LZ experiment detects potential dark matter
A new analysis of LUX-ZEPLIN experiment data found a single high-energy event that might be the first detection of a WIMP, a leading dark matter candidate.
Researchers analyzing data from the LUX-ZEPLIN (LZ) experiment have identified a single particle candidate that may be the first sign of a dark matter particle. The finding, if confirmed, would be one of the most monumental discoveries in the history of physics.
Dark matter constitutes about 85 percent of the universe, yet its fundamental nature has eluded physicists for decades. The leading theoretical candidate has long been the weakly interacting massive particle, or WIMP. JiJi Fan at Brown University, who was not part of the analysis, told New Scientist this result is exciting because it provides a potential positive hint for what dark matter could be.
The LZ experiment's search
The LZ experiment is located more than a kilometre underground in South Dakota. It uses a 7-tonne tank of liquid xenon, surrounded by layers of shielding, to search for dark matter. When a particle strikes a xenon atom, it produces a flash of light measured by sensitive detectors. Researchers use this signal to reconstruct the particle's path and the interaction's energy.
Initially, the search focused on events with energies below about 30 kiloelectronvolts (keV), corresponding to the simplest type of interaction where a WIMP bounces off a single nucleon. Finding no hints in that range, the LZ team reanalyzed the first 220 days of data, looking for higher-energy events.
A high-energy candidate
Analysis of the data revealed a dark matter particle candidate at an energy of about 248 keV. LZ spokesperson Richard Gaitskell, also at Brown University, stated that one must never assume nature will do things the easy way. Such a high-energy event cannot result from the simplest WIMP-nucleon interaction. Instead, it suggests a more complex coupling between the WIMP and the entire xenon nucleus. This mechanism implies the WIMP must have a mass more than 200 times that of a proton.
Wick Haxton at the University of California, Berkeley, who was not involved in the analysis, noted the significance lies not just in seeing something, but in the hint at the underlying mechanism. He said if this stands up, the learning curve will be steep.
Statistical significance and the path forward
In particle physics, a solid discovery requires a statistical significance of 5 sigma, meaning there is about a 1-in-3.5 million chance the signal is a fluke. The current LZ detection sits at 2.6 sigma, corresponding to about a 1-in-200 chance it could appear randomly. Gaitskell cautioned that in science, one comes across a 1-in-100 event fairly often.
Haxton said that before declaring victory, more data points are needed, but now researchers have something to aim for and know where to look. The analysis covered only about one-third of the data LZ has already collected. Other global dark matter detectors also hold relevant data. A few more events in the same energy range could elevate the finding to the 5 sigma threshold, forever altering our understanding of physics and the universe.
Detecting a dark matter particle could reveal secrets about the early universe and drastically shake up the standard model of particle physics. The search continues.
