Final analysis of proton form factor ratio data at Q(2)=4.0, 4.8, and 5.6 GeV2

By A. Puckett, E. Brash, O. Gayou, M. Jones, L. Pentchev, C. Perdrisat, V. Punjabi, K. Aniol, T. Averett, F. Benmokhtar and 64 more. 2011

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Precise measurements of the proton electromagnetic form factor ratio R = mu(p)G(E)(p)/G(M)(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(2) for Q(2) greater than or similar to 1 GeV2, 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(2) values in the range 3.5 GeV2 <= Q(2) <= 5.6 GeV2. A possible discrepancy between the originally published GEp-II results and more recent measurements at higher Q(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(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. (Less)

Subfields: Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/e00c9b66dc062af2f54fe66ce81747257f15b32b

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