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


Abstract in English

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 $H(vec{e},evec{p})$ 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 $epsilon$.

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