A. J. R. Puckett, E. J. Brash, O. Gayou, M. K. Jones, L. Pentchev, C. F. Perdrisat, V. Punjabi, K. A. Aniol, T. Averett, F. Benmokhtar, W. Bertozzi, L. Bimbot, J. R. Calarco, C. Cavata, Z. Chai, C. -C. Chang, T. Chang, J. P. Chen, E. Chudakov, R. De Leo, S. Dieterich, R. Endres, M. B. Epstein, S. Escoffier, K. G. Fissum. H. Fonvieille, S. Frullani, J. Gao, F. Garibaldi, S. Gilad, R. Gilman, A. Glamazdin, C. Glashausser, J. Gomez, J. -O. Hansen, D. Higinbotham, G. M. Huber, M. Iodice, C. W. de Jager, X. Jiang, M. Khandaker, S. Kozlov, K. M. Kramer, G. Kumbartzki, J. J. LeRose, D. Lhuillier, R. A. Lindgren, N. Liyanage, G. J. Lolos, D. J. Margaziotis, F. Marie, P. Markowitz, K. McCormick, R. Michaels, B. D. Milbrath, S. K. Nanda, D. Neyret, N. M. Piskunov, R. D. Ransome, B. A. Raue, R. Roché, M. Rvachev, A. Saha, C. Salgado, S. Sirca, I. Sitnik, S. Strauch, L. Todor, E. Tomasi-Gustafsson, G. M. Urciuoli, H. Voskanyan, K. Wijesooriya, B. B. Wojtsekhowski, X. Zheng, L. Zhu
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 $Q^2 \gtrsim 1$ GeV$^2$, 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 $Q^2$ values in the range 3.5 GeV$^2 \le Q^2 \le 5.6$ GeV$^2$. 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.
View original:
http://arxiv.org/abs/1102.5737
No comments:
Post a Comment