Media release

From: ARC Centre of Excellence for Dark Matter Particle Physics

LZ experiment sets a new record in the hunt for galactic dark matter and gets a new look at neutrinos from the sun’s core

There’s more to the universe than meets the eye. Dark matter, the invisible substance making up most of the universe’s mass, is all around us — but what it is remains one of science’s biggest mysteries.

The newest results from LUX-ZEPLIN (LZ), released at 4am on Tuesday, 9 December (10am on Monday, 8 December US Mountain Time), extend the experiment’s search for low-mass dark matter and set world-leading limits on one of the prime dark matter candidates: weakly interacting massive particles, or WIMPs. They also mark the first time LZ has picked up signals from neutrinos from the sun, a milestone in sensitivity.

University of Melbourne and ARC Centre of Excellence for Dark Matter Particle Physics postdoctoral researcher Dr Robert James has played a leading role in achieving these results.

Dr James led the statistical analysis that produced the world’s most significant observation of boron-8 solar neutrinos interacting via neutrino-nucleus scattering, and set world-leading dark matter exclusion limits for masses above 5 GeV/c², roughly the mass of five protons.

“It was a challenge to model our detector for the first time in this low-energy regime, but these are extremely rewarding results,” he said.

The results were presented in a scientific webinar in the Sanford Underground Research Facility (SURF) in South Dakota and the paper will be submitted to the journal Physical Review Letters. This is the first LUX-ZEPLIN (LZ) publication to list the University of Melbourne as an LZ institution, reflecting Dr James’ role in the analysis.

LZ is an international collaboration of 250 scientists across six countries, operating nearly a mile underground in the underground laboratory.

The experiment uses 10 tonnes of ultrapure liquid xenon to detect tiny recoils caused by dark matter or neutrino interactions, isolated by deep shielding and low-radioactivity materials. The new results use the largest dataset ever collected by a dark matter detector.

LZ’s extreme sensitivity, designed to hunt dark matter, also allows it to detect neutrinos – fundamental, nearly massless particles that are notoriously hard to catch – in a new way. This analysis showed a new look at neutrinos from a particular source: the boron-8 solar neutrinos produced by fusion in our sun’s core.

This data is a window into how neutrinos interact and the nuclear reactions in stars that produce them. But the signal also mimics what researchers could expect to see from dark matter. That background noise, sometimes called the “neutrino fog,” could start to compete with dark matter interactions as researchers look for lower mass particles. Reaching into the neutrino fog for the first time highlights LZ’s performance, with the ability to sense incredibly tiny amounts of energy from individual particle interactions.

“It's the first time that this neutrino signal has been observed at a level signifying statistical significance. We expected to see it, and whilst that might not sound very exciting, it’s a milestone result because it demonstrates our ability to discover things with these detectors. We’re still looking for dark matter and this is almost a dry run of what would happen if we were to discover a dark matter signal, because it could look very similar to one of these neutrino signals,” Dr James said.

Dr James was one of two Australian and ARC Centre of Excellence for Dark Matter Particle Physics scientists involved in the collaboration, also including University of Sydney researcher Dr Theresa Fruth, who played a key role in commissioning the detector.

“It’s extraordinary that our detector can now observe neutrinos from the Sun,” Dr Fruth said. “We are opening a new window into solar and neutrino physics while continuing the search for dark matter.”

LZ will continue data collection through 2028, lowering thresholds for low-mass dark matter and exploring exotic interactions. Lessons from LZ are guiding the design of next-generation detectors, such as XLZD, which will study neutrinos, dark matter candidates, and other rare phenomena at an unprecedented scale.

For interviews, contact: Fleur Morrison | 0421 118 233 | fleur.morrison@unimelb.edu.au