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 $\epsilon$.
Categories: nucl-ex nucl-th