This paper presents a new theoretical way to describe a subtle effect in elastic electron scattering on nuclei. The authors include a two-photon-exchange mechanism. That mechanism can produce a small difference in scattering when the incoming electron beam is polarized perpendicular to the scattering plane. This difference is called the beam normal-spin asymmetry, and it conserves parity (it is not a parity-violating effect).

To study this, the researchers built a model for the two-photon-exchange amplitude at small scattering angles. They treat the amplitude in a diffractive framework, a description that emphasizes wave-like diffraction at small momentum transfer. The approach is similar to methods used for pion-nucleus elastic scattering. The paper gives calculations for three nuclei: carbon-12 (12C), calcium-40 (40Ca) and lead-208 (208Pb).

The calculations produce specific predictions for how the beam polarization asymmetry depends on the scattering kinematics for these nuclei. In plain terms, the model says how the asymmetry should change with angle and energy in the small-angle, diffractive regime. The two-photon-exchange contribution is the culprit behind these parity-conserving single-spin asymmetries in the authors’ picture.

Why this matters: recent measurements at Jefferson Lab show results at finite scattering angles that do not obviously match expectations in the extreme forward direction. The forward limit is constrained by the optical theorem, a general relation that links the forward scattering amplitude to the total cross section. The authors point out that their diffractive two-photon-exchange predictions may reconcile the finite-angle data with that forward limit. They also suggest this could potentially resolve the so-called “PREX Puzzle” for lead-208.