Measurement of the beam-normal single-spin asymmetry for elastic electron scattering from C 12 and Al 27

We report measurements of the parity-conserving beam-normal single-spin elastic scattering asymmetries Bn on C12 and Al27, 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 Bn on C12 and provide a first measurement on Al27. The experiment utilized the Qweak apparatus at Jefferson Lab with a beam energy of 1.158 GeV. The average laboratory scattering angle for both targets was 7.7∘, and the average Q2 for both targets was 0.024 37 GeV2 (Q=0.1561 GeV). The asymmetries are Bn=-10.68±0.90(stat)±0.57(syst) ppm for C12 and Bn=-12.16±0.58(stat)±0.62(syst) ppm for Al27. The results are consistent with theoretical predictions, and are compared to existing data. When scaled by Z/A, the Q dependence of all the far-forward angle (θ\u3c10∘) data from H1 to Al27 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