  Two-photon exchange contributions have potentially broad ranging impact on
several charged lepton scattering measurements. Previously believed to be
extremely small, based in part on comparisons of positron scattering and
electron scattering in the 1950s and 1960s, recent data suggest that the
corrections may be larger than expected, in particular in kinematic regions
that were inaccessible in these early positron scattering measurements.
Additional measurements using positron beams at Jefferson Lab would allow for a
detailed investigation of these contributions in a range of reactions and
observables.
