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

We report measurements of the parity-conserving beam-normal single-spin elastic scattering asymmetries B_nB n ​ on ^{; ; 12}; ; 12 C and ^{; ; 27}; ; 27 Al, 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_nB n ​ on ^{; ; 12}; ; 12 C and provide a first measurement on ^{; ; 27}; ; 27 Al. 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 Q^2Q 2 for both targets was 0.02437 GeV^2 2 (Q=0.1561 GeV). The asymmetries are B_nB n ​ = -10.68 \pm± 0.90 stat) \pm± 0.57 (syst) ppm for ^{; ; 12}; ; 12 C and B_nB n ​ = -12.16 \pm± 0.58 (stat) \pm± 0.62 (syst) ppm for ^{; ; 27}; ; 27 Al. 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 ^{; ; 1}; ; 1 H to ^{; ; 27}; ; 27 Al can be described by the same slope out to Q \approx 0.35Q≈0.35 GeV. Larger-angle data from other experiments in the same Q range are consistent with a slope about twice as steep.

single-spin asymmetry ; elastic electron scattering

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