Search for effects beyond the Born approximation in polarization transfer observables in $\vec{e}p$ elastic scattering

TL;DR

This study measures polarization transfer in elastic electron-proton scattering at high momentum transfer to investigate effects beyond the Born approximation, specifically two-photon exchange contributions, and finds a small but significant epsilon dependence in polarization transfer observables.

Contribution

It provides new polarization transfer measurements at fixed Q^2 across different epsilon values, revealing potential two-photon exchange effects beyond the Born approximation.

Findings

The ratio R is epsilon-independent within 1.5%.

The longitudinal polarization transfer P_l shows a 2.3% enhancement at large epsilon.

Results suggest possible effects beyond the one-photon exchange approximation.

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 (TPEX) 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 $H(\vec{e},e'\vec{p})$ reaction for three different beam energies at a fixed squared momentum transfer $Q^2 = 2.5$ GeV$^2$, spanning a wide range of the virtual photon polarization parameter, $\epsilon$. From these measured polarization observables, we have obtained separately the ratio $R$, which equals $\mu_p G_{E}/G_{M}$ in the Born approximation, and the longitudinal polarization transfer component $P_\ell$, with statistical and systematic uncertainties of $\Delta R \approx \pm 0.01 \mbox{(stat)} \pm 0.013 \mbox{(syst)}$ and $\Delta P_\ell/P^{Born}_{\ell} \approx \pm 0.006 \mbox{(stat)}\pm 0.01 \mbox{(syst)}$. The ratio $R$ is found to be independent of $\epsilon$ at the 1.5% level, while the $\epsilon$ dependence of $P_\ell$ shows an enhancement of $(2.3 \pm 0.6) %$ relative to the Born approximation at large $\epsilon$.