Physics
LZ dark matter signal fuels higgsino theories
A recent hint of dark matter from the LZ experiment has revived interest in supersymmetry's long-favored particle, the higgsino, Science News reports.
Science News reports that scientists with the LUX-ZEPLIN, or LZ, collaboration announced on September 1 they had seen a signal in their detector that could be a single particle of dark matter. It could also be a blip from known physics. The detector, sitting in a tank of liquid xenon at the Sanford Underground Research Facility in Lead, S.D., looks for dark matter speeding through and knocking into a xenon nucleus. In this event, the nucleus recoiled with unusually high energy, and no low-energy recoils were seen — a pattern that fits some dark matter candidates better than others.
If the signal is real, it would be a huge deal. It would be the first direct detection of a particle that has never been seen but is thought to make up most of the matter in the universe. It would also, according to researchers, be the first discovery of supersymmetry — a theory that pairs every known particle with a heavier superpartner. That idea, popular in the early 2000s, has lost ground as particle colliders failed to find evidence for it. But the LZ event may give it new life.
The higgsino candidate
The leading candidate among the hypothesised superpartners is the higgsino, the partner of the Higgs boson. It has a mass roughly 1,000 times that of a proton. Theoretical physicist Tracy Slatyer of MIT told Science News that in 2000, many physicists would have named the higgsino as their favorite dark matter candidate. Its appeal: unlike typical candidates, it comes in two mass states. A collision only happens if there's enough energy to bump it from the lighter to the heavier state, which explains why LZ saw only high-energy recoils.
Within a day of LZ's announcement, several papers on arXiv.org proposed the higgsino as the source. “I got pretty excited,” Katherine Freese of the University of Texas at Austin, a coauthor of one such paper, told Science News. “First paper I ever wrote in 24 hours.”
Problems pile up
But the higgsino explanation soon faced trouble. If higgsinos exist, they would collect in the sun and annihilate, producing neutrinos. The IceCube neutrino detector at the South Pole should have seen those neutrinos, but it hasn't, according to a paper by Harikrishnan Ramani and a colleague. Also, Slatyer and colleagues noticed a plot in LZ's paper that looks at even higher energies. If LZ saw one higgsino at the reported energy, they'd expect to see more at higher energies — but LZ didn't report any. LZ spokesperson Rick Gaitskell of Brown University told Science News that the detector wasn't calibrated for those higher energies and that they made no formal claim about detection or non-detection in that region.
“It comes pretty close to working,” Slatyer told Science News, but the combined evidence makes the higgsino a stretch.
Possible workarounds
Physicists have found some wiggle room. If the higgsino's mass is around 2,000 times a proton's mass — double the usual assumption — and if dark matter particles in the Milky Way can travel faster than typically modelled, the data could still fit. Freese and a coauthor posted an October 2 arXiv paper laying out that scenario. “Big picture: We love [the] higgsino, and what we're doing now is putting in the details that we think are important,” she told Science News.
Other theorists are exploring particles that aren't bound by supersymmetry's rules. Dan Hooper of the University of Wisconsin–Madison told Science News that such particles wouldn't necessarily create neutrinos in the sun, sidestepping IceCube's silence. His team proposed a version of a different dark matter particle that could also explain mysterious gamma rays from the Milky Way's center.
Meanwhile, LZ and other experiments like XENONnT in Italy are combing through their data for similar high-energy events. “It clearly requires a cross-check,” XENONnT spokesperson Elena Aprile told Science News.
Even if this signal isn't dark matter, researchers are enjoying the puzzle. “I think it's wrong to throw tantrums over the fact that it's not the higgsino the way we want it to be,” Ramani told Science News. “Nature might not give us everything we want in one go.”
Related stories
Astronomy
IceCube: how the South Pole became a neutrino detector
Almost 100 holes melted near the South Pole make up the largest neutrino detector ever built, and a founding leader of the collaboration is to receive the Nobel Prize in Physics.
Space
Dark Matter showrunner Blake Crouch on season 2's challenge
Season 2 of the Apple TV thriller moves beyond the 2016 novel, and showrunner Blake Crouch tells Space.com what that required.
Physics
Thorium nuclear clock ticks steadily for 24 hours
A Vienna-led team kept a thorium clock's laser steady for more than a day with no conventional atomic clock holding it in place.