Search for Effects Beyond the Born Approximation in Polarization Transfer Observables ine→pElastic Scattering

Abstract

Intensive theoretical and experimental efforts over the past decade have aimed at explaining the discrepancy between data for the proton electric to magnetic form factor ratio, G(E)/G(M), obtained separately from cross section and polarization transfer measurements. One possible explanation for this difference is a two-photon-exchange contribution. In an effort to search for effects beyond the one-photon-exchange or Born approximation, we report measurements of polarization transfer observables in the elastic H(e[over →],e(')p[over →]) reaction for three different beam energies at a Q(2)=2.5 GeV(2), spanning a wide range of the kinematic parameter ε. The ratio R, which equals μ(p)G(E)/G(M) in the Born approximation, is found to be independent of ε at the 1.5% level. The ε dependence of the longitudinal polarization transfer component P(ℓ) shows an enhancement of (2.3±0.6)% relative to the Born approximation at large ε.

A 3% Measurement of the Beam Normal Single Spin Asymmetry in Forward Angle Elastic Electron-Proton Scattering using the Qweak Setup

“…2.12) by the GEp2γ experiment [56] recently, the two-photon exchange effects on the polarization transfer observables may not be negligible at forward angles as initially thought to be. Quoting authors in Ref.…”

“…Since the neutron electric form-factor is small compared to that of the proton, two-photon exchange contribution can be more prominent in the extraction of the neutron's form factor is calculated using the Born approximation at a Q 2 of 2.5 (GeV/c) 2 . The results show a 2.3 ± 0.6% enhancement in P L at large ε relative to the Born approximation [56]. The curves are calculations of P L with two-photon exchange correction from a GPD based model and a hadronic model.…”

Study of Two-Photon Exchange via the Beam Transverse Single Spin Asymmetry in Electron-Proton Elastic Scattering at Forward Angles over a Wide Energy Range

“…In the Born approximation arXiv:2002.06252v1 [nucl-ex] 14 Feb 2020 G E (Q 2 , ε) andG M (Q 2 , ε) reduce to the usual electric and magnetic Sachs form factors G E (Q 2 ) and G M (Q 2 ) which are independent of ε, while the remaining four amplitudes F i vanish. The modified ε dependence of the transverse and longitudinal transferred polarizations in polarized ep scattering has been investigated [14]. The interference term of the one-photon-and two-photon-exchange amplitudes, which is proportional to the 3 rd power of the lepton charge, contributes to the cross section with opposite signs in the case of electron-proton and positronproton scattering.…”

Updated analysis of recent results on electron and positron elastic scattering on the proton

“…We do not enter here in the comparison and the virtues of the model dependent 2γE calculations. Let us note that, if a qualitative agreement is found on reproducing the difference between polarized and unpolarized FF ratio, the agreement disappear when compared to another observable, the ǫ dependence of P L /P t [37]. Does the discrepancy between the unpolarized and polarized form factor ratio experiments really exist?…”