Measurement of Elastic Electron Scattering Transverse Asymmetries on ¹H, ¹²C and ²⁷Al

We report measurements of the parity-conserving beam-normal single-spin elastic scattering asymmetries B_n on ¹H, ¹²C and ²⁷Al, obtained with an electron beam polarized transverse to its momentum direction. These measurements add an additional kinematic point to a series of previous measurements of B_n on ¹H and ¹²C, and provide a first measurement on ²⁷Al. The experiment utilized the Qweak apparatus at Jefferson Lab with a beam energy of ~1.15 GeV. The average laboratory scattering angle was 7.8°, and the average Q² was 0.024 GeV² (Q = 0.156 GeV). The results are consistent with theoretical predictions, and are compared to existing data. The ¹H result is the most precise measurement of B_n to date and therefore provides a stringent test of two-photon exchange models at far-forward scattering angles (θ_lab→0) where they should be most reliable. When scaled by Z/A, the Q dependence of all the far-forward angle (q <10°) data from ¹H to ²⁷Al can be described by the same slope out to Q = 0.35 GeV. Larger-angle data from other experiments in the same Q range are consistent with a slope about twice as steep.