Search for effects beyond the born approximation in polarization transfer observables in e→p elastic 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, GE/GM, 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(e→,e′p→) reaction for three different beam energies at a Q2=2.5GeV2, spanning a wide range of the kinematic parameter μ. The ratio R, which equals μpGE/GM in the Born approximation, is found to be independent of μ at the 1.5% level. The μ dependence of the longitudinal polarization transfer component P ℓ shows an enhancement of (2.3±0.6)% relative to the Born approximation at large μ. © 2011 American Physical Society.