Polarization transfer observables in elastic electron-proton scattering at Q~2 = 2.5, 5.2, 6.8, and 8.5 GeV2

摘要

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 approx> 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 Laboratory’s (Jlab’s) Hall C from 2007 to 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 theBorn 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procedures, and reports the results of a final reanalysis of the data from both experiments, including the previously unpublished results of the full-acceptance dataset of the Gep-2γ experiment.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 → ep 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_E~p/G_M~p 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_E~p/G_M~p remains positive but is consistent with zero. At Q~2 = 2.5 GeV~2, G~p_E/G_M~p derived from the polarization component ratio R ∝ P_t/P_ℓ shows no statistically significant ∈ dependence, as expected in the Born approximation. On the other hand, the ratio P_ℓ/P ~(Born)_ℓ 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 P_t that cancels in the ratio R.