Among recent nuclear physics experiments, the measurements of the neutron skin thicknesses of Ca-48 (the CREX experiment) and Pb-208 (the PREX experiment) at Jefferson Laboratory have received the most attention. In nuclei that have many more neutrons than protons, the neutrons tend to be pushed outwards relative to the protons to form a 'neutron skin'. Since it costs energy (called the symmetry energy) to maintain matter with more neutrons than protons, the greater the neutron excess and the greater the symmetry energy, the more pressure matter exerts. Nuclear models assuming greater symmetry pressure predict nuclei with greater neutron skin thicknesses compared to other models, and, analogously, predict neutron stars with larger radii than other models. Interestingly, the CREX Ca-48 experiment predicts that the symmetry pressure is small, while the PREX Pb-208 experiment predicts the opposite. I will discuss how nuclear models that best fit both experimental results, however, turn out to be compatible with other nuclear experiments, the theory of neutron matter, and observations of neutron stars from radio, optical, X-ray and gravitational radiation. As a result, neutron stars are now understood better than ever before.
Prof. Lattimer, a Distinguished Professor in the Physics & Astronomy Department of Stony Brook University, is a long-time resident of East Setauket and a former Chairperson of the Earth and Space Sciences Department. He has received Sloan and Guggenheim Fellowships, is a Fellow of the American Physical Society, and has received their highest award in nuclear astrophysics, the Hans A. Bethe Prize. His outside interests include his grandchildren and ferroequinology.