Axions are a hypothetical particle that might explain the existence of dark matter. But it might occasionally interact with normal matter, especially in the cores of stars. A team of physicists have searched for evidence of axions in Betelgeuse and come up with nothing. It doesn’t mean that the axion doesn’t exist, but it does mean that it will be harder to find.
We don’t know what the dark matter is, which is the name we give to the component of the universe that makes up 85% of all the mass. Whatever it is, it’s a form of particle unknown to the standard model of physics. It also must hardly ever interact with photons or normal matter, otherwise we would have seen it by now. There are a lot of candidates for the dark matter, and one family of candidate particles is known as the axions, which are predicted in various theories of high energy physics.
At first glance the axion fits the bill, since it’s small, lightweight, and almost never interacts with light or normal matter. Almost. If the axion is of a certain kind known as “ultralight”, it can potentially become detectable through a process known as the Primakoff effect. This effect needs a few ingredients to work. If a photon is embedded in a super strong magnetic field, it can occasionally decide to turn into an axion. That axion would then travel wherever it wants, since it doesn’t really interact with anything else. But if that axion encounters another magnetic field, it can turn back into a photon, usually something high energy like an x-ray, giving off a flash of light that betrays the existence of the axion and the Primakoff effect.
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Stars have really strong magnetic fields in their cores, so they might be axion factories. And these axions may give off x-ray radiation. We would love to use our sun as a laboratory for this, but unfortunately it’s busy releasing all sorts of x-rays for its own reasons, and so it’s hard to tell if they’re coming from axions or something far less exotic.
But Betelgeuse is a different story. It’s a giant star near the end of its life. It is much cooler than the sun, so it hardly ever emits x-rays. So if axions exist, and the Primakoff effect is correct, then Betelgeuse ought to be emitting a detectable amount of x-rays.
A team of researchers based on MIT did exactly that…and found squat. No x-ray. No signs of axions.
“What our results say is, if you want to look for these really light particles, which we looked for, they’re not going to talk very much to photons,” says Kerstin Perez, assistant professor of physics at MIT. “We’re basically making everyone’s lives harder because we’re saying, ‘you’re going to have to think of something else that would give you an axion signal.’”
These results don’t rule out the existence of axions altogether. It just means that if they’re ultralight, then their ability to transform into photons and vice-versa has to be very small. In fact, these results using Betelgeuse alone provide constraints on this ease of transition that are three times greater than any laboratory experiment.
So we’ll keep on hunting.