Search for Effects Beyond the Born Approximation in Polarization Transfer Observables in e-vectorp 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, G{sub E}/G{sub 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(e-vector,e{sup '}p-vector) reaction for three different beam energies at a Q{sup 2}=2.5 GeV{sup 2}, spanning a wide range of the kinematic parameter {epsilon}. The ratio R, which equals {mu}{sub p}G{sub E}/G{sub M} in the Born approximation, is found to be independent of {epsilon} at the 1.5% level. The {epsilon} dependence of the longitudinal polarization transfer component P{sub l} shows an enhancement of (2.3{+-}0.6)% relative to the Born approximation at large {epsilon}.