Search for effects beyond the Born approximation in polarization transfer observables in $\vec{e}p$ elastic 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 (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 $H(\vec{e},e^{\prime }\vec{p})$ 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 $ΔR \approx \pm 0.01 \mbox{(stat)} \pm 0.013 \mbox{(syst)}$ and $ΔP_\ell/P^{Born}_{\ell} \approx \pm 0.006 \mbox{(stat)}\pm 0.01 \mbox{(syst)}$. 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 $\epsilon$.

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• Two-Photon Physics in Hadronic Processes
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