A mysterious signal has appeared in an extremely sensitive dark matter-hunting experiment. And after more than a year of trying to convince themselves that they were looking at noise, scientists think they might have found something new.
Today, the scientists behind the XENON1T dark matter experiment are reporting the result of a search through its data for particles that interacted with the electrons in its detecting medium. They’ve spotted evidence of an excess, or more interactions with the detector than the Standard Model of particle physics would predict. They don’t know what’s causing the signal yet; it might be a rare but mundane event, or it could be evidence of some other unknown physics phenomenon.
“We were wracking our brain around what it could be,” Laura Baudis, University of Zurich particle astrophysicist, told Gizmodo. She explained that University of Zurich researcher Michelle Galloway stayed up late into the night in order to work alongside University of Chicago graduate student Evan Shockley and University of California, San Diego student Jingqiang Ye on completing the analysis.
XENON1T is an experiment containing 3.2 metric tons of the element xenon, buried deep inside a mountain at the INFN Laboratori Nazionali del Gran Sasso in Italy, which collected data from 2016 to 2018. The experiment sits in wait for particles that barely interact with matter to cause a slight knock to the xenon nuclei or their electrons, releasing tiny flashes of light detected by sensors in the walls of the experiment. The XENON collaboration’s scientists hope to uncover the true identity of dark matter, the mysterious stuff whose gravity seems to serve as the universe’s scaffolding but that our particle physics theories fail to explain. Up until now, the experiment’s data haven’t revealed any new physical phenomena.
But XENON1T data taken from February 2017 to February 2018 has revealed an unexpected excess of low-energy interactions with the xenon’s electrons—285 events instead of the expected 232 events, according to the paper posted today on the XENON1T website.
Physicists have hypothesized various potential sources of the excess, testing their ideas against the data. They arrived at three most likely sources: Perhaps an unobserved, theoretical particle called an axion struck the detector after traveling from the Sun. Maybe a property of the neutrino particle, called its magnetic moment, is higher than previously predicted. Or, perhaps they’d simply spotted an unaccounted-for background radioactive process, the decay of the hydrogen isotope called tritium. Just a few tritium atoms sprinkled into the two tons of otherwise ultra-pure xenon could have produced the signal.
Axions are a theorized, low-mass particle meant to solve a physics problem called the strong-CP problem, which asks why subatomic particles called quarks follow the same laws of physics when you replace them with their mirror image with the opposite charge, when there’s no reason that they have to. If axions did exist, then scientists predict that the Sun would produce them in its core, and we’d be possibly be able to detect them from Earth. Axions are also a proposed culprit to explain at least some of dark matter.
Solar axions passing through the detector would look most like the signals that the researchers observed, with a 3.5-sigma significance, meaning that there’s a 99.98% chance that the observed signal wasn’t caused by typical physics processes. However, introducing tritium decay as another background process decreases the significance to 2 sigma, or a 95% chance that typical physics interactions plus decaying tritium didn’t cause the signal, Baudis said. More data could easily wipe out such a fluctuation and has in the past. Particle physicists strive for a 5-sigma significance in order to proclaim a discovery.
Other physicists were impressed with the amount of work and thought that went into the analyses and with the extreme sensitivity of the detector—but they urged caution with interpretations of the results. “They’ve done a tremendously good job of understanding their background,” Bob Jacobsen, University of California, Berkeley professor not involved in this work, told Gizmodo. But he pointed out that there simply isn’t a lot of data to look at here. “It’s just really interesting hints.”
Javier Redondo, physicist at the University of Zargoza in Spain, told Gizmodo in an email that the signal looked just like solar axions were passing through XENON1T. However, he said, if hypothetical axion particles produced by the Sun were to create this signal, then it implies a stronger interaction between axions and electrons than theory predicts today.
“Even our Sun would not agree with the best theoretical models and experiments as well as it does now,” he said. The solar axion properties implied by the XENON1T experiment would result in the Sun being hotter than astronomers predict and producing far more neutrinos than astronomers observe. He said that in order for the analysis to convince him that solar axions were causing the excess, he’d want to see a smoking-gun signature, such as solar axions appearing in dedicated experiments called solar magnetic helioscopes. Perhaps some other, unknown particle is causing the signal, he said.
Uncovering the true cause of the signal will take more work. A suite of similar experiments, such as the LZ experiment at the Sanford Underground Research Facility in South Dakota and PandaX at the China Jinping Underground Laboratory in Sichuan, China, will likely follow with their own analyses to see if they find the same signal. The XENON1T experiment will soon get an upgrade to become XENONnT—now with even more liquid xenon—after which physicists can repeat this analysis. XENONnT and solar magnetic helioscopes should be able to spot and explain this signal more easily, if the signal holds, of course. “There’s a lot of ifs,” Baudis said.
Such is the way of physics; sometimes, the most interesting signals lie at the furthest reaches of an experiment’s sensitivity and don’t occur often enough for physicists to claim a discovery. While we all hope that these signals will turn out to be some unexplained feature of the universe, sometimes, they’re just the ordinary decay of a couple of pesky radioactive atoms. At the moment, physicists don’t know what they’re looking at. All they can say is that it’s definitely interesting.