When a star dies in a supernova, one possible outcome is for the remains to become a neutron star. Inside a neutron star, the protons and electrons combine into uncharged neutrons. This substance is called neutron matter.

A team of researchers from the United States, China, Turkey, and Germany has performed ab initio (i.e., from the most fundamental principles) simulations to calculate spin and density correlations in neutron matter. They used realistic nuclear interactions at higher densities of neutrons than previously explored. Spin and density are the probability of finding a neutron in a particular position with a particular direction of spin. These correlations determine key aspects of how neutrinos scatter and heat up in a core-collapse supernova.

The research is published in the journal Physical Review Letters.

Particle physicists have been looking for so-called “sterile neutrinos” for a few decades now. They are a hypothesized particle that would have a tiny mass like the three known neutrinos but would not interact by the weak force or any other Standard Model force, only through gravitational interactions.

Its existence—or their existence—would solve some anomalies seen in neutrino experiments, help answer questions beyond the Standard Model of particle physics, and, if massive enough, could explain cold dark matter or warm dark matter.

But sterile neutrinos have not been seen in any particle experiments, despite many attempts. Now an experiment by the IceCube Collaboration has used 10.7 years of data from their detector near the Amundsen-Scott South Pole Station to lower the probability that at least one sterile neutrino does not exist. Their paper appears in Physical Review Letters.

Researchers at UC Berkeley proposed that axions, hypothetical particles, could be detected shortly after a supernova’s gamma rays. They suggest that the Fermi Gamma-ray Space Telescope has a 1 in 10 chance of observing this phenomenon. Axions would be produced during the early moments of a star’s collapse and would then transform into high-energy gamma rays in the star’s magnetic field.

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