Finding new physics in debris from colliding neutron stars

Neutron star mergers are a treasure trove for new physics signals, with implications for determining the true nature of dark matter, according to research from Washington University in St. Louis.

On Aug. 17, 2017, the Laser Interferometer Gravitational-wave Observatory (LIGO), in the United States, and Virgo, a detector in Italy, detected gravitational waves 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 light by dozens of telescopes on the ground and in space.

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

Axions and axion-like particles are leading candidates to compose part or all of the "missing" matter, or dark matter, of the universe that scientists have not been able to account for yet. At the very least, these feebly-interacting particles can serve as a kind of portal, connecting the visible sector that humans know much about to the unknown dark sector of the universe.

"We have good reason to suspect that new physics beyond the standard model might be lurking just around the corner," said Dev, first author of the study in Physical Review Letters and a faculty fellow of the university's McDonnell Center for the Space Sciences.

When two neutron stars merge, a hot, dense remnant is formed for a brief period of time. This remnant is an ideal breeding ground for exotic particle production, Dev said. "The remnant gets much hotter than the individual stars for about a second before settling down into a bigger neutron star or a black hole, depending on the initial masses," he said.

These new particles quietly escape the debris of the collision and, far away from their source, can decay into known particles, typically photons. Dev and his team -- including WashU alum Steven Harris (now NP3M fellow at Indiana University), as well as Jean-Francois Fortin, Kuver Sinha and Yongchao Zhang -- showed that these escaped particles give rise to unique electromagnetic signals that can be detected by gamma-ray telescopes, such as NASA's Fermi-LAT.

The research team analyzed spectral and temporal information from these electromagnetic signals and determined that they could distinguish the signals from the known astrophysical background. Then they used Fermi-LAT data on GW170817 to derive new constraints on the axion-photon coupling as a function of the axion mass. These astrophysical constraints are complementary to those coming from laboratory experiments, such as ADMX, which probe a different region of the axion parameter space.

In the future, scientists could use existing gamma-ray space telescopes, like the Fermi-LAT, or proposed gamma-ray missions, like the WashU-led Advanced Particle-astrophysics Telescope (APT), to take other measurements during neutron star collisions and help improve upon their understanding of axion-like particles.

"Extreme astrophysical environments, like neutron star mergers, provide a new window of opportunity in our quest for dark sector particles like axions, which might hold the key to understanding the missing 85% of all the matter in the universe," Dev said.

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Revealing the mysteries of the universe under the skin of an atomic nucleus

Massive neutron stars colliding in space are thought to be able to create precious metals such as gold and platinum. The properties of these stars are still an enigma, but the answer may lie beneath the skin of one of the smallest building blocks on Earth -- an atomic nucleus of lead. Getting the nucleus of the atom to reveal the secrets of the strong force that governs the interior of neutron stars has proven difficult. Now a new computer model from Chalmers University of Technology, Sweden, can provide answers.

In a recently published article in the scientific journal Nature Physics, Chalmers researchers present a breakthrough in the calculation of the atomic nucleus of the heavy and stable element lead.

The strong force plays the main role

Despite the huge size difference between a microscopic atomic nucleus and a neutron star several kilometers in size, it is largely the same physics that governs their properties. The common denominator is the strong force that holds the particles -- the protons and neutrons -- together in an atomic nucleus. The same force also prevents a neutron star from collapsing. The strong force is fundamental in the universe, but it is difficult to include in computational models, not least when it comes to heavy neutron-rich atomic nuclei such as lead. Therefore, the researchers have wrestled with many unanswered questions in their challenging calculations.

A reliable way to make calculations

"To understand how the strong force works in neutron-rich matter, we need meaningful comparisons between theory and experiment. In addition to the observations made in laboratories and with telescopes, reliable theoretical simulations are therefore also needed. Our breakthrough means that we have been able to carry out such calculations for the heaviest stable element -- lead," says Andreas Ekström, Associate Professor at the Department of Physics at Chalmers and one of the main authors of the article.

The new computer model from Chalmers, developed together with colleagues in North America and England, now shows the way forward. It enables high precision predictions of properties for the isotope* lead-208 and its so-called 'neutron skin'.

The thickness of the skin matters

It is the 126 neutrons in the atomic nucleus that form an outer envelope, which can be described as a skin. How thick the skin is, is linked to the properties of the strong force. By predicting the thickness of the neutron skin, knowledge can increase about how the strong force works -- both in atomic nuclei and in neutron stars.

"We predict that the neutron skin is surprisingly thin, which can provide new insights into the force between the neutrons. A groundbreaking aspect of our model is that it not only provides predictions, but also has the ability to assess theoretical margins of error. This is crucial for being able to make scientific progress," says research leader Christian Forssén, Professor at the Department of Physics at Chalmers.

Model used for the spread of the coronavirus

To develop the new computational model, the researchers have combined theories with existing data from experimental studies. The complex calculations have then been combined with a statistical method previously used to simulate the possible spread of the coronavirus.

With the new model for lead, it is now possible to evaluate different assumptions about the strong force. The model also makes it possible to make predictions for other atomic nuclei, from the lightest to the heaviest.

The breakthrough could lead to much more precise models of, for example, neutron stars and increased knowledge of how these are formed.

"The goal for us is to gain a greater understanding of how the strong force behaves in both neutron stars and atomic nuclei. It takes the research one step closer to understanding how, for example, gold and other elements could be created in neutron stars- and at the end of the day it is about understanding the universe," says Christian Forssén.

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