Being a particle physicist must sometimes feel like I imagine it feels to support certain football teams (hello, Spurs fans). You live in constant hope that things could turn out wonderfully, while knowing deep down always to expect disappointment. Many researchers will be wrestling with such emotions on hearing the news that a particle detector buried underground in a former gold mine in South Dakota might, just maybe, have detected a single particle of dark matter.
Experts will know not to get too excited about this development concerning one of the biggest mysteries in modern physics. Dark matter seems to be demanded by countless astronomical observations, which imply that it should greatly outweigh all the regular matter in the universe. But no one has the slightest idea what it is, and all previous efforts to detect it directly have failed. The latest results from an international experiment called LUX-ZEPLIN have revealed a solitary particle that doesn’t fit the profile of any known to science.
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Such findings have a habit of being chimeras, vanishing on closer inspection. It’s very possible that this will be the eventual outcome here. The probability that the detection is just a random signal with other causes is around one in 200 – which might sound small until you realise that particle physicists have much higher standards for detections they consider convincing. For that, the chance must be one in several million.
Dark matter is thought to interact with ordinary matter just (or nearly just) via its gravity, and to interact with light not at all – hence “dark”. It is thought to pervade the universe, and its presence in galaxies is crucial to binding them together gravitationally, preventing the stars from flying apart as the galaxies rotate.
We can deduce just a few things about what the particles of such matter should be like – they must, for example, have a mass at least as great (roughly) as that of silver atoms. In one view, such particles could also interact with ordinary matter via the “weak force” that acts in atomic nuclei, making them “weakly interacting massive particles” or WIMPs. Some still unproven theories of fundamental physics predict that these particles exist – but if so, they will be very hard to spot.
All the same, WIMPs might be expected very, very rarely to collide with ordinary atoms. That’s the rationale for the LUX-ZEPLIN detector, a vat of 10 tons of ultracold liquid xenon monitored constantly for little flashes of light that might signal such a collision. The detector team has seen one apparent collision at an energy considerably higher than they’d expected, having previously seen nothing at lower energies. It doesn’t seem to have been caused by a stray, known particle such as a neutron or gamma ray (particles that might be produced from cosmic-ray collisions; the experiment is housed underground to reduce such “background” events).
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But if the lone event was indeed caused by a dark-matter WIMP, that’s a real puzzle. It doesn’t seem possible for such a high-energy collision to happen without there being many more at lower energies – of which there has been no sign. Cosmologist Neal Weiner of New York University says that other explanations are possible, but these might imply that dark matter is more complicated than supposed.
Perhaps it is itself a composite of several types of unknown particle. But in the end, says Weiner, there’s “not a lot you can say with one event… we are left waiting for more data.” Happily, more should be coming. There are lots of other dark-matter detection experiments besides LUX-ZEPLIN, and now they’ll be looking at the same energies to see if they can corroborate – or dismantle – the finding.
