Constraining the properties of neutron star matter

Combining heavy-ion experiments, astrophysical observations, and nuclear theory.

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Sometimes neutron stars collide with each other, producing most of the heavy elements, such as silver and gold. Therefore, neutron stars and their collisions are considered unique laboratories to study the properties of matter at densities far beyond the densities inside atomic nuclei.

Heavy-ion collision experiments conducted with particle accelerators are a complementary way to produce and probe matter at high densities and under extreme conditions.

Recent progress in multi-messenger astronomy allowed the international research team, involving scientists from Germany, the Netherlands, the US, and Sweden, to gain new insights into the fundamental interactions in nuclear matter. By combining data from heavy-ion experiments, gravitational-wave measurements, and other astronomical observations, scientists constrain nuclear matter’s properties found in the interior of neutron stars.

Sabrina Huth, Institute for Nuclear Physics at Technical University Darmstadt, said“Combining knowledge from nuclear theory, nuclear experiment, and astrophysical observations is essential to shedding light on the properties of neutron-rich matter over the entire density range probed in neutron stars. We find that constraints from collisions of gold ions with particle accelerators show a remarkable consistency with astrophysical observations even though they are obtained with completely different methods.”

In this study, scientists included information obtained in heavy-ion collisions into a framework combining astronomical observations of electromagnetic signals, measurements of gravitational waves, and high-performance astrophysics computations with theoretical nuclear physics calculations.

The authors used data from gold-ion collision experiments at the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, as well as Brookhaven National Laboratory and Lawrence Berkeley National Laboratory in the United States, in their multi-step procedure that analyses constraints from nuclear theory and astrophysical observations, such as neutron star mass measurements from radio observations and information from the Neutron Star Interior Composition.

Additional limitations in the density area where nuclear theory and astrophysical observations are less sensitive have been enabled by including data from heavy-ion collisions in the analysis. This has aided in developing a more comprehensive knowledge of the dense matter. Improved constraints from heavy-ion collisions should help bridge the gap between nuclear theory and astrophysical observations in the future by giving complementary data.

Experiments that investigate higher densities while lowering experimental uncertainties, in particular, offer a lot of promise for providing new constraints on neutron star features. In the next years, new knowledge from either side can be incorporated into the framework to increase our understanding of dense matter.

Journal Reference:

  1. Huth, S., Pang, P.T.H., Tews, I. et al. Constraining neutron-star matter with microscopic and macroscopic collisions. Nature 606, 276–280 (2022). DOI: 10.1038/s41586-022-04750-w