Scientists are turning the tuning knob on an experiment that's essentially a radio receiver inside of a magnet. It's been around for years, but now it might finally be sensitive enough to hear a whole new kind of particle - one that could explain the mystery of the universe's dark matter.
The ADMX experiment in its experimental hall. Photo: G. Rybka
There are various hypothetical candidates that could explain dark matter which, if it exists, should outnumber regular matter by a factor of five to six to one. None of these proposed particles have been spotted in a lab. But some folks are excited about an especially tiny (and especially weird) candidate that happens to be named after a laundry detergent: The axion. The scientists at the Axion Dark Matter eXperiment (ADMX) at the University of Washington think they're ready to spot this theoretical particle.
"The prime real estate to explore is still there, and that's what we're doing with ADMX," Grey Rybka, a professor from the University of Washington, told Gizmodo.
Here's the deal with dark matter: Several observations imply that there are weird gravitational effects in space without enough mass to cause them. Physicists are now hunting for "dark matter", some sort of previously undetected particle or physical effect that could account for the excess mass and explain that weirdness.
There are several dark matter candidates. Vats of liquid xenon or other detectors buried deep underground are hunting for the most popular idea, called WIMPs, or Weakly Interacting Massive Particles. These hypothetical particles would interact only very weakly with regular matter through the tiniest nudges.
But some are frustrated that WIMPs haven't yet been discovered, either by these big underground experiments or the Large Hadron Collider, an atom smasher in Switzerland. So instead, they're looking for axions.
Axions aren't just a random guess at what dark matter might be. They come from a proposed solution to another problem in physics completely unrelated to dark matter, called the strong-CP problem.
You might know that there are four different ways by which particles communicate: The ones that you can feel, called gravity and electromagnetism, and the ones only atoms can feel, called the strong and the weak force. Physicists long thought that if you gave a particle the opposite charge and handedness, the physics governing its behaviour would be the same. It should have Charge and Parity (handedness), or CP symmetry.
But physicists in the 1960s realised that this was not the case with the weak force; the "symmetry" was "broken", since the laws of physics didn't work the same way under the swap. To some, this meant that the strong force, too, should break this CP symmetry, and the strong-CP problem was born. The most popular solution, called the Peccei-Quinn theory, requires an extremely light new particle called the axion to exist. Physicists later realised that despite its teeny mass, the axion could also account for all of the universe's dark matter.
ADMX. Photo: G. Rybka
Rybka and his team are looking for axions with their ADMX. Theories suggest that if axions existed, a big magnetic field could cause them to produce microwave or radio wave light particles, called photons. That means that ADMX physicists are essentially operating a radio receiver in a magnetic field, slowly turning the tuner and waiting to see if they can find the right station to hear the axion's photon song.
The researchers haven't discovered the axion yet, but they're reporting today in Physical Review Letters that their experiment is officially sensitive enough to probe the most likely areas. Basically, if the particle does exist, they will find it soon. "It's been going for two decades, but it's only now that we're sensitive to the plausible theoretically predicted axion," said Rybka. "Maybe someone predicted it wrong - but now we have the power to find it."
One researcher who studies another wild potential manifestation of the axion thought that this paper was a "momentous achievement". "If it is seen in the ADMX experiment we will finally know part of the answer to the puzzle of dark matter," Masha Baryakhtar, a postdoctoral researcher at the Perimeter Institute for Theoretical Physics in Canada, told Gizmodo.
But particle physics requires all sorts of different experiments, looking for particles of a lot of different masses and a lot of different interaction strengths. "As with any experiment, ADMX can only look for axions in a particular range of masses," she said. "This range is well motivated to be the dark matter, but until we find it, we will not know if we are looking in the right place."
Physicists are deep in the trenches of the dark matter search. Many would have placed their bets on the WIMP. Physicists sometimes discuss the "WIMP miracle", since a WIMP would answer several seemingly unrelated physics questions simultaneously. But as WIMP searches turn up empty-handed, some have begun to think about hunting for particles once thought to be less likely.
Rybka thinks the axion is as likely to exist as anything else. "Different physicists have different opinions on what's more compelling," he said. "I like the axion miracle better."