First Determination of the 27Al Neutron Distribution Radius from a Parity-Violating Electron Scattering Measurement
QWeak Collaboration: D. Androic🇭🇷 · D.S. Armstrong🇺🇸 · K. Bartlett🇺🇸 · R.S. Beminiwattha🇺🇸 · J. Benesch🇺🇸 · F. Benmokhtar🇺🇸 · J. Birchall🇨🇦 · R.D. Carlini🇺🇸 · J.C. Cornejo🇺🇸 · S. Covrig Dusa🇺🇸 · M.M. Dalton🇺🇸 · C.A. Davis🇨🇦 and 81 other authors
We report the first measurement of the parity-violating elastic electron scattering asymmetry on 27Al. The 27Al elastic asymmetry is A_PV = 2.16 +- 0.11 (stat) +- 0.16 (syst) ppm, and was measured at <Q^2> =0.02357 +- 0.0001 GeV^2, <theta_lab> = 7.61 +- 0.02 degrees, and <E_lab> = 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 27Al 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 27Al 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.