New Measurements of the Transverse Beam Asymmetry for Elastic Electron Scattering from Selected Nuclei

We have measured the beam-normal single-spin asymmetry $A_n$ in the elastic scattering of 1–3 GeV transversely polarized electrons from ${}^1\mathrm{H}$ and for the first time from ${}^4\mathrm{He}$, ${}^{12}\mathrm{C}$, and ${}^{208}\mathrm{Pb}$. For ${}^1\mathrm{H}$, ${}^4\mathrm{He}$, and ${}^{12}\mathrm{C}$, the measurements are in agreement with calculations that relate $A_n$ to the imaginary part of the two-photon exchange amplitude including inelastic intermediate states. Surprisingly, the ${}^{208}\mathrm{Pb}$ result is significantly smaller than the corresponding prediction using the same formalism. These results suggest that a systematic set of new $A_n$ measurements might emerge as a new and sensitive probe of the structure of heavy nuclei.

Figure 1

Plots of the asymmetries for carbon and lead. The top row shows for the ${}^{12}\mathrm{C}$ target the left high resolution spectrometer (HRS) and right HRS asymmetries in the left and right panels, respectively. The bottom row shows the same sequence for the ${}^{208}\mathrm{Pb}$ target. The data have been sign-corrected for the $\lambda /2$ plate insertions.

Figure 2

Extracted physics asymmetries $A_n$ vs $Q$. Each curve, specific to a particular nucleus as indicated, is a theoretical calculation from Ref. [19].

Figure 3

Percent fractional deviation of $A_n$ measurements normalized to the respective theory prediction vs target nucleus $Z$.