Purpose: The GEp-III and GEp-2γ experiments were carried out in Jefferson Laboratory’s (JLab’s) Hall C from\ud 2007 to 2008, to extend the knowledge of Gp\ud E/Gp\ud M to the highest practically achievable Q2 given the maximum\ud beam energy of 6 GeV and to search for effects beyond the Born approximation in polarization transfer observables\ud of elastic ep scattering. This article provides an expanded description of the common experimental apparatus and\ud data analysis procedures, and reports the results of a final reanalysis of the data from both experiments, including\ud the previously unpublished results of the full-acceptance dataset of the GEp-2γ experiment.\ud Methods: Polarization transfer observables in elastic ep → ep scattering were measured at central Q2 values\ud of 2.5, 5.2, 6.8, and 8.54 GeV2\ud . At Q2 = 2.5 GeV2\ud , data were obtained for central values of the virtual photon\ud polarization parameter of 0.149, 0.632, and 0.783. The Hall C High Momentum Spectrometer detected and\ud measured the polarization of protons recoiling elastically from collisions of JLab’s polarized electron beam\ud with a liquid hydrogen target. A large-acceptance electromagnetic calorimeter detected the elastically scattered\ud electrons in coincidence to suppress inelastic backgrounds.\ud Results: The final GEp-III data are largely unchanged relative to the originally published results. The statistical\ud uncertainties of the final GEp-2γ data are significantly reduced at = 0.632 and 0.783 relative to the original\ud publication.\ud Conclusions: The final GEp-III results show that the decrease with Q2 of Gp\ud E/Gp\ud M continues to Q2 = 8.5 GeV2, but at a slowing rate relative to the approximately linear decrease observed in earlier Hall A measurements. At\ud Q2 = 8.5 GeV2\ud , Gp\ud E/Gp\ud M remains positive but is consistent with zero. At Q2 = 2.5 GeV2\ud , Gp\ud E/Gp\ud M derived from\ud the polarization component ratio R ∝ Pt/P shows no statistically significant dependence, as expected in the\ud Born approximation. On the other hand, the ratio Pl/P Born\ud l of the longitudinal polarization transfer component\ud to its Born value shows an enhancement of roughly 1.7% at = 0.783 relative to = 0.149, with ≈ 2.2σ\ud significance based on the total uncertainty, implying a similar effect in the transverse component Pt that cancels\ud in the ratio R.
