PREX, CREX, and nuclear models: the plot thickens

In a recent Physical Review Letter (PRL) paper, a team of theorists from the University of Erlangen–Nuremberg in Germany, the University of Milan in Italy, and FRIB extended their previous critical analysis of the Lead Radius Experiment (PREX) at the Thomas Jefferson National Accelerator Facility (Jefferson Lab). The experiment involved deducing the neutron size of a lead atom’s core, or nucleus, through a measurement of a tiny left-right asymmetry in electron scattering off lead-208. In a recent experiment (CREX) at Jefferson Lab, the asymmetry measurement was carried out for calcium-48.

To better understand the impact of these measurements, the theoretical team extended their analysis to both PREX and CREX. The study was complemented by inspecting another key observable: the static electric dipole polarizability that is theoretically expected to correlate with the neutron skin.

The team found a tension between the CREX and PREX results and predictions of global nuclear models. This result (shown in the figure) calls for a critical search of limitations of current nuclear models and/or possible other sources of uncertainty in experiment.

Witek Nazarewicz, John A. Hannah Distinguished Professor of Physics and chief scientist at FRIB, is one of the authors of the PRL paper.