Measurement of the two-photon exchange with quasi-elastic electron-neutron and positron-neutron scattering

The first measurements of elastic electron nucleon scattering with polarization transfer, allowing to separate the electric and magnetic nucleon form factors, showed a significant discrepancy with Rosenbluth measurement of the cross section of the same process. The two-photon exchange, suspected to be the cause of this observation, got a major focus since then. The two-photon exchange is expected to depend on the charge of the lepton, meaning that a Rosenbluth measurement on positron-nucleon scattering should clarify the role of the two photon exchange in the aforementioned discrepancy. We propose a measurement of the Rosenbluth slope in positron neutron scattering at Q2 = 3.0 (GeV/c)2 and Q2 = 4.5 (GeV/c)2. This measurement purports to complete and extend the measurement of the two-photon exchange in electron-neutron scattering submitted to and approved by PAC48 in 2020, and recorded in 2022 (experiment E12-20-010, currently under analysis). It would be performed with the Super BigBite Spectrometer installed in Hall C and will measure simultaneous positron-proton and positron-neutron scattering off deuterium. I will present the motivations and details of the proposed measurement, and provide a few aspects of the ongoing analysis of experiment E12-20-010 relevant to this new measurement.