Search for effects beyond the Born approximation in polarization transfer observables in e⃗p 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_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(e⃗,e'p⃗) 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, ϵ. From these measured polarization observables, we have obtained separately the ratio R, which equals μ_p G_E/G_M in the Born approximation, and the longitudinal polarization transfer component P_ℓ, with statistical and systematic uncertainties of Δ R ≈± 0.01 ± 0.013 and Δ P_ℓ/P^Born_ℓ≈± 0.006 ± 0.01. The ratio R is found to be independent of ϵ at the 1.5 shows an enhancement of (2.3 ± 0.6) largeϵ.