Preliminary calculations for a two photon exchange experiment with electrons at Jefferson Lab*

Decades of elastic electron scattering experiments on proton targets have helped to map out the proton’s electromagnetic form factors, which encode the spatial distributions of its electric charge and magnetism. Currently, there remains an unresolved discrepancy between measurements made using polarization transfer method and those made using the Rosenbluth separation technique. One possible explanation is that two-photon exchange, an effect previously assumed to be negligible, is biasing the two techniques differently. To isolate the effect of two-photon exchange, we intend to compare the transverse single-spin asymmetry for electrons and positrons scattering off a polarized proton target in Hall B at Jefferson Lab. A non-zero asymmetry that changes sign between electrons and positrons would be strong evidence for  two photon exchange. Using a Monte Carlo simulation to account for the CLAS detector acceptance, the shift of particle trajectories due to the target holding field, and factoring in uncertainties from target dilution and detector resolution we are able to estimate the required beamtime to measure the asymmetry with uncertainty sufficient to put stringent constraints on two photon exchange.