Beyond the Born approximation: A precise comparison of positron-proton and electron-proton elastic scattering in the CEBAF large acceptance spectrometer (CLAS)

How well we know the structure of the proton depends on our knowledge of the form factors of the proton. The ratio of the electromagnetic form factors of the proton measured by the Rosenbluth and the polarization transfer methods differ by a factor of 3 at four momentum transfer squared (Q²)=5.6 GeV². The two photon exchange (TPE) effect is the leading candidate to explain this discrepancy. The theoretical estimates of the TPE effect are model dependent so precise measurement is required to resolve this problem. The TPE effect can be measured in a model independent way by measuring the ratio of positron-proton to electron-proton elastic scattering cross-sections. We produced a simultaneously mixed electron-positron beam in the engineering test run computed in October 2006 and measured the e +p/e- p ratio using the CEBAF large acceptance spectrometer (CLAS). Due to the luminosity constraint our kinematic coverage is limited to low Q² and high epsilon (longitudinal polarization of the virtual photon). We continued our background study through GEANT4 simulation developed for the test run design in order to find more background sources and to design required shielding. The simulation is validated by using the test run data and is used further to optimize the luminosity for the final experiment. We are able to increase the luminosity by an order of magnitude for the upcoming final run. The final experiment will extend the data in high Q² and low epsilon region where TPE effect is expected to be large.