Measurement of the Beam-Normal Single-Spin Asymmetry for Elastic Electron Scattering from 12C and 27Al
Abstract
We report measurements of the parity-conserving beam-normal single-spin elastic scattering asymmetries Bn on 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 Bn on 12C and provide a first measurement on 27Al. The experiment utilized the Qweak apparatus at Jefferson Lab with a beam energy of 1.158 GeV. The average lab scattering angle for both targets was 7.7 degrees, and the average Q2 for both targets was 0.02437 GeV2 (Q=0.1561 GeV). The asymmetries are Bn = -10.68 0.90 stat) 0.57 (syst) ppm for 12C and Bn = -12.16 0.58 (stat) 0.62 (syst) ppm for 27Al. 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 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.