| Summary: | Abstract Hadronic box diagrams pose a significant source of radiative corrections to lepton scattering and $$\beta $$ β -decay measurements, yet their calculation remains highly model-dependent. Two-photon exchange is a box diagram generally relevant to most lepton scattering experiments, and is directly detectable via multiple observables. Single-spin asymmetries (SSA) are sensitive to the imaginary part of the TPE amplitude, while the ratio of $$e^+/e^-$$ e + / e - elastic scattering cross sections is sensitive to the real part of the TPE amplitude. While SSA measurements have been reported for several complex nuclei, measurements of the $$e^+/e^-$$ e + / e - ratio exist only for the proton. Proposed here is a measurement of the $$e^+/e^-$$ e + / e - ratio on $$^4$$ 4 He, a feasible nuclear target for which the ratio's deviation from unity due to TPE is expected to be on order of 1-2%. Choosing a low Z nucleus minimizes Coulomb distortions, which could greatly overwhelm TPE for heavier nuclei. The proposed 19-day program, including measurements at three kinematic settings between $$0.3< Q^2 < 0.4$$ 0.3 < Q 2 < 0.4 GeV $$^2$$ 2 , would be made possible by the addition of a positron source at Jefferson Lab.
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