Proton form factor ratio, μ<sub>p</sub>G<sub>E</sub><sup>P</sup>/G<sub>M</sub><sup>P</sup> from double spin asymmetry

Abstract

The form factors are fundamental properties of the nucleon representing the effect of its structure on its response to electromagnetic probes such as electrons. They are functions of the four-momentum transfer squared Q2 between the electron and the proton. This thesis reports the results of a new measurement of the ratio of the electric and magnetic form factors of the proton up to Q2 = 5.66 (GeV/c)2 using the double spin asymmetry with a polarized beam and target. Experiment E07-003 (SANE, Spin Asymmetries of the Nucleon Experiment) was carried out in Hall C at Jefferson Lab in 2009 to study the proton spin structure functions with a dynamically polarized ammonia target and longitudinally polarized electron beam. By detecting elastically scattered protons in the High-Momentum Spectrometer (HMS) in coincidence with the electrons in the Big Electron Telescope Array (BETA), elastic measurements were carried out in parallel. The elastic double spin asymmetry allows one to extract the proton electric to magnetic form factor ratio GpE/GpM at high-momentum transfer, Q2= 5.66 (GeV/c)2. In addition to the coincidence data, inclusively scattered electrons from the polarized ammonia target were detected by HMS, which allows to measure the beam-target asymmetry in the elastic region with the target spin nearly perpendicular to the momentum transfer, and to extract GpE/GpM at low Q2= 2.06 (GeV/c)2. This alternative measurement of GpE/GpM has verified and confirmed the dramatic discrepancy at high Q2 between the Rosenbluth and the recoil-polarization-transfer iv method with a different measurement technique and systematic uncertainties uncorrelated to those of the recoil-polarization measurements. The measurement of the form factor ratio at Q2 = 2.06 (GeV/c)2 has been determined as {mu}{sub p}GpE/GpM = 0.605{+-}0.178stat{+-}0.033sys which is in agreement with an earlier measurement with the polarized target technique at similar kinematics. The measurement of the form factor ratio at Q2 = 5.66 (GeV/c)2 has been determined as μpGpE/GpM = 0.672 {+-} 0.362stat which represents the highest Q2 reach with the double spin asymmetry to date.