. Ahmed, P. Blunden, and W. Melnitchouk, Phys. Rev. C 102, 045205 (2020).