Neutron Skins of Nuclei

What are the properties of nuclear matter which lead to the spectacular phenomena of supernovae explosions? What is the nature of neutron stars? How does subatomic matter organize itself? These and other fundamental questions about the nature of some of the most fascinating astrophysical objects as well as the characteristics of dense nuclear matter are determined by the nuclear equation of state (EOS). A key unknown of the EOS is the density dependence of the symmetry energy, which quantifies the change in nuclear energy associated with modifying the neutron-proton asymmetry. An accurate determination of the thickness of the neutron skin of a nucleus would provide a unique experimental constraint on the symmetry energy and ultimately on some critical properties of supernovae and neutron stars.

Recent developments in Effective Field Theory (EFT) for nuclear forces enable controlled calculations of matter at nuclear densities. This advance has opened up a new era in the study of strongly interacting nuclear matter. However, these theoretical approaches need guidance from experiment.

The Lead Radius EXperiment (PREX) has for the first time established the existence of a neutron skin in a clean and model-independent way at the 95% confidence level. The commissioning of new experimental programs on neutron skins of both stable and exotic beams, at JLab, MESA, FAIR and FRIB, makes urgent the need to bring together all stakeholders to review the recent progress, critically examine the new data, and suggest new directions in order to find common answers to some fundamental questions, both from the experimental and the theoretical side.