Abstract
We discuss recent experimental results concerning the cross section ratio of positron over electron elastic scattering on protons, and compare with the predictions of a pre-existent calculation. The deviation from unity of this ratio, $i.e.$, a charge asymmetry different from zero, is the signature of contributions beyond the Born approximation. After reviewing the published results, we compare the elastic data to a calculation which includes the diagram corresponding to two-photon exchange. It turns out that all the data on the cross section ratio, in the limit of their precision, do not show evidence of enhanced two-photon contribution beyond the expected percent level. Our results confirm that experimental evidence for a large contribution of two-photon exchange is not yet found.
Highlights
Elastic electron-proton scattering has been the subject of large experimental and theoretical efforts for many decades
The development of 100% duty cycle electron machines at Jefferson Laboratory (JLab), with highly polarized electron beams, the construction of large solid angle spectrometers, and detectors, the development of proton polarimetry in the GeV region made possible to apply the polarization method suggested by Akhiezer and Rekalo at the end of the 1960s [2,3]
As radiative corrections applied to the data may differ from one paper to another by some finite expression, in order to be less sensitive to model corrections, we consider the total odd contribution from Ref. [24] and remove the odd correction from the calculations used in the data
Summary
Elastic electron-proton scattering has been the subject of large experimental and theoretical efforts for many decades. Summarizing, it can be stated from these general features that, in the presence of a sizable 2γ E contribution, one expects: nonlinearities in the Rosenbluth plot, a charge asymmetry (differences in e± p elastic cross sections, in the same kinematical conditions), and nonvanishing parity-odd polarization observables All these effects would increase with Q2. In this work we compile and discuss the results of three recent experiments that were especially built to detect a possible charge asymmetry through the measurement of the cross section ratio of electron and positron elastic scattering on the proton. This observable is sensitive to the real part of the 2γ E amplitude. The recent data are compared with a calculation [24], where no specific model dependent enhancement of the 2γ E contribution is added
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