Experimental search for two-hard-photon exchange in elastic ep experiments

A remarkable change of paradigm was precipitated by the results, obtained in the last 12 years, of new measurements of the proton form factor ratio; the two form factors, G{sub Ep} and G{sub Mp}, are not in a constant ratio, as had been concluded from previous cross section experiments, and would be the case if charge and magnetization spatial distributions were the same. Rather, as the series of polarization measurements at JLab shows, the ratio {mu}{sub p}G{sub Ep}/G{sub Mp} decreases smoothly from 1 at Q{sup 2} = 0, to about 0.15 at Q{sup 2} = 8.5 GeV{sup 2}, with Q{sup 2} the negative of the four-momentum transfer squared. The interesting question is then: how can the results using two methods both related through the Born approximation, be found to lead to a different form factor ratio? The short answer is that cross sections require large radiative corrections, which tend to mask G{sub Ep} for increasing Q{sup 2}, whereas recoil polarization experiments measure the ratio of two components of the recoil polarization, which tends to cancel the effect of radiative corrections. Radiative corrections to the cross section of ep scattering have a long history. They may just not be accurate enoughmore » when the ratio G{sub Ep}/G{sub Mp} {approx} 0.05 and {tau} = Q{sup 2}/4m{sub p}{sup 2} {approx} 2.5, as is the case for the largest Q{sup 2} for which we have double-polarization data, 8.5 GeV{sup 2}. For this Q{sup 2} and with the Born cross section given by d{sigma} {approx} G{sub Ep}{sup 2} + ({tau}/{var_epsilon})G{sub Mp}{sup 2}, with {var_epsilon} the kinematic factor, the contribution to the cross section of G{sub Ep} becomes smaller than 0.1%, i.e. non-measurable in cross section experiments. The other hypothesis, is that the radiative corrections are incomplete, and that the exchange of two hard photons is the source of the discrepancy; the idea has been pursued in numerous works, but to this day there is no direct, experimental evidence that two-hard-photons exchange is the major source of the discrepancy.« less