Physicists are scrambling to make sense of possible dark matter signal

The LUX-ZEPLIN main detector in a surface lab before installation underground

Sanford Underground Research Facility

We may have finally spotted a dark matter particle – but figuring out exactly what kind of particle it was is a daunting task. Even as researchers race to confirm they really did detect an unknown particle, physicists around the world have already started working out what sort of dark matter particle it might have been.

The potential particle in question was found at the LUX-ZEPLIN (LZ) detector in South Dakota and announced on 1 September. The detector uses a huge tank of liquid xenon to look for signatures of dark matter particles smashing into the nuclei of xenon atoms, which are expected to produce a burst of light in response to any collision that would then be picked up by the arrays of cameras surrounding the tank.

The LZ group recorded only a single burst of light, and there is about a 1-in-200 chance that the signal was a fluke event rather than evidence that dark matter was there – but nevertheless it is tantalising.

What makes the signal even more intriguing is that it was at a higher energy than would be expected if the particle were a standard weakly interacting massive particle, or WIMP, the simplest proposed type of dark matter. “It’s an anomalous event,” says Henning Flaecher at the University of Bristol in the UK, part of the LZ group. “It’s not what you would expect from the most general dark matter models.” The fact that this high-energy event was spotted, with no accompanying lower-energy collisions, makes it exceedingly unlikely that this particle, if it exists, is a “vanilla” WIMP, he says.

In the weeks since the announcement, theorists have released scores of papers digging into the data, searching for answers. “The large number of papers is less a statement that theorists believe the event is a [true] dark matter discovery and more a reflection of the fact that, if it persists, its energy already tells us quite a lot about types of physical processes that could produce it,” says Juri Smirnov at the University of Liverpool in the UK.

Many of those papers debate the possibility that the particle could be what is called a higgsino, a type of non-“vanilla” WIMP. It arises from a theoretical framework called supersymmetry, in which every known particle is proposed to have a “superpartner”, with some similar properties but different spin. In supersymmetry, the higgsino would be the superpartner of the Higgs boson.

A higgsino is the simplest WIMP explanation still available to explain the LZ signal, according to JiJi Fan at Brown University in Rhode Island. “It gives rise to a wealth of signals that could be searched for at different experimental frontiers.” Plus, a higgsino detection event is predicted to be unaccompanied by a host of lower-energy collisions, matching the signal detected by the LZ group.

However, the higgsino has its problems. The most basic model of that sort of particle that could explain the LZ detection seems to be ruled out by constraints from other experiments. This doesn’t mean the particle couldn’t have been a higgsino, but it does suggest the higgsino in question would need to have had a much higher mass than expected.

Many other models have been proposed to explain the detection, though. Among them are several models of dark matter particles originating in extra dimensions. While this may seem an unnecessarily complicated line of enquiry, these models were already being studied to solve other problems in particle physics, says Lisa Randall at Harvard University, a co-author of one such paper. “In some sense, the data looks like it’s calling out for something like this: it avoids the pre-existing constraints and fits quite naturally.”

An extra-dimensional dark matter candidate could both fit the LZ detection and explain why the fundamental particles have the masses they do, a long-standing puzzle, she says. Other models propose a whole dark matter sector, with multiple types of dark particles.

Right now, there is no way to tell which, if any, of these models might be correct. “I don’t think the data currently justify saying one of these is ‘the’ explanation,” says Smirnov. Some of them do come with additional predictions, though, which will be particularly useful when trying to pick through so many ideas, he says.

As the LZ team and other teams at detectors around the world continue to analyse their data, looking for additional events to confirm that the dark matter particle was real, all of these ideas could help them know exactly what it is they are looking for. “Anomalies could come and go, as we all know,” says Fan. “My take is that they provide us with some concrete targets to think: is this expected? Are there new territories/questions in theory or experiments we should cover? No matter whether the LZ event is confirmed, some of these topics would lead to long-term research.”