Neutron-proton effective mass splitting in neutron-rich matter and its impacts on nuclear reactions

TL;DR

This paper reviews the neutron-proton effective mass splitting in neutron-rich matter, its theoretical calculations, experimental constraints, and implications for nuclear reactions and astrophysical phenomena.

Contribution

It synthesizes recent progress in understanding the neutron-proton effective mass splitting, highlighting its theoretical foundations, experimental constraints, and impact on nuclear reaction dynamics.

Findings

Optical model analyses provide reliable data at saturation density.

Several heavy-ion collision observables are sensitive to mass splitting.

Progress has been made in calculating the splitting using many-body theories.

Abstract

The neutron-proton effective mass splitting in neutron-rich nucleonic matter reflects the space-time nonlocality of the isovector nuclear interaction. It affects the neutron/proton ratio during the earlier evolution of the Universe, cooling of protoneutron stars, structure of rare isotopes and dynamics of heavy-ion collisions. While there is still no consensus on whether the neutron-proton effective mass splitting is negative, zero or positive and how it depends on the density as well as the isospin-asymmetry of the medium, significant progress has been made in recent yeas in addressing these issues. We first recall the connections among the neutron-proton effective mass splitting, the momentum dependence of the isovector potential and the density dependence of the symmetry energy. We then make a few observations about the progress in calculating the neutron-proton effective mass splitting using various nuclear many-body theories and its effects on the isospin-dependence of in-medium nucleon-nucleon cross sections. Perhaps, our most reliable knowledge so far about the neutron-proton effective mass splitting at saturation density of nuclear matter comes from optical model analyses of huge sets of nucleon-nucleus scattering data accumulated over the last five decades. The momentum dependence of the symmetry potential from these analyses provide a useful boundary condition at saturation density for calibrating nuclear many-body calculations. Several observables in heavy-ion collisions have been identified as sensitive probes of the neutron-proton effective mass splitting in dense neutron-rich matter based on transport model simulations. We review these observables and comment on the latest experimental findings.