New Light on Dark Matter?

From New Scientist, The first signs of dark matter particles may finally have been spotted

Researchers may have spotted the first signs of a dark matter particle. If the result is confirmed, it could be one of the most monumental discoveries in the history of physics. Dark matter makes up about 85 per cent of the universe, but for decades physicists have been unable to determine what it itself is made of. Hypotheses have ranged from different sorts of particles and forces to strangely behaving gravity and everything in between – but the leading idea has always been that of the weakly interacting massive particle, or WIMP. As we have built more and more powerful detectors to search for WIMPs, though, their failure to show up has led many to doubt their existence and move on to more exotic ideas.

That doubt may soon come to an end. A new analysis of data from the LUX-ZEPLIN (LZ) experiment in South Dakota has revealed a single particle that may be the first WIMP ever detected. “This result is certainly an exciting one, because it provides some potential positive hint for what dark matter could be, not just what dark matter could not be,” says JiJi Fan at Brown University in Rhode Island, who wasn’t part of the analysis.

LZ looks for dark matter using a 7-tonne tank of liquid xenon, surrounded by several layers of shielding and buried more than a kilometre down to keep out other types of particles. When an outside particle hits a xenon atom, it creates a tiny burst of light that is measured by sensitive detectors surrounding the tank. Researchers can then use that light to reconstruct the path of the incoming particle and the energy level of the interaction.

In the search for WIMPs, we generally look for events with energies below about 30 kiloelectronvolts (keV), says LZ spokesperson Richard Gaitskell, also at Brown University. That assumes the simplest sort of interaction, in which the WIMP is essentially bouncing off an individual nucleon in the xenon nucleus. Having not found any hints of WIMPs in that energy range, the LZ team decided to reanalyse the first 220 days of data from the detector, searching for events with higher energies. That is where they found their dark matter particle candidate, at about 248 keV. “You must never make an assumption that nature is going to do something the easy way,” says Gaitskell. “Nobody can accuse our universe of making the simplest and most natural-seeming choices.”

A WIMP Detector

In particle physics, the statistical threshold for a finding to go from an intriguing hint to a solid discovery is a number called 5 sigma, which means there is about a 1-in-3.5 million chance that a signal like this would show up as a fluke rather than a true sign of dark matter. Right now, this detection from LZ sits at 2.6 sigma, which means that there is about a 1-in-200 chance it could appear as a fluke.

That might still seem fairly secure, but in the search for dark matter it is far from enough. “In science we sit down and we do so many damned experiments, I’m afraid you come across a 1-in-100 event fairly often,” says Gaitskell.

Back in my biology days we were pretty happy with a 2.5 sigma result, provided you had done adequate correction for multiple tests. Still, basing the result on the alleged detection of a single WIMP seems a little sketchy. Your not going to get very good statistics with n=1. That is certainly a weakly interacting particle.