Particle physics experiments

Combining heavy ion experiments and nuclear theory

Artist’s rendering showing the simulation of the merger of two neutron stars (left) and the traces of emerging particles visible during a collision of heavy ions (right) which creates matter under similar conditions in the laboratory . Credit: Tim Dietrich, Arnaud Le Fevre, Kees Huyser; background: ESA/Hubble, Sloan Digital Sky Survey

Combination of heavy ion experiments, astrophysical observations and nuclear theory.

When a massive star explodes as a supernova, if it is not completely destroyed, it will leave behind either a black hole or a neutron star. These enigmatic cosmic objects are particularly mysterious because of the overwhelming internal pressures resulting from the incredible density of neutron stars and the bewildering properties of the nuclear matter they are made of.

Today, an international team of researchers has for the first time combined data from heavy ion experiments, gravitational wave measurements and other astronomical observations using advanced theoretical modeling to constrain more precisely the properties of nuclear matter as it can be found inside neutron stars. The results were published on June 8, 2022 in the journal Nature.

Neutron stars form when a giant star runs out of fuel and collapses. They are among the densest objects in the cosmos, with a single cube-sized piece weighing 1 billion tonnes (1 trillion kg).

Throughout the Universe, neutron stars are born in supernova explosions that mark the end of the life of massive stars. Sometimes neutron stars are linked in binary systems and will eventually collide with each other. These high-energy astrophysical phenomena exhibit such extreme conditions that they produce most of the heavy elements, such as silver and gold. Therefore, neutron stars and their collisions are unique laboratories for studying the properties of matter at densities far beyond the densities inside atomic nuclei. Heavy ion collision experiments conducted with particle accelerators are a complementary means of producing and probing matter at high density and under extreme conditions.

New insights into the fundamental interactions at play in nuclear matter

“Combining insights from nuclear theory, nuclear experiments and astrophysical observations is essential to shed light on the properties of neutron-rich matter across the full density range probed in neutron stars,” said Sabrina Huth, from the Institute of Nuclear Physics of the Technical University of Darmstadt, which is one of the main authors of the publication. Peter TH Pang, another lead author from the Institute of Gravitational and Subatomic Physics (GRASP) at Utrecht University, added: “We find that the stresses from gold ion collisions with particle accelerators show a remarkable consistency with astrophysical observations, even if they are obtained with completely different methods.

Artistic representation of the neutron star

Artist’s rendering of a neutron star. Credit: ESO / L. Calçada

Recent advances in multi-messenger astronomy have provided the international research team, made up of researchers from Germany, the Netherlands, the United States and Sweden, with new insights into fundamental interactions involved in nuclear matter. In an interdisciplinary effort, the researchers included the information obtained during heavy ion collisions in a framework combining astronomical observations of electromagnetic signals, measurements of gravitational waves, and powerful astrophysical calculations with theoretical nuclear physics calculations. Their systematic study combines all of these individual disciplines for the first time, indicating higher pressure at intermediate densities in neutron stars.

Heavy ion collision data included

The authors incorporated information from gold ion collision experiments performed at GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt as well as Brookhaven National Laboratory and Lawrence Berkeley National Laboratory in the United States into their multi-step procedure which analyzes the stresses of nuclear theory and astrophysical observations, including neutron star mass measurements through radio observations, information from the Neutron Star Interior Composition Explorer (NICER) mission on the International Space Station (ISS), and multi-messenger observations of binary neutron star mergers.

Nuclear theorists Sabrina Huth and Achim Schwenk of the Technical University of Darmstadt and Ingo Tews of Los Alamos National Laboratory have played a key role in translating information obtained from heavy ion collisions into matter of neutron stars , necessary to integrate the astrophysical constraints.

The inclusion of heavy ion collision data in the analyzes added constraints in the density region where nuclear theory and astrophysical observations are less sensitive. This helped provide a more complete understanding of dense matter. In the future, improved constraints of heavy ion collisions may play an important role in linking nuclear theory and astrophysical observations by providing complementary information. In particular, experiments that probe higher densities while reducing experimental uncertainties have great potential to provide new constraints for the properties of neutron stars. New information from either side can easily be included in the framework to further enhance the understanding of dense matter in years to come.

Reference: “Constraining neutron star matter with microscopic and macroscopic collisions” by Sabrina Huth, Peter TH Pang, Ingo Tews, Tim Dietrich, Arnaud Le Fèvre, Achim Schwenk, Wolfgang Trautmann, Kshitij Agarwal, Mattia Bulla, Michael W. Coughlin and Chris Van Den Broeck, June 8, 2022, Nature.
DOI: 10.1038/s41586-022-04750-w