As we continue to search for dark matter particles, one thing is very clear: they cannot be any of the elementary particles we have discovered so far. The particles should have mass, but interact only weakly with light. Of the known particles, neutrinos fit this description, but neutrinos have tiny mass and are nowhere near enough to explain dark matter. Another type of particle must make up most of the dark matter.
Physicists have looked for possible candidates beyond the Standard Model of Particle Physics, and one that has looked promising is known as the Axion. Axions were first proposed in the 1970s to solve a symmetry problem in particle physics. As Emmy Noether first pointed out, symmetry is fundamental to the nature of physics. There are three important symmetries in particle physics: charge, parity, and time.
A demonstration of how CP symmetry is violated in particle decay. Photo credit: Kavli IPMU
Time symmetry is concerned with how interactions look forward and backward in time. Imagine two billiard balls colliding on a pool table. If you saw a video of the collision, it would be difficult to tell if the video was reversed without any other indication. Billiard balls obey the symmetry of time. Charge symmetry deals with the interaction of charges. Two positive charges repel each other, but so do two negative charges. So when you see two charged particles pushing away from each other, all you know is that they have the same type of charge. Parity has to deal with mirror image interactions. When a moon orbits a planet, it behaves the same way, whether it is orbiting clockwise or counterclockwise. These symmetries can also be combined. For example, an interaction that is symmetric under an inversion of charge and parity would obey CP symmetry.
It turns out that these symmetries are not perfect. It is known that the weak interaction violates CP symmetry about once in a thousand in certain interactions. The Standard Model predicts this, but it also predicts that there should be a similar injury in the strong force. Physicists have looked for such an injury but found none. This is where the Axion comes in. When axions exist, they suppress the CP violation in the strong force and solve the symmetry problem in the Standard Model. In theory, axions should be abundant, have mass and no charge, making them a good candidate for dark matter particles. And a recent study tried to find them.
An artistic impression of a magnetar with a very complicated magnetic field inside and a simple little dipolar field outside. Credits: ESA – Author: Christophe Carreau
Since axions have no charge, they would normally not interact with light at all. However, if they are in an intense magnetic field, they can be triggered to emit photons. When enough axions are clumped together, these photons should produce a detectable radio signal. So the team looked for these signals from neutron stars. It is known that neutron stars have intense magnetic fields and, with their stellar mass and high density, would likely attract many axions.
Using data from the Green Bank Telescope in West Virginia and the Effelsberg Telescope in southwest Germany, the team examined radio signals from two neutron stars. They also found no axion signal. This actually dampens the idea that axions exist, but does not completely rule out axions.
So it looks like both physicists and astronomers have cause for disappointment. Physicists will likely have to look elsewhere for a solution to their symmetry problem, and astronomers will continue to search for dark matter particles.
Reference: Foster, Joshua W. et al. "Green Bank and Effelsberg Radio Telescope are looking for Axion Dark Matter Conversion in neutron star magnetospheres." Physical Review Letters 125.17 (2020): 171301.