Two-Photon Exchange Effects in Elastic Electron-Proton Scattering.

摘要

Two methods, Rosenbluth separation and polarization transfer, can be used to extract the proton form factor ratio mupG Ep/GMp, but they do not yield the same results. It is thought that the disagreement is due to two photon exchange corrections to the dierential cross sections. High precision proton Rosenbluth extractions were carried out at 102 kinematics points spanning 16 values of momentum transfer Q2, from 0.40 to 5.76 GeV 2. Reduced cross sections were found to 1.1% or better for Q 2 less than 3 GeV2, increasing to 4% at 5.76 GeV 2. The form factor ratios were determined to 1:5-3% for Q2 < 1.5 GeV2, increasing to 9% by 3 GeV2 and rapidly above. Our data agrees with prior Rosenbluth, improving upon it the 1.0 - 2.0 GeV2 range to conclusively show a separation from polarization transfer where it had not been certain before. In addition, reduced cross sections at each Q2 were tested for nonlinearity in the angular variable. Such a departure from linearity would be a signature of two photon exchange effects, and prior data had not been suciently precise to show nonzero curvature. Our data begins to hint at negative curvature but does not yet show a signicant departure from zero.