UC scientists help advance study of nuclear matter in neutron star mergers

A team of researchers from the Department of Physics of the Faculty of Sciences and Technology of the University of Coimbra (FCTUC), in collaboration with the GANIL laboratory in France, has succeeded in identifying the properties of inhomogeneous matter similar to that which forms in supernovae or neutron star mergers: low-density nuclear matter composed of protons, neutrons and light nuclei, such as hydrogen and helium isotopes.

Understanding the equation of state of nuclear matter under extreme conditions of temperature, density, and proton fraction is essential for interpreting observations of neutron stars, supernova explosions, and neutron star mergers. However, recreating these conditions in the laboratory is not always feasible.

Using an innovative approach to analyse data from particles resulting from heavy-ion collisions of xenon with tin at intermediate energies—measured with the INDRA@GANIL detector—Tiago Custódio, a PhD student at FCTUC, has described their abundances without making any prior assumptions about the density of the system. He used a theoretical model developed by Constança Providência and Helena Pais, researchers at the Centre for Physics of the University of Coimbra (CFisUC).

"We analysed thirteen velocity bins, representing particles that decouple from the hot source at different times. In each bin," explains Constança Providência, professor at FCTUC, "the temperature and density were determined using a comprehensive Bayesian analysis developed by the student in collaboration with Tuhin Malik, a researcher at CFisUC.

The analysis was based on a microscopic thermal model grounded in the relativistic mean-field theory for nuclear matter, considering medium effects essential for an accurate description. “The results showed an excellent match with experimental data using two distinct nuclear models. The statistical sets revealed a cooling curve at an almost constant density, interpreted as a freeze-out density below which particles stop interacting via nuclear forces,” says Tiago Custódio.

As the researchers point out, previous analyses have always assumed an ideal gas - or modified ideal gas - of free nucleons and light nuclei to estimate the system density, making the density associated with each bin a direct consequence of this initial assumption. However, the results obtained allow us to construct a realistic equation of state that accurately describes light nuclei in low-density nuclear matter. “These equations of state are a key contribution to more realistic simulations of neutron star mergers, bringing new advances to the study of nuclear astrophysics,” the team concludes.

The scientific article Calibrating the Medium Effects of Light Clusters in Heavy-Ion Collisions, presenting these results, has been published in Physical Review Letters and is available here.