Speaker
Description
The energy density functional for dense nuclear matter can be presented as the sum of two parts. The isospin-symmetric part determines the equation of state of symmetric nuclear matter. The isospin-asymmetric part is commonly considered to be proportional to the squared isospin asymmetry, with the proportionality coefficient referred to as the nuclear symmetry energy. There are two ways to probe nuclear matter properties at densities well above the saturation point. Observations of neutron star properties such as mass and radius constrain the beta equilibrium equation of state of superdense matter, while laboratory experiments on heavy-ion collisions provide the equation of state of symmetric nuclear matter. Neither of these approaches alone allows one to determine the nuclear symmetry energy at high densities. However, by combining them as input for the inverse beta-equilibrium problem, one can infer the symmetry energy as a function of baryon number density in a model-independent way. This idea was proposed several years ago [1] for minimum-composition (npe) nuclear matter. In this work, we extend it to the realistic npe\mu composition of the neutron star core. Using renowned heavy-ion collision data [2] and state-of-the-art astrophysical observations [3], we reconstruct the symmetry energy—baryon density relation with remarkable accuracy. The resulting inferences on particle fractions strongly support the operation of the direct Urca neutrino emission process in neutron stars.
[1] Li B.-A., Xie W.-J., 2020, PhLB, 806, 135517
[2] Danielewicz P., Lacey R., Lynch W. G., 2002, Sci, 298, 1592
[3] Rutherford N. et al., 2024, ApJL, 971, L19
