A team of scientists, including a University of Minnesota researcher, have found that mysterious x-rays detected from nearby neutron stars may be the first evidence of axions, hypothetical particles that many physicists believe make up dark matter. If their theory is confirmed, the researchers’ findings could help physicists unravel several mysteries of the universe.
Their paper is published in Physical Review Letters, a peer-reviewed academic journal published by the American Physical Society.
There are many kinds of particles that make up matter in the universe. The most common are protons, neutrons, and electrons. These particles collide with each other in certain settings, such as inside a star’s core or in particle accelerators built by scientists on Earth. Axions have long been elusive to physicists because they are “weakly interacting,” which means they rarely collide with other particles and instead often pass through them.
“Finding axions has been one of the major efforts in high-energy particle physics, both in theory and in experiments,” said Raymond Co, an author on the paper and postdoctoral researcher in the University of Minnesota’s School of Physics and Astronomy. “We think axions could exist, but we haven’t discovered them yet. You can think of axions as ghost particles. They can be anywhere in the universe, but they don’t interact strongly with us so we don’t have any observations of them yet.”
Theoretically, axions can be created by other particles colliding or exist naturally as dark matter, which physicists believe makes up a large percentage of the universe that we cannot directly see. The discovery of axions would answer many questions about dark matter and other particle physics mysteries. Axions are also predicted by string theory, or the idea that all the forces and particles in the universe are tied together as part of the same framework.
In 2019, Co’s former colleagues at the University of Michigan observed a mysterious, inexplicable increase in x-rays emitted from several neutron stars, which are extremely dense stars made up mostly of neutrons. In their recent paper, Co worked with his former colleagues to propose that these extra x-rays are caused by axions being produced in the neutron stars’ cores.
The researchers used a previously proposed theory about axions to explain this phenomenon. The theory states that axions are produced in the core of a neutron star as byproducts of colliding neutrons and protons. The particles then shoot out into the star’s strong magnetic field, where they are converted into photons—particles of light—which make up the x-rays detected by telescopes on Earth. Since axions carry much more energy than the photons these neutron stars typically emit, the photons produced from the axions would yield more energy as well, explaining the unexpected increase in x-rays.
“We’re not claiming that we’ve made the discovery of the axion yet, but we’re saying that the extra x-ray photons can be explained by axions,” Co said. “It is an exciting discovery of the excess in the x-ray photons, and it’s an exciting possibility that’s already consistent with our interpretation of axions.”
The current telescope data is not sufficient enough to prove that the x-rays come from axions, but the researchers hope that more data from other telescopes may provide further insight in the future. Other scientists in the particle physics community will also be able to branch off this research in their searches for axions.
Other members of the research team include Malte Buschmann, a postdoctoral researcher at Princeton University; Christopher Dessert, a graduate student at the University of Michigan; and Benjamin Safdi, a researcher at Lawrence Berkeley National Lab.
This research was funded by grants from the Department of Energy’s Office of Science and supported by Advanced Research Computing at the University of Michigan; the National Science Foundation; the Mainz Institute for Theoretical Physics (MITP) of the Cluster of Excellence PRISMA+; the Munich Institute for Astro- and Particle Physics (MIAPP) of the DFG Excellence Cluster Origins; and the CERN Theory department.
Read the research paper entitled, “Axion Emission Can Explain a New Hard X-ray Excess from Nearby Isolated Neutron Stars” on the Physical Review Letters website.