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

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 $H(\overrightarrow{e},e^{\prime }\overrightarrow{p})$ reaction for three different beam energies at a $Q^2=2.5{\mathrm{GeV}}^2$, spanning a wide range of the kinematic parameter $ϵ$. The ratio $R$, which equals ${\mu }_p{G}_E/G_M$ 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 $P_{\ell }$ shows an enhancement of $(2.3\pm 0.6)\%$ relative to the Born approximation at large $ϵ$.