  One of the most surprising discoveries made at Jefferson Lab has been the
discrepancy in the determinations of the proton's form factor ratio $\mu_p
G_E^p/G_M^p$ between unpolarized cross section measurements and the
polarization transfer technique. Over two decades later, the discrepancy not
only persists but has been confirmed at higher momentum transfers now
accessible in the 12-GeV era. The leading hypothesis for the cause of this
discrepancy, a non-negligible contribution from hard two-photon exchange, has
neither been conclusively proven or disproven. This state of uncertainty not
only clouds our knowledge of one-dimensional nucleon structure but also poses a
major concern for our field's efforts to map out the three-dimensional nuclear
structure. A better understanding of multi-photon exchange over a wide phase
space is needed. We propose making comprehensive measurements of two-photon
exchange over a wide range in momentum transfer and scattering angle using the
CLAS12 detector. Specifically, we will measure the ratio of positron-proton to
electron-proton elastic scattering cross sections, using the proposed positron
beam upgrade for CEBAF. The experiment will use 2.2, 4.4, and 6.6 GeV lepton
beams incident on the standard CLAS12 unpolarized hydrogen target. Data will be
collected by the CLAS12 detector in its standard configuration, except for a
modified trigger to allow the recording of events with beam leptons scattered
into the CLAS12 central detector. The sign of the beam charge, as well as the
polarity of the CLAS12 solenoid and toroid, will be reversed several times in
order to suppress systematics associated with local detector efficiency and
time-dependent detector performance. The proposed high-precision determination
of two-photon effects will be...

Nuclear Experiment