Parity-violating electron scattering and the neutron distribution in ²⁷Al.

Parity-violating elastic electron scattering can be used as a probe of the neutron distributions in nuclei, because the weak charge of the neutron is much larger than that of the proton. The electroweak response of the nucleus is then dominated by the neutrons, and thus the radius of the neutron distribution can be extracted from a measurement of the parity-violating asymmetry. The PREX-2 collaboration has used this technique to recently report [Phys. Rev. Lett. 126, 172502 (2021)] an essentially model-independent determination of the neutron skin in ²⁰⁸Pb. Here we report final results from the Qweak experiment for the parity-violating asymmetry from electron scattering on ²⁷Al, accounting for contributions from non-elastic processes, and discuss the implications for the neutron distribution radius in ²⁷Al. This represents only the second extraction of a neutron radius from parity violation, and provides a valuable check of both theory and experiment in a case where we expect the neutron and proton distributions to have similar radii.