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

摘要

Purpose: The GEp-III and GEp-2γ experiments were carried out in Jefferson Laboratory’s (JLab’s) Hall C fromud2007 to 2008, to extend the knowledge of GpudE/GpudM to the highest practically achievable Q2 given the maximumudbeam energy of 6 GeV and to search for effects beyond the Born approximation in polarization transfer observablesudof elastic ep scattering. This article provides an expanded description of the common experimental apparatus anduddata analysis procedures, and reports the results of a final reanalysis of the data from both experiments, includingudthe previously unpublished results of the full-acceptance dataset of the GEp-2γ experiment.udMethods: Polarization transfer observables in elastic ep → ep scattering were measured at central Q2 valuesudof 2.5, 5.2, 6.8, and 8.54 GeV2ud. At Q2 = 2.5 GeV2ud, data were obtained for central values of the virtual photonudpolarization parameter of 0.149, 0.632, and 0.783. The Hall C High Momentum Spectrometer detected andudmeasured the polarization of protons recoiling elastically from collisions of JLab’s polarized electron beamudwith a liquid hydrogen target. A large-acceptance electromagnetic calorimeter detected the elastically scatteredudelectrons in coincidence to suppress inelastic backgrounds.udResults: The final GEp-III data are largely unchanged relative to the originally published results. The statisticaluduncertainties of the final GEp-2γ data are significantly reduced at = 0.632 and 0.783 relative to the originaludpublication.udConclusions: The final GEp-III results show that the decrease with Q2 of GpudE/GpudM continues to Q2 = 8.5 GeV2, but at a slowing rate relative to the approximately linear decrease observed in earlier Hall A measurements. AtudQ2 = 8.5 GeV2ud, GpudE/GpudM remains positive but is consistent with zero. At Q2 = 2.5 GeV2ud, GpudE/GpudM derived fromudthe polarization component ratio R ∝ Pt/P shows no statistically significant dependence, as expected in theudBorn approximation. On the other hand, the ratio Pl/P Bornudl of the longitudinal polarization transfer componentudto its Born value shows an enhancement of roughly 1.7% at = 0.783 relative to = 0.149, with ≈ 2.2σudsignificance based on the total uncertainty, implying a similar effect in the transverse component Pt that cancelsudin the ratio R.