Among the most fundamental observables of nucleon structure, electromagnetic form factors are a crucial benchmark for modern calculations describing the strong interaction dynamics of the nucleon's quark constituents; indeed, recent proton data have attracted intense theoretical interest. In this Letter, we report new measurements of the proton electromagnetic form factor ratio using the recoil polarization method, at momentum transfers Q2=5.2, 6.7, and 8.5  GeV2. By extending the range of Q2 for which G(E)(p) is accurately determined by more than 50%, these measurements will provide significant constraints on models of nucleon structure in the nonperturbative regime.

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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 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[over →],e(')p[over →]) reaction for three different beam energies at a Q(2)=2.5  GeV(2), spanning a wide range of the kinematic parameter ε. The ratio R, which equals μ(p)G(E)/G(M) in the Born approximation, is found to be independent of ε at the 1.5% level. The ε dependence of the longitudinal polarization transfer component P(ℓ) shows an enhancement of (2.3±0.6)% relative to the Born approximation at large ε.

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Background: Interest in the behavior of nucleon electromagnetic form factors at large momentum transfers has steadily increased since the discovery, using polarization observables, of the rapid decrease of the ratio G p E /G p M of the proton's electric and magnetic form factors for momentum transfers Q 2 1 GeV 2 , in strong disagreement with previous extractions of this ratio using the traditional Rosenbluth separation technique. Purpose: The GEp-III and GEp-2γ experiments were carried out in Jefferson Lab's (JLab's) Hall C from 2007-2008, to extend the knowledge of G p E /G p M to the highest practically achievable Q 2 given the maximum beam energy of 6 GeV, and to search for effects beyond the Born approximation in polarization transfer observables of elastic ep scattering. This article provides an expanded description of the common experimental apparatus and data analysis procedures, and reports the results of a final reanalysis of the data from both experiments, including the previously unpublished results of the full-acceptance dataset of the GEp-2γ experiment. Methods: Polarization transfer observables in elastic ep → e p scattering were measured at central Q 2 values of 2.5, 5.2, 6.8, and 8.54 GeV 2. At Q 2 = 2.5 GeV 2 , data were obtained for central values of the virtual photon polarization parameter of 0.149, 0.632, and 0.783. The Hall C High Momentum Spectrometer detected and measured the polarization of protons recoiling elastically from collisions of JLab's polarized electron beam with a liquid hydrogen target. A large-acceptance electromagnetic calorimeter detected the elastically scattered electrons in coincidence to suppress inelastic backgrounds. Results: The final GEp-III data are largely unchanged relative to the originally published results. The statistical uncertainties of the final GEp-2γ data are significantly reduced at = 0.632 and 0.783 relative to the original publication. Conclusions: The final GEp-III results show that the decrease with Q 2 of G p E /G p M continues to Q 2 = 8.5 GeV 2 , but at a slowing rate relative to the approximately linear decrease observed in earlier Hall A measurements. At Q 2 = 8.5 GeV 2 , G p E /G p M remains positive, but is consistent with zero. At Q 2 = 2.5 GeV 2 , G p E /G p M derived from the polarization component ratio R ∝ Pt/P shows no statistically significant-dependence, as expected in the Born approximation. On the other hand, the ratio P /P Born of the longitudinal polarization transfer component to its Born value shows an enhancement of roughly 1.7% at = 0.783 relative to = 0.149, with ≈ 2.2σ significance based on the total uncertainty, implying a similar effect in the transverse component Pt that cancels in the ratio R.