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

TL;DR

This study measures the beam-normal single-spin asymmetry in elastic electron scattering off lead-208 at low energy, revealing a nonzero asymmetry that challenges existing theoretical models and suggests a kinematic dependence of two-photon exchange effects.

Contribution

The paper provides the first low-energy measurement of $A_n$ in $^{208}$Pb, highlighting discrepancies with prior high-energy results and prompting a reevaluation of TPE effects in heavy nuclei.

Findings

Measured $A_n$ at 570 MeV and $Q^2$=0.04 GeV$^2$/c$^2$ as -9.1 ppm

Contrasts with previous higher-energy results, indicating kinematic dependence

Suggests current theories do not fully capture TPE effects in heavy nuclei

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 $GeV^2$/$c^2$, 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.