Incorporating the weak mixing angle dependence to reconcile the neutron skin measurement on Pb208 by PREX-II

Abstract

The only available electroweak measurement of the $^{208}\mathrm{Pb}$ neutron skin $\mathrm{\ensuremath{\Delta}}{R}_{\mathrm{np}}$, performed by the PREX-II Collaboration through polarized electron-lead scattering, shows a mild tension with respect to both the theoretical nuclear-model predictions and a host of measurements. However, the dependence on the weak mixing angle should be incorporated in the calculation, since its low-energy value is experimentally poorly known. We first repeat the PREX-II analysis confirming their measurement by fixing the weak mixing angle to its standard model value. Then, we show the explicit dependence of the PREX-II measurement on the weak mixing angle, obtaining that it is fully degenerate with the neutron skin. To break this degeneracy, we exploit the weak mixing angle measurement from atomic parity violation on lead, obtaining a slightly thinner neutron skin but with about doubled uncertainties, possibly easing the PREX tension. Relying on the theoretical prediction, $\mathrm{\ensuremath{\Delta}}{R}_{\mathrm{np}}^{\mathrm{th}}\ensuremath{\approx}0.13\text{--}0.19\phantom{\rule{4pt}{0ex}}\mathrm{fm}$, and using it as a prior in the fit, we find a weak mixing angle value about $1.2\ensuremath{\sigma}$ smaller than the standard model prediction. Thus, we suggest a possible solution of the PREX-II tension by showing that, considering its underlying dependence on the weak mixing angle, the PREX-II neutron skin determination could be in agreement with the other available measurements and predictions if the weak mixing angle at the proper energy scale is smaller than the standard model prediction.