We report precision determinations of the beam-normal single spin asymmetries (An) in the elastic scattering of 0.95 and 2.18 GeV electrons off C12, Ca40, Ca48, and Pb208 at very forward angles where the most detailed theoretical calculations have been performed. The first measurements of An for Ca40 and Ca48 are found to be similar to that of C12, consistent with expectations and thus demonstrating the validity of theoretical calculations for nuclei with Z≤20. We also report An for Pb208 at two new momentum transfers (Q2) 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≤20 nuclei. These data confirm our basic understanding of the underlying dynamics that govern An for nuclei containing 50 nucleons, but point to the need for further investigation to understand the unusual An behavior discovered for scattering off Pb208.

Original languageEnglish
Article number142501
JournalPhysical Review Letters
Issue number14
StatePublished - 8 Apr 2022
Externally publishedYes

Bibliographical note

Publisher Copyright:
© 2022 American Physical Society.


We thank the entire staff of JLab for their efforts to develop and maintain the polarized beam and the experimental apparatus, and acknowledge the support of the U.S. Department of Energy, the National Science Foundation, and NSERC (Canada). This material is based upon the work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics Contract No. DE-AC05-06OR23177.

FundersFunder number
National Science Foundation
U.S. Department of Energy
Office of Science
Nuclear PhysicsDE-AC05-06OR23177
Natural Sciences and Engineering Research Council of Canada


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