LZ Experiment Detects Intriguing Dark Matter Candidate Signal

The event, observed in the region where dark matter is expected to appear, represents the most compelling hint of dark matter that LZ has reported to date. However, it does not meet the statistical threshold required to claim a discovery. “We’re very intrigued to see this event in the data, in the region where we expect dark matter to show up, and the competing backgrounds are very low,” said Rick Gaitskell, a physics professor at Brown University and the spokesperson for LZ. “With only one event, we don’t want to get ahead of ourselves. We are not claiming to have seen dark matter. But we have seen something interesting that we want to share with the scientific community for their input.”

The anomalous event, if caused by dark matter, would likely have come from a WIMP with a mass of at least 200 GeV/c2, more than 200 times the mass of a proton. The analysis has a significance of 2.6 sigma, meaning there is approximately a 0.5% chance the event could be explained by known backgrounds. Physics experiments generally require a 5-sigma significance to claim a discovery. With additional data, researchers can test whether the signal grows in significance or fades away.

LZ, co-led by Brown University faculty and students, has already accumulated the world’s largest dark matter dataset. The experiment will continue to accrue WIMP search data at the Sanford Underground Research Facility (SURF), substantially improving its search statistics.

JiJi Fan, an associate professor of physics at Brown who was not involved in the LZ experimental research but whose theoretical work is cited in the new paper, called the findings “very intriguing.” She noted that the single high-energy event could not have resulted from the simplest type of WIMP interaction, since that would have produced many lower-energy recoil events. Other interaction types, such as inelastic scattering or momentum-dependent elastic scattering, could potentially account for the observation, she said. “This also sets up a target for on-going dark matter hunting, not only at direct detection experiments such as LZ but also at other complementary experimental frontiers,” Fan added.

The LZ paper detailing the finding is available as a PDF. The team says it will continue collecting data to determine whether the hint becomes a definitive discovery.