New record in search for dark matter

The latest results from the LUX-ZEPLIN (LZ) experiment, the world's most sensitive dark matter detector located nearly a mile underground in South Dakota, narrow down the possibilities for a leading dark matter candidate called Weakly Interacting Massive Particles (WIMPs). The experiment’s results explore weaker dark matter interactions than ever searched before and further limit what WIMPs could be.

Dark matter, the invisible substance that makes 85% of the mass in our universe, has never been directly detected; its presence is only inferred from its gravitational effects on galaxies and cosmic structures.

LZ, led by the Department of Energy's Lawrence Berkeley National Laboratory, found no evidence of WIMPs above around 9 gigaelectronvolts/c² (GeV/c²) – for comparison, a proton's mass is slightly less than 1 GeV/c². This significantly constrains the mass range where these hypothetical particles could exist.

‘These are new world-leading constraints by a sizeable margin on dark matter and WIMPs,’ said Chamkaur Ghag, spokesperson for LZ and a professor at University College London (UCL). He noted that the detector and analysis techniques are performing even better than the collaboration expected. ‘If WIMPs had been within the region we searched, we’d have been able to robustly say something about them. We know we have the sensitivity and tools to see whether they’re there as we search lower energies and accrue the bulk of this experiment’s lifetime.’

The detector hunts for dark matter by looking for faint flashes of light that would result if a WIMP collided with a xenon atom in its 10 tons of liquid xenon. Over 280 days of highly sensitive data, no convincing WIMP signals were observed. LZ's unprecedented sensitivity comes from its multi-layered design that blocks out all other radiation sources. Only potential dark matter signals can pass through to be detected. Advanced analysis techniques also help reject any false signals mimicking a WIMP interaction.

Associate Professor Kimberly Palladino from the Department of Physics at the University of Oxford is a member of the LZ executive board; she shared: ‘Along with Professor Hans Kraus, I am thrilled to see the LZ detector and our analysis performing so well. The Oxford LZ group played numerous roles in this result, especially in the removal of pernicious accidental background events and the final statistical checks of the result. Both of those areas of study were especially important for our method of mitigation for unconscious bias: we used a "salting" technique where fake WIMP signals are added during data collection – this way we blindly analysed both real and fake signals together.’

While no WIMPs were found yet, the 250-scientist LZ collaboration plans to keep taking data over the next few years, probing lower mass ranges with new analysis methods. In addition to WIMPs, LZ can search for other exotic phenomena that could reveal new physics beyond the Standard Model.

In parallel with the operation and analysis of LZ, the Oxford xenon dark matter group is actively planning the next liquid xenon time projection chamber, XLZD. Such an experiment would be an observatory for rare physics, able to look for dark matter and neutrino physics signals over a wide range of energies. Professor Palladino is a steering committee member: ‘XLZD is a like a supergroup, bringing together the Xenon, LZ, and DARWIN collaborations to build the ultimate detector. One exciting prospect is that the UK could host this experiment at Boulby Underground Laboratory, where the ZEPLIN dark matter programme that helped pioneer the liquid xenon technology had previously operated.’