Precision Neutron Skins of ${}^{208}$Pb and ${}^{48}$Ca from Parity-Violating Electron Scattering

Abstract

We have measured the parity-violating elastic electron scattering asymmetry in the PREX and CREX experiments on ${}^{208}$Pb and ${}^{48}$Ca respectively; these are both doubly-magic nuclei whose excited states can be discriminated from the ground state by the high resolution spectrometers in Hall A at Jefferson Lab. This asymmetry provides a precise determination of the weak charge form factor at one $Q^2$ and pins down the neutron radius in these two nuclei in a relatively clean and model-independent way. This is because the $Z^0$ boson of the weak interaction couples primarily to neutrons. The heavier lead nucleus, with a neutron excess, provides an interpretation of the neutron skin thickness in terms of properties of bulk neutron matter. For the lighter ${}^{48}$Ca nucleus, which is also rich in neutrons, comparisons to microscopic nuclear theory calculations are sensitive to poorly constrained 3-neutron forces. The weak neutral form factors $F_W(Q^2)$ were extracted to be $0.368\pm 0.013$ at $Q=0.3977{\mathrm{fm}}^{-1}$ for ${}^{208}$Pb from PREX-2 and $0.1304\pm 0.0055$ at $Q=0.8733{\mathrm{fm}}^{-1}$ for ${}^{48}$Ca. The form factor differences $(F_{ch}-F_W)(Q^2)$ were calculated to be $0.041\pm 0.013$ at $Q=0.3977{\mathrm{fm}}^{-1}$ for ${}^{208}$Pb from PREX-2 and $0.0277\pm 0.0055$ at $Q=0.8733{\mathrm{fm}}^{-1}$ for ${}^{48}$Ca. Correcting for Coulomb distortions and using nuclear model information, we find the neutron skin thicknesses to be $R_{\mathrm{skin}}^{208}=0.283\pm 0.071$ fm combining PREX-1 and PREX-2 and $R_{\mathrm{skin}}^{48}=0.121\pm 0.035$ fm. This paper provides a full description of the special experimental and data analysis techniques employed for precisely measuring these small asymmetries.

51 pages, 43 figures