If we could see it (Image: Max Planck Institute For Astrophysics/Science Photo Library)

Oops. One of the universe’s most-wanted particles may have shown its face in a simple tabletop experiment nearly a decade ago – only no one noticed at the time.

“If other experiments confirm the effect, then it could be an immense step forward in our understanding of the matter contents of the universe,” says Christian Beck of Queen Mary, University of London, UK.

Beck has reanalysed an unexplained signal in an electrical circuit, first reported in 2004, and says it is just what you would expect to see if dark matter takes the form of hypothetical particles called axions. It’s too soon to known if the signal actually is dark matter, but these circuits – Josephson junctions – may present a promising new way to hunt for the mysterious stuff.


Though we know from gravity’s effects on galaxies that dark matter must make up about 85 per cent of the universe’s matter, its identity is still a puzzle. Topping the list of candidates are weakly interacting massive particles, or WIMPs. Axions, which are much lighter and colder, are the runner-up. Various detectors around the world are on the hunt for both. WIMP signals have been spotted, but these are not definitive, while the only axion sighting was later ruled out.

Axion profile

Beck reckons Josephson junctions provide a new way to hunt axions. In these circuits, pairs of electrons tunnel from one superconducting layer to another with no resistance, even though a thin layer of ordinary metal lies sandwiched in the middle.

Noting that the equations describing this current and the motion of an axion are similar, Beck suggested that when an axion hits a Josephson junction, it should trigger extra electron pairs to flow between the two layers. Since axions would constantly rain down on us from space, he reasoned that they might already have left a signature in Josephson junctions, and set about searching results from labs that use them.

Now he has found what he was looking for. In 2004, Francois Lefloch of CEA, the French atomic energy and alternative energies commission, and colleagues reported an excess current of unknown origin in a Josephson junction. Beck has shown that the signal fits the profile of an axion.

He says that Lefloch’s team, who weren’t looking for dark matter, happened to use just the right parameters in their experiments to make glimpsing axions likely. “Only special types of Josephson junctions are suitable as good axion detectors,” Beck says.

Rogue current

He has used the signal to estimate the number of axions hitting the junction – 1011 and their mass, 0.11 millielectronvolts. That’s less than a billionth the mass of an electron.

Lefloch says the unknown peak could still be due to rogue electrical effects, so someone needs to repeat the experiment and see if the signal persists.

Pasquale Serpico of the CNRS, the French national centre for scientific research, says Beck’s claim deserves further scrutiny.

If a similar signal is glimpsed in other Josephson junctions, then Beck’s mass estimate could be used to guide axion searches in bigger experiments such as ADMX at the University of Washington.

Journal reference: Physical Review Letters, doi.org/p9z

This article will appear in print under the headline “Hints of dark matter pop up in 10-year-old circuit”