Determination of the Al-27 Neutron Distribution Radius from a Parity-Violating Electron Scattering Measurement

We report the first measurement of the parity-violating elastic electron scattering asymmetry on Al-27. The Al-27 elastic asymmetry is A(PV) = 2.16 +/- 0.11(stat) +/- 0.16(syst) ppm, and was measured at < Q(2)> = 0.02357 +/- 0.00010 GeV2, = 7.61 degrees +/- 0.02 degrees, and < E-lab > = 1.157 GeV with the Q(weak) apparatus at Jefferson Lab. Predictions using a simple Born approximation as well as more sophisticated distorted-wave calculations are in good agreement with this result. From this asymmetry the Al-27 neutron radius R-n = 2.89 +/- 0.12 fm was determined using a many-models correlation technique. The corresponding neutron skin thickness R-n - R-p = -0.04 +/- 0.12 fm is small, as expected for a light nucleus with a neutron excess of only 1. This result thus serves as a successful benchmark for electroweak determinations of neutron radii on heavier nuclei. A tree-level approach was used to extract the Al-27 weak radius R-w = 3.00 +/- 0.15 fm, and the weak skin thickness R-wk - R-ch = -0.04 +/- 0.15 fm. The weak form factor at this Q(2) is F-wk = 0.39 +/- 0.04.