At last we might have found a clue to one of the greatest unexplained mysteries in physics

The world of fundamental physics has been set abuzz with the announcement of a possible sighting of something unknown. The prominent XENON1T collaboration, which operates a detector buried beneath Gran Sasso mountain in central Italy, announced on 17th June that it had seen a signal above “known backgrounds,” meaning that it may have detected particles of unknown origin—perhaps with some of the characteristics expected of a hypothetical candidate particle for dark matter. To put it in the simplest terms, it is possible that the project has finally seen some of the elusive particles thought to constitute about 85 per cent of the matter in the universe.

If so, the implications are immense, and the discovery would certainly rank alongside Nobel-Prize-winning detections of hitherto unknown particles and phenomena, such as the gravitational waves that garnered the 2017 Nobel, or the Higgs boson seen in the Large Hadron Collider at Cern in Geneva in 2012. But no one is cracking open the champagne yet, and for good reason. If past experience is any guide, there is always the risk of a false alarm: the signal could have more mundane origins.

Hitting the right tone here is quite a challenge. The easy response would be a cynical one: we’ve been here before with mistaken “sightings” of dark matter, so it’s all a storm in a teacup. But you don’t need to go to the other extreme of credulous, breathless headlines declaring a revolution in physics. A degree of cautious excitement is perfectly warranted while the XENON1T scientists comb through the details, and await independent verification, to see if the result will stand up to scrutiny.

As I explained last year for Prospect, dark matter is so-called because it seems to be almost totally invisible, not interacting either with light or with the ordinary matter from which all atoms are made. Pretty much its sole effect on the visible universe is via gravity: because it has mass, it exerts a gravitational tug on ordinary matter. The main reason dark matter is thought to exist at all is that there seems to be an unexplained gravitational influence in galaxies that holds them together as they rotate: the movements of the visible stars and gas can’t by wholly accounted for by their own gravity alone. There also seems to be something unseen that occasionally bends light rays travelling through the cosmos as if they are passing through a lens—a gravitational effect predicted by Einstein’s theory of general relativity—and clumps of dark matter might be the cause.

From the size of these effects, and from the amount of dark matter needed in theoretical models of how the cosmos has evolved and galaxies formed since the Big Bang, it’s estimated that dark matter outweighs visible matter by a factor of around five. Yet despite this, no one has ever detected dark-matter particles directly. Because it passes through ordinary matter as if was not there, these particles might be constantly streaming right through the Earth without leaving any trace.

Or almost none. Dark-matter detectors are designed on the assumption that the interactions with ordinary matter—with regular atoms and their subatomic constituents—are not entirely impossible, but just very, very rare. If the volume of the detector is big enough, there’s just a chance that such collisions might be seen, typically via the other particles or flashes of light generated in the process.

To do so, however, you need to rule out all “background” events due to collisions between known particles, such as the cosmic rays (largely high-energy protons) that streak through space. That’s why these detectors are buried underground: to reduce this background by ensuring that such ordinary particles are mostly absorbed in the rock before they penetrate that deep.

XENON1T, an elaboration of an earlier, smaller device, has been operating since 2016. It is basically a tank of around two tons of liquid xenon, a highly unreactive element, fitted with ultra-sensitive instruments for measuring light and other radiation at the top and bottom. A dark-matter particle passing through the tank might undergo a collision with a xenon atom that would be picked up by the detectors. The international XENON1T collaboration has just released a preprint—a paper that has not yet been vetted by peer review—describing measurements between February 2017 and February 2018. It says that the number of detection events seen within a particular range of energies was significantly higher than that predicted when all known sources of background signal are taken into account. Something unknown is being seen.

But “unknown” here doesn’t necessarily mean new. In an experiment demanding this level of sensitivity, it’s extremely hard to exclude all the potential confounding factors. The researchers say that one possible mundane source of the “excess” detections at these energies is beta particles (electrons) created by radioactive decay of a rare form of hydrogen called tritium, which could itself be generated by, for example, cosmic-ray collisions with xenon atoms or with impurities such as ordinary hydrogen atoms in water. So little tritium would be needed to produce the excess signal that it couldn’t be directly detected within the liquid xenon.

But what if the signal is indeed from dark matter? What sort of particle then is it? The favourite candidates for many theorists are so-called weakly interacting massive particles (WIMPs) that lie outside the “Standard Model” describing all known particles and forces. XENON1T was designed primarily to spot these—but has previously found no sign of them, and the latest finding doesn’t at all match what would be expected from WIMPs either.

It was partly because of that dispiriting (if not entirely unsurprising) failure that the collaboration widened its net over the past few years to look for other, less massive candidate particles, such as so-called axions. These have been postulated to solve other puzzles connected to the “nuclear strong force” that binds protons and neutrons in atomic nuclei—but if they exist, they might also account for dark matter. The theories predict that axions should be generated by the nuclear reactions in the heart of the Sun, and that they have a tiny chance of colliding with electrons like those in xenon atoms. Such a collision would produce an “electron recoil,” rather like that of between billiard balls, generating a distinctive signature in the XENON1T detectors.

Solar axions would not themselves be dark matter. But if there were proof that axions exist at all, these particles could have been formed in the early universe and, dispersed through the cosmos, might make up some of the dark matter.

Axions aren’t the only possible “exotic” source of the unexplained signal. One possibility, which would be very interesting in itself, though no answer to the dark-matter riddle, is its generation by collisions of known (but also ultra-light) particles called neutrinos, which the Sun also produces in abundance. This would only work, though, if the neutrinos had certain magnetic properties that no one has seen before—which would itself be a hint for how to extend fundamental physics beyond the Standard Model.

At the moment, the smart money would be on the new findings having some prosaic explanation, such as contamination of the xenon or some unrecognised quirk of the instrumentation. But XENON1T is not the only such experiment seeking dark matter, and so the unexplained outcome could prompt others to turn their attention towards the same territory. In particular, a successor to XENON1T itself, using 8.3 tonnes of xenon and called XENONnT, will surely look there when it begins to collect data later this year.

Dark-matter seekers have failed to find anything interesting for so long that it would take a hard heart to begrudge them this surge of excitement now. It might all be literally a flash in the pan—but if so, that’s because figuring out what our universe is made of has proved to be so extraordinarily difficult.