Parity-Violating and Parity-Conserving Asymmetries in Electron Scattering from ²⁷Al

The QWeak collaboration at Jefferson Lab has made the first measurements of parity-violating and parity-conserving single-spin asymmetries from the scattering of 1.16 GeV electrons from ²⁷Al. The parity-violating asymmetry is sensitive to the neutral weak form factor of ²⁷Al, which can be used to extract the radius of the neutron distribution in this nucleus. Our result is compared with a relativistic mean-field calculation by C.J. Horowitz, and provides a useful benchmark of the theoretical approach used to extract neutron skins from heavier nuclei (²⁰⁸Pb and ⁴⁸Ca) in the upcoming PREx-II and CREx experiments. The parity-conserving asymmetry arises from two-photon exchange diagrams, and our result is compared to a calculation by M. Gorchtein, which uses the optical theorem to model the inelastic intermediate state contributions using the total photo-absorption cross section.