LZ Sets a World’s Best in the Hunt for Galactic Dark Matter and Gets a New Look at Neutrinos from the Sun’s Core

LZ's extreme sensitivity, designed to hunt dark matter, now also allows it to detect neutrinos -fundamental, nearly massless particles that are notoriously hard to catch -in a new way. (Fittingly, LZ sits in the same underground cavern where Ray Davis ran his decades-long, Nobel Prize-winning experiment on neutrinos).

The analysis showed a new look at neutrinos from a particular source: the boron-8 solar neutrino 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 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.

"To maximize our dark matter sensitivity, we had to reduce and carefully model our instrumental backgrounds, and worked hard in calibrating our detector to understand what types of signals solar neutrinos would produce," said Ann Wang, associate staff scientist at SLAC National Accelerator Laboratory and co-lead of the analysis. "With this dataset, we have officially entered the neutrino fog, but only when searching for dark matter with these smaller masses. If dark matter is heavier -say, 100 times the mass of a proton -we're still far away from neutrinos being a significant background, and our discovery power there is unaffected."

The boron-8 solar neutrinos interact in the detector through a process that was only observed for the first time in 2017: coherent elastic neutrino-nucleus scattering, or CEvNS. In this process, a neutrino interacts with an atomic nucleus as a whole, rather than just one of the particles inside it (a proton or neutron). Hints of boron-8 solar neutrinos interacting with xenon appeared in two detectors last year: PandaX-4T and XENONnT. Those experiments were shy of the standard threshold for a physics discovery, a confidence level known as "5 sigma," reporting 2.64 and 2.73 sigma (respectively). The new LZ result improves the significance to 4.5 sigma, passing the 3-sigma threshold that is considered "evidence."

"Seeing these neutrino interactions is a pivotal milestone," said Dan Kodroff, a Chamberlain Fellow at Berkeley Lab and co-lead of the analysis. "It simultaneously showcases LZ's ability to detect signals of cosmic origin while also giving us new avenues for probing solar and neutrino physics to test the Standard Model."