With a Recycled LHC Magnet, the Axion Solar Telescope Hunts for Ghost Photons

CERN's axion hunt drills down toward New Physics

by Michael Byrne
May 1 2017, 3:00pm

CERN's Axion Solar Telescope (CAST) experiment has pushed past a critical boundary in its hunt for the elusive axion, a hypothesized subatomic particle with the potential to explain lingering problems in quantum physics, possibly even dark matter. While the particle remains MIA, this is the first time physicists have been able to probe below a critical astrophysical boundary that may hide physics extending or transcending the Standard Model. The results are described in a paper published Monday in the journal Nature Physics by the CAST Collaboration.  

First off, no, the Large Hadron Collider is not the sole physics experiment living at CERN, though CAST is based around a giant magnet harvested from said collider. It's basic operation to is stare at the Sun for a few hours a day while using said 9-meter magnet to convert incoming axions into X-rays. A mirror system then focuses these X-rays for detection.

But what's an axion anyway? Basically, it solves a problem in quantum chromodynamics, a field concerned with the strong force—one of the four fundamental forces, along with the weak force, gravity, and electromagnetism—and its constituent particles, quarks and gluons. Within the strong force, things are unexpectedly symmetric with respect to time. There should be a very slight different in how the force behaves forward through time and backward through time, but that doesn't seem to be the case. That's pretty dang weird. 

So, physicists came up with the axion, a really tiny particle that could add the neccessary something to make time reversal symmetry plausible.  

That's what the axion was proposed for anyhow. Nowadays it's often considered within the context of dark matter, the mysterious something that makes up about 85 percent of all of the mass in the universe. It's not the only candidate for dark matter, but it's promising enough that there are several detection experiments underway across the globe. It helps that axion detection experiments are relatively cheap—CAST is one of the cheapest astrophysics experiments out there.

Axions can be understood as a very peculiar form of light, a form that doesn't really interact with the same things that normal light does. So, it just passes through everything like a ghost. It's thought that the axion may represent an entirely new family of particles called WISPS, or Weakly Interacting Slim Particles. We might be able to find axions using the magnet trick employed by CAST, where axion particles are converted to photons in the presence of a very strong magnetic field. Thanks to its giant recycled magnet, CAST can provide such a field.

Another variation on axion-hunting is represented by the ALPS experiment, described in the video above. In that case, photons are passed through a strong magnetic field, hopefully producing axions (the magnetic field thing works both ways), and then fired at a wall. The wall is meant to block out any photons while still allowing axions to pass through. On the other side is another strong magnetic field, which should turn those axions back into photons. CAST is sort of the same idea, but instead of generating its own axions from photons, it looks to the Sun as its axion source.

"CAST can still provide interesting clues on the physics beyond the standard model, but it is unlikely it will be able to push much further the exploration of the axion parameters," explains US physicist Maurizio Giannotti in a commentary accompanying the CAST results. "The magnet, not specifically designed for axion searches, is difficult to manoeuvre much outside its horizontal position, limiting the system to follow the Sun for only a couple hours each day. Moreover, the magnet surface area is small."

Actual detection is then left to next-generation experiments, but CAST demonstrates that we can indeed go deep enough to find axions in the first place.