Final analysis of proton form factor ratio data at Q[superscript 2]=4.0, 4.8, and 5.6 GeV[superscript 2]

Abstract

Precise measurements of the proton electromagnetic form factor ratio R=μ[subscript p]G[subscript E][superscript p]/G[subscript M][superscript p] using the polarization transfer method at Jefferson Lab have revolutionized the understanding of nucleon structure by revealing the strong decrease of R with momentum transfer Q[superscript 2] for Q[superscript 2]≳1 GeV[superscript 2], in strong disagreement with previous extractions of R from cross-section measurements. In particular, the polarization transfer results have exposed the limits of applicability of the one-photon-exchange approximation and highlighted the role of quark orbital angular momentum in the nucleon structure. The GEp-II experiment in Jefferson Lab's Hall A measured R at four Q[superscript 2] values in the range 3.5 GeV[superscript 2]≤Q[superscript 2]≤5.6 GeV[superscript 2]. A possible discrepancy between the originally published GEp-II results and more recent measurements at higher Q[superscript 2] motivated a new analysis of the GEp-II data. This article presents the final results of the GEp-II experiment, including details of the new analysis, an expanded description of the apparatus, and an overview of theoretical progress since the original publication. The key result of the final analysis is a systematic increase in the results for R, improving the consistency of the polarization transfer data in the high-Q[superscript 2] region. This increase is the result of an improved selection of elastic events which largely removes the systematic effect of the inelastic contamination, underestimated by the original analysis.