Beam-Normal Single-Spin Asymmetry in ${}^{208}$Pb at low energy: discrepancy resolved or new kinematic puzzle?

Abstract

A longstanding discrepancy between measured and predicted beam-normal single-spin asymmetries $A_n$ in elastic electron scattering off ${}^{208}$Pb has challenged our understanding of two-photon exchange (TPE) in heavy nuclei. We report a new measurement at 570 MeV and $Q^2$=0.04 $Ge{V}^2$/$c^2$, yielding$A_n=(-9.1\pm 2.1\text{(stat)}\pm 0.7\text{(syst)})\mathrm{ppm}$. This nonzero value contrasts with previous results at higher energies and suggests a kinematic dependence of TPE effects not captured by current theory, prompting a reevaluation of earlier interpretations.