Measuring the 2γ Exchange Effect in Electron-Proton Scattering at CLAS

Abstract

Recent results from experiments at Jefferson Laboratory, Newport News VA, which measure the ratio of the electric to magnetic form factors of the proton, GE/GM, have forced us to reexamine the single photon exchange approximation in lepton‐proton elastic scattering. Discrepancies between the ratio obtained via the time‐tested Rosenbluth separation method and new polarization transfer measurements, which differ by as much as a factor of three, may be resolved by considering the effect of two photon exchange (TPE). Theoretical corrections for the TPE effect are difficult due to the large number of excited nucleon states that can contribute to the process. The TPE effect can, however, be determined directly by measuring the ratio of the positron‐proton to electron‐proton elastic scattering cross sections, R = σ(e+)/σ(e−), since the TPE effect changes sign with the charge of the incident particle. The CLAS TPE experiment at Jefferson Laboratory, will determine the effect of two‐photon exchange in elastic lepton‐proton scattering by precisely measuring the ratio of positron‐proton to electron‐proton elastic cross sections over a large kinematic range (0.1<ε<0.96, 0.02≤Q2≤2.0). We will accomplish this by directing the 5.5 GeV primary electron beam, provided by the Continuous Electron Beam Accelerator Facility (CEBAF), onto a set of radiators and converters to produce simultaneous and identical beams of electron and positrons. Acceptance and efficiency concerns are minimized by only considering the ratios of the elastic cross sections and switching polarity of magnets in the beam line and the spectrometer. Guided by the results of a short 2006 test run and extensive GEANT based modeling, new shielding and beam line components were designed to maximize luminosity. The unique experimental design and challenges of the TPE experiment and the projected luminosities will be presented.