Polarization transfer observables in elastic electron-proton scattering at Q2=2.5, 5.2, 6.8, and 8.5 GeV2

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(E)(p)/G(M)(p) of the proton's electric and magnetic form factors for momentum transfers Q(2) greater than or similar to 1 GeV2, in strong disagreement with previous extractions of this ratio using the traditional Rosenbluth separation technique. Purpose: The GEp-III and GEp-2 gamma experiments were carried out in Jefferson Laboratory's ( JLab's) Hall C from 2007 to 2008, to extend the knowledge of G(E)(p) /G(M)(p) 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 (e) over right arrowp 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procedures, and reports the results of a final reanalysis of the data from both experiments, including the previously unpublished results of the full-acceptance dataset of the GEp-2 gamma experiment. Methods: Polarization transfer observables in elastic (e) over right arrowp -> e (p) over right arrow scattering were measured at central Q(2) values of 2.5, 5.2, 6.8, and 8.54 GeV2. At Q 2 = 2.5 GeV2, data were obtained for central values of the virtual photon polarization parameter is an element of 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-2 gamma data are significantly reduced at is an element of = 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(E)(p)/G(M)(p) continues to Q(2) = 8.5 GeV2, but at a slowing rate relative to the approximately linear decrease observed in earlier Hall A measurements. At Q(2) = 8.5 GeV2, G(E)(p)/G(M)(p) remains positive but is consistent with zero. At Q(2) = 2.5 GeV2, G(E)(p)/G(M)(p) derived from the polarization component ratio R alpha P-t/P-l shows no statistically significant is an element of dependence, as expected in the Born approximation. On the other hand, the ratio P-l/P-l(Born) of the longitudinal polarization transfer component to its Born value shows an enhancement of roughly 1.7% at is an element of = 0.783 relative to is an element of = 0.149, with approximate to 2.2 sigma significance based on the total uncertainty, implying a similar effect in the transverse component Pt that cancels in the ratio R.