New Measurements of the Beam-Normal Single Spin Asymmetry in Elastic Electron Scattering Over a Range of Spin-0 Nuclei

TL;DR

This paper presents precise measurements of beam-normal single spin asymmetries in elastic electron scattering off various nuclei, confirming theoretical predictions for lighter nuclei and revealing unexpected results for lead, indicating complex underlying nuclear dynamics.

Contribution

First measurements of $A_n$ for $^{40}$Ca and $^{48}$Ca, validating theoretical models for nuclei with Z ≤ 20, and extending measurements for $^{208}$Pb to new momentum transfers.

Findings

$A_n$ for $^{40}$Ca and $^{48}$Ca similar to $^{12}$C, consistent with theory.

$A_n$ for $^{208}$Pb shows significant disagreement with lighter nuclei results.

Data suggests complex nuclear effects in heavy nuclei like lead.

Abstract

We report precision determinations of the beam normal single spin asymmetries ($A_n$) in the elastic scattering of 0.95 and 2.18~GeV electrons off $^{12}$C, $^{40}$Ca, $^{48}$Ca, and $^{208}$Pb at very forward angles where the most detailed theoretical calculations have been performed. The first measurements of $A_n$ for $^{40}$Ca and $^{48}$Ca are found to be similar to that of $^{12}$C, consistent with expectations thus demonstrating the validity of theoretical calculations for nuclei with Z~$\leq20$. We also report $A_n$ for $^{208}$Pb at two new momentum transfers (Q$^2$) extending the previous measurement. Our new data confirm the surprising result previously reported, with all three data points showing significant disagreement with the results from the $Z\leq 20$ nuclei. These data confirm our basic understanding of the underlying dynamics that govern $A_n$ for nuclei containing $\lesssim 50$ nucleons, but point to the need for further investigation to understand the unusual $A_n$ behaviour discovered for scattering off $^{208}$Pb.