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 B-n on C-12 and Al-27, 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 C-12 and provide a first measurement on Al-27. The experiment utilized the Q(weak) apparatus at Jefferson Lab with a beam energy of 1.158 GeV. The average laboratory scattering angle for both targets was 7.7 degrees, and the average Q(2) for both targets was 0.024 37 GeV2 (Q = 0.1561 GeV). The asymmetries are B-n = -10.68 +/- 0.90 (stat) +/- 0.57 (syst) ppm for C-12 and B-n = -12.16 +/- 0.58 (stat) +/- 0.62 (syst) ppm for Al-27. 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 (theta < 10 degrees) data from H-1 to Al-27 can be described by the same slope out to Q approximate to 0.35 GeV. Larger-angle data from other experiments in the same Q range are consistent with a slope about twice as steep.