Abstract
The parity-conserving single-spin beam asymmetry of elastic electron–proton scattering is induced by an absorptive part of the two-photon exchange amplitude. We demonstrate that this asymmetry has logarithmic and double-logarithmic enhancement due to contributions of hard collinear quasi-real photons. An optical theorem is used to evaluate the asymmetry in terms of the total photoproduction cross section on the proton, predicting its magnitude at 20–30 parts per million for high electron beam energies and small scattering angles. At fixed 4-momentum transfers, the asymmetry is rising logarithmically with increasing electron beam energy, following the high-energy diffractive behavior of total photoproduction cross section on the proton.
Highlights
It has been known for a long time [1, 2, 3] that the two photon exchange (TPE) mechanism can generate the single-spin normal asymmetry (SSNA) of electron scattering due to a nonzero imaginary part of the TPE amplitude A2γ, An
In Ref.[6] the beam SSNA at large momentum transfers was estimated at the level of one ppm, using the partonic picture developed in Ref.[7] for TPE effects but not related to the electron helicity flip
In this paper we demonstrate that collinear photon exchange in the TPE amplitude results in single- and double-logarithmic enhancement of the beam SSNA, whereas such enhancement does not take place for the target SSNA and spin correlations caused by longitudinal polarization of the scattering electrons
Summary
It has been known for a long time [1, 2, 3] that the two photon exchange (TPE) mechanism can generate the single-spin normal asymmetry (SSNA) of electron scattering due to a nonzero imaginary part of the TPE amplitude A2γ , An. Current experiments designed for parity-violating electron scattering allow to measure the beam asymmetry with a fraction of ppm accuracy [8, 9, 10] and may provide data on the parity-conserving beam SSNA Such measurements are needed because beam SSNA is a source of systematic corrections in the measurements of parity-violating observables. It was noted in [4] that while considering excitation of inelastic intermediate hadronic states, the beam SSNA (Eq(11) of Ref.[4]), after factoring out the electron mass, has an enhancement when at least one of the photons in the TPE loop diagram is collinear to its parent electron. For large electron energies and small scattering angles, we use an optical theorem to relate the nucleon Compton amplitude to the total photoproduction cross section and obtain a simple analytic formula for the beam SSNA in this kinematics
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