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

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~$\le 20$. 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 $Z\le 20$ 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.

7 pages, 2 figures