This talk will describe how to make intense identical positron and electron beams at Jefferson Lab and use them to measure two-photon exchange contributions to elastic electron-proton scattering. It will cover the results of a recent test run in CLAS where we produced about 10 pA each of electrons and positrons.
The proton electric form factor describes the charge distribution of the proton. This has been measured extensively with electron scattering using Rosenbluth separations and polarization measurements. These measurements of the proton electric form factor disagree by a factor of three at Q 2 = 6 GeV 2 . Since alpha, the fine structure constant, is less than 1%, electron-proton scattering should be almost exclusively one-photon exchange. However, a two-photon exchange contribution of about 5% could explain the discrepancy between the measurements. We will determine the two-photon exchange contribution by measuring the ratio of the electron-proton and positron-proton elastic scattering cross sections to 1%.