10.2172/1047735

Abstract

Among the fundamental observables of nucleon structure, electromagnetic form factors are a crucial benchmark for modern calculations describing the strong interaction dyna mics of the nucleon's quark constituents. Electromagnetic probes are traditionally preferered to the hadronic beams. The electromagnetic interaction is a powerful tool for investigating the nucleon structure since it is well understood and it reveals observables that can be directly interpreted in terms of the current carried by the quarks. Elastic scattering leads to the form factors that describe the spatial charge a nd current distributions inside the nucleon. The reaction mechanism is assumed to be one photon exchange, the electromagnetic interaction is exactly calculable in QED, and one can safely extract the information on the hadronic vertex. The most important feature of early measurements of proton form factor ratio G{sub E}{sup p}/G{sub M}{sup p} with recoil polarization technique at Q{sup 2} up to 5.6 (GeV/c){sup 2} is the sharp decline of the ratio with Q{sup 2} increases, indicating that G{sub E}{sup p} falls much faster than G{sub M}{sup p}. This contradicts to data obtained by Rosenbluth separation method. An intriguing question was whether G{sub E}{sup p} will continue to decrease or become constant when Q{sup 2} increases. more » New set of measurements of proton form factor ratio G{sub E}{sup p}/G{sub M}{sup p} at Q{sup 2} = 2.5, 5.2, 6.7 and 8.5 (GeV/c){sup 2} have been conducted at JLab Hall C using {approx}85% longitudinally polarized electron elastic scattering from unpolarized hydrogen target. Recoil protons were detected in the HMS magnetic spectrometer with the standard detector package, combined with newly installed trigger scintillators and Focal Plane Polarimeter. The BigCal electromagnetic calorimeter (1744 channel) have been used for electron detection. Data obtained in this experiment show that G{sub E}{sup p}/G{sub M}{sup p} ratio continued to drop with Q{sup 2} and may cross 'zero' at Q{sup 2} > 10-15 (GeV/c){sup 2}. Intensive theoretical and experimental efforts over the past decade have aimed at explaining the discrepancy between data for the proton form factor ratio G{sub E}{sup p}/G{sub M}{sup p} obtained from cross section and polarization measurements. It was assumed that the two photon exchange contribution might be responsible for difference of G{sub E}{sup p}/G{sub M}{sup p} ratio obtained by Rosenbluth separation method and recoil polarization technique. The kinematical dependence of polarization transfer observables in elastic electron-proton scattering at Q{sup 2} = 2.5 (GeV/c){sup 2} have been used in search of effects of 2{gamma} contribution. For a wide range of values of the virtual photon polarization {epsilon} ({epsilon} = 0.15, 0.63, and 0.77), the proton form factor ratio and longitudinal polarization transfer component were measured with statistical uncertainties of {+-}0.01 and {+-}0.005, respectively. Our data provide significant constraints on models of nucleon structure. « less