Much of the search for dark matter—the invisible substance comprising 80 percent of all the mass in the cosmos—has focused on one type: weakly interacting massive particles (WIMPs). However, there's another strong contender: the axion, a very low-mass particle that could be numerous enough to play the part of dark matter. Axions were predicted in the 1970s because they provide a fix for a problem in particle physics, but they have yet to be seen experimentally.

Or so we've thought. Maybe they have been found, and we just didn't know what we were looking at. That's the premise of a provocative paper by Christian Beck. A certain anomaly that's been detected in superconducting experiments could be the result of axions passing through the apparatus. If this thesis is correct, then researchers have already detected axions with a tiny mass of about 0.11 thousandths of an electron volt (meV), or about 2×10-10 times the mass of an electron.

Of course we must insert all the appropriate precautions and weasel words. This doesn't mean we've definitely detected axions; the superconducting effect could be caused by something else, which would say little about the existence or nonexistence of axions. However, the hypothesis has a major advantage: it's relatively easy to check, since superconducting experiments of this type are far easier to perform and don't require the expensive, large-scale detectors that have been built to find WIMPs. Follow-up investigations using existing experimental setups could be able to distinguish between signals produced by axions and noise from other more mundane sources.

What are axions, anyway?

Axions were not originally proposed as a solution to the dark matter problem. Instead, they are a possible way to solve a pressing problem in quantum chromodynamics (QCD), the theory of the strong force, which governs quarks and their interactions.

Most interactions in physics work the same if you simultaneously exchange particles with their antimatter counterparts and reverse the direction in which they occur. This is known as CP symmetry (where C stands for charge and P stands for parity). CP symmetry is violated in many interactions involving the weak force, but, experimentally, it appears to be preserved by the strong force. The problem is that QCD appears to break CP symmetry badly, in stark contrast to experimental results.



The CP problem in QCD is akin to the one in electroweak theory, which describes the electromagnetic and weak forces. In the simplest form of the model, electrons, quarks, and force carrier particles are massless. To solve the problem, physicists introduced the Higgs field and its associated particle, the Higgs boson; the Higgs mechanism turns massless particles into massive ones, making the theory match what we observe in nature. However, the Higgs boson is very massive and decays rapidly into lighter particles. Axions are much lower mass than any normal particle (save perhaps neutrinos) and thus should be stable.



Physicists have proposed several possible solutions to this problem, extensions to the Standard Model of particles and interactions. The axion is perhaps the simplest of these, as it explains why CP symmetry is mostly upheld in QCD. If this idea is correct, many axions were produced in the moments after the Big Bang, meaning the Universe could be full of them. Thus, if axions exist, they could simultaneously solve the CP and the dark matter problem.

Axions have a very low mass (unlike predictions for WIMPs), are electrically neutral, and don't interact via the weak or strong forces. (Neutrinos also have low mass, interact primarily through the weak force, but aren't numerous enough to handle dark matter.) However, if they pass through a strong magnetic field, axions can transform into photons and vice versa. That's the basis of experiments such as the Axion Dark Matter eXperiment (ADMX) at the University of Washington. However, no experiment has detected axions, despite the theory having been around since 1977.

Ice cold: superconducting axion detectors?

However, if Beck's hypothesis is correct, there could be a much simpler way to hunt axions. This method would use devices called Josephson junctions, which involve two superconductors with some sort of separation between them, in this case a thin insulating layer. The two superconductors exchange electron pairs across the division in an oscillatory way at a rate called the Josephson frequency. Josephson junctions are used in a wide variety of applications, from sensitive quantum experiments to detectors in telescopes.

In the proposed scheme, axions entering the insulating layer of the Josephson junction create a spike in the current when the Josephson frequency matches the energy that corresponds to the mass of the particles. This process is the inverse of the standard axion-hunting scheme: axions transform into photons inside the junction, these enhance the superconducting signal, and they then decay back into axions on the other side.

Beck proposed that effect is the reason for an anomalous result described in a 2004 paper by C. Hoffmann and colleagues. This article examined the sources of various forms of noise in Josephson junctions, most of which the authors could explain with existing models. However, at a particular energy, the noise increased dramatically, which they proposed was due to multiple simultaneous transfers of electron pairs for an unknown reason. Other researchers followed up on this phenomenon without providing a full explanation for it, though they generally felt that it could be understood within the context of superconducting theory.

If Beck's idea is correct, however, the spike in noise was actually due to axions with a mass about 200 billion times smaller than an electron's. That's an exciting prospect, since it means it would be relatively easy to replicate the spike and might be possible to determine whether it comes from axions or some other source. For example, if microwaves are responsible for the boost in noise, then isolating the Josephson junction from such interference would make the signal vanish. The shield would not stop axions, though, which pass through pretty much everything. Similarly, if the energy at which the effect occurs isn't consistent, it's obviously not axions causing it.

So, as usual, we must be scientific skeptics and call for more research before we can say this settles anything. However, unlike the massive detectors required for WIMP hunting, Josephson junctions are relatively common and simple to fabricate in the lab. If Beck's hypothesis is correct, the long hunt for dark matter could be nearing an end, bringing us into the next phase: direct experiments on the particles. And the QCD folks would probably be pretty excited, too.

Physical Review Letters, 2013. DOI: 10.1103/PhysRevLett.111.231801 (About DOIs).