There's more to the universe than meets the eye. Dark matter, the invisible substance that accounts for 85 percent of the mass in the universe, is hiding all around us - and figuring out exactly what it is remains one of the biggest questions about how our world works.
The newest results from LUX-ZEPLIN (LZ) 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.
"We have been able to further increase the incredible sensitivity of the LUX-ZEPLIN detector with this new run and extended analysis. While we don't see any direct evidence of dark matter events at this time, our detector continues to perform well, and we will continue to push its sensitivity to explore new models of dark matter," said Rick Gaitskell, a professor at Brown University and the spokesperson for LZ. "As with so much of science, it can take many deliberate steps before you reach a discovery, and it's remarkable to realize how far we've come. Our latest detector is over 3 million times more sensitive than the ones I used when I started working in this field."
LZ is an international collaboration of 250 scientists and engineers from 37 institutions, including Lawrence Livermore National Laboratory (LLNL). Researchers at LLNL have played key roles in data acquisition, calibration, background mitigation and data analysis.
"This latest result reaffirms LZ as the world's most sensitive direct detection dark matter experiment. LZ is discovery-ready," said LLNL and LZ scientist Jingke Xu. "By observing evidence of solar neutrino interactions, which strongly resemble expected dark matter signals, we are confident in our ability to detect interactions of dark matter if its mass and interactions strength fall in the sensitivity region of LZ."
A detector with unmatched sensitivity
The new results use the largest dataset ever collected by a dark matter detector and have unmatched sensitivity. The analysis found no sign of WIMPs with a mass between 3 GeV/c2 (gigaelectronvolts/c2), roughly the mass of three protons, and 9 GeV/c2. It's the first time LZ researchers have looked for WIMPs below 9 GeV/c2, and the world-leading results above 5 GeV/c2 further narrow down possibilities of what dark matter might be and how it might interact with ordinary matter.
The results were presented today in a scientific talk at the Sanford Underground Research Facility (SURF) and will be released on the online repository arXiv. The paper will also be submitted to the journal Physical Review Letters.
Dark matter has never been directly detected, but its gravitational influence shapes how galaxies form and stay together; without it, the universe as we know it wouldn't exist. Because dark matter doesn't emit, absorb or reflect light, researchers have to find a different way to "see" it.
LZ uses 10 tonnes of ultrapure, ultracold liquid xenon. If a WIMP hits a xenon nucleus, it deposits energy, causing the xenon to recoil and emit light and electrons that the sensors record. Deep underground, the detector is shielded from cosmic rays and built from low-radioactivity materials, with multiple layers to block (or account for) other particle interactions - letting the rare dark matter interactions stand out.
Peering into the neutrino fog
LZ's extreme sensitivity also allows it to detect neutrinos - fundamental, nearly massless particles that are notoriously hard to catch - in a new way. The analysis showed boron-8 neutrinos coming from a particular source: the fusion in our sun's core.
This data is a window into how fusion in stars produces neutrinos and how the particles interact. But the signal also creates background noise, sometimes called the "neutrino fog," that competes with dark matter interactions as researchers look for lower-mass particles.
LLNL scientist Rachel Mannino serves as the Level-2 Run Manager of LZ. She oversees the detector data acquisition plans, particularly calibrations and mitigation of detector instabilities.
"The detector is regularly calibrated to measure its response to signal-like nuclear recoils and background-like electron recoils across a range of energies. We then use these calibration bands to identify whether a signal has the characteristics of a WIMP or solar neutrino or whether it should be excluded as background noise," said Mannino. "This is especially important at low energies where we start to encounter the neutrino fog."
While the background signal from neutrinos presents challenges for the dark matter detector at low masses (3-9 GeV/c2), its new secondary role as a solar neutrino observatory gives theorists more information for their models of neutrinos, which still hold many mysteries themselves.
LZ can provide an independent measurement of how many boron-8 neutrinos are coming from the sun, detect future neutrino bursts to better understand supernovae, and help study one of the fundamental parameters that describe how particles interact.
Reaching into the neutrino fog also highlights LZ's performance, with the ability to sense incredibly tiny amounts of energy from individual particle interactions.
A bright future
LZ is scheduled to collect more than 1,000 days of live search data by 2028, more than doubling its current exposure. With that enormous and high-quality dataset, LZ will become more sensitive to dark matter at higher masses in the 100 GeV/c2 to 100 TeV/c2 (teraelectronvolt) range. Collaborators will also work to reduce the energy threshold to search for low-mass dark matter below 3 GeV/c2, and search for unexpected or "exotic" ways that dark matter might interact with xenon.
"Analysis of future data in these larger data sets may reveal exciting new physics in this expanded energy range," said Mannino.
Many of the researchers from LZ are designing a next-generation WIMP hunter that can also study neutrinos, the sun, cosmic rays and other unusual candidates for dark matter, such as dark photons and axion-like particles.
More details on LZ's latest results can be found here.
