St. Merging neutron stars are a treasure trove of new physical signals that have implications for determining the true nature of dark matter, according to research from Washington University in St. Louis.

On August 17, 2017, the Laser Interferometer Gravitational Wave Observatory (LIGO) in the United States and the Virgo detector in Italy detected gravitational waves resulting from the collision of two neutron stars. For the first time, this astronomical event was not only heard in gravitational waves, but also seen in the light of dozens of telescopes on Earth and in space.

Study of action-like particles

Arts and Sciences physicist Bhupal Dev used observations of this neutron star merger, an event identified in astronomy circles as GW170817, to derive new constraints on axial-like particles. These hypothetical particles have not been directly observed, but they appear in many extensions of the standard model of physics.

Acions and axion-like particles are leading candidates to make up some or all of the “missing” matter in the universe, or dark matter, that scientists have yet to explain. At the very least, these weakly interacting particles could serve as a kind of portal connecting the visible sector, about which humans know so much, with the unknown dark sector of the universe.

“We have good reason to suspect that new physics beyond the Standard Model may be imminent,” said Dev, the study’s first author. Physical Examination Letters and a fellow at the university’s McDonnell Center for Space Science.

Research on the fusion of neutron stars

When two neutron stars merge, a hot, dense remnant is briefly formed. Dev said this remnant is an ideal environment for the production of exotic particles. “The remnant becomes much hotter than the individual stars for about a second before settling into a larger neutron star or black hole, depending on the initial masses,” he said.

These new particles drift silently away from the debris of the collision and can decay into known particles, usually photons, far from their source. Dev and his team, which included WashU graduate Stephen Harris (now an NP3M fellow at Indiana University) as well as Jean-Francois Fortin, Coover Xingh, and Yongchao Zhang, showed that these runaway particles create unique electromagnetic signals that can be detected by Gamma-ray telescopes like NASA’s Fermi-LAT.

The research team analyzed the spectral and temporal information from these electromagnetic signals and determined that they were able to distinguish the signals from the known astrophysical background. They then used Fermi-LAT data for GW170817 to obtain new constraints on axon-photon coupling as a function of axon mass. These astrophysical constraints complement those obtained from laboratory experiments such as the Axion Dark Matter eXperiment (ADMX), which explores a different region of the axion parameter space.

Future prospects of elementary particle physics

In the future, scientists will be able to use existing space-based gamma-ray telescopes, such as Fermi-LAT, or proposed gamma-ray missions, such as the WashU-led Advanced Particle-Astrophysics Telescope (APT), to make neutron star collisions and other measurements. Help improve their understanding of axion-like particles.

“Extraordinary astrophysical environments such as neutron star mergers open a new window of opportunity in our search for dark sector particles such as axons that may hold the key to understanding the missing 85% of all matter in the universe,” Dev said.