Axions don’t show up yet, but that doesn’t mean they’re not out there.
With the identification of the Higgs boson at CERN’s Large Hadron Collider, scientists put the last piece of the Standard Model of physics in place. What they haven’t found is any hint of something beyond the Standard Model. And that hasn’t been for lack of trying. Supersymmetry, the most popular extension to the Standard Model, predicts a large collection of additional particles. We’ve looked for them and, so far, they have not shown up.
But some extensions of the Standard Model don’t predict the sorts of heavy particles that the LHC is designed to identify. Instead, they suggest there’s a very light force-carrying particle called an axion. With the right properties, an axion could solve issues in everything from particle interactions up to the scale of galaxy clusters. But its tiny mass and odd behavior means it won’t be detected in the LHC.
But that doesn’t mean the LHC’s hardware can’t find it. Clever engineers at CERN took magnets originally designed for the LHC, combined them with X-ray focusing technology originally designed for space, and built a device that could spot axions arriving here from the Sun. So far, it has seen no sign of them, which places some strict limits on the properties of these hypothetical particles.
Putting limits on our imagination
Physicists don’t just come up with hypothetical particles for fun. (Well, they might enjoy doing it, but it’s not solely for fun.) They prefer their particles to be what they call “well motivated,” meaning there’s a good reason for proposing them. In the case of axions, that motivation came from quantum chromodynamics, which describes the interactions of quarks and gluons. Axions were proposed to provide a theoretical explanation for why these particles appear to be indifferent to the direction of time (technically called “time-reversal invariance”).
Since then, other types of axions have been proposed, but they all share a critical property: they have mass (although not very much). This makes them possible dark matter candidates, since they should be present in our Universe in very large numbers. (Click to Article)