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. Unexpected hints of dark matter should breathe life into particle physics 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.
In the weeks since the announcement, theorists have released scores of papers digging into the data, searching for answers. 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. 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. Sarah Alam Malik explores whether preserving more of it could help us spot signs of dark matter and other new physics 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.
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