Beam-normal single-spin asymmetry in elastic scattering of electrons from a spin-0 nucleus

We study the beam-normal single-spin asymmetry (BNSSA) in high-energy elastic electron scattering from several spin-0 nuclei. Existing theoretical approaches work in the plane-wave formalism and predict the BNSSA to scale as similar to A/Z with the atomic number Z and nuclear mass number A. While this prediction holds for light and intermediate nuclei, a striking disagreement in both the sign and the magnitude of BNSSA was observed by the PREX collaboration for Pb-208, coined the PREX puzzle. To shed light on this disagreement, we go beyond the plane-wave approach which neglects Coulomb distortions known to be significant for heavy nuclei. We explicitly investigate the dependence of BNSSA on A and Z by (i) including inelastic intermediate states' contributions into the Coulomb problem in the form of an optical potential, (ii) by accounting for the experimental information on the A-dependence of the Compton slope parameter, and (iii) giving a thorough account of the uncertainties of the calculation. Despite of these improvements, the PREX puzzle remains unexplained. We discuss further strategies to resolve this riddle.

Automated summary

The study reveals that the prediction of beam-normal single-spin asymmetry scales with the atomic number and nuclear mass number for light and intermediate nuclei, but a significant discrepancy is observed for Pb-208. Despite attempts to improve the theoretical approach, such as considering Coulomb distortions and experimental information, the PREX puzzle remains unexplained, requiring further investigation and discussion.