https://doi.org/10.1140/epja/s10050-022-00682-1
Regular Article - Experimental Physics
Two photon exchange with nuclei from 
elastic cross section ratios
1
Massachusetts Institute of Technology, 02139, Cambridge, MA, USA
2
George Washington University, 20052, Washington, DC, USA
Received:
1
March
2021
Accepted:
8
February
2022
Published online:
28
February
2022
Hadronic box diagrams pose a significant source of radiative corrections to lepton scattering and 
-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 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 ratio exist only for the proton. Proposed here is a measurement of the ratio on 
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 
GeV
, would be made possible by the addition of a positron source at Jefferson Lab.
© The Author(s) 2022
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