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 Al27. The Al27 elastic asymmetry is APV=2.16±0.11(stat)±0.16(syst) ppm, and was measured at «Q2»=0.02357±0.00010 GeV2, «θlab»=7.61°±0.02°, and «Elab»=1.157 GeV with the Qweak 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 Al27 neutron radius Rn=2.89±0.12 fm was determined using a many-models correlation technique. The corresponding neutron skin thickness Rn-Rp=-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 Al27 weak radius Rw=3.00±0.15 fm, and the weak skin thickness Rwk-Rch=-0.04±0.15 fm. The weak form factor at this Q2 is Fwk=0.39±0.04.