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 鈥� 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. 鈥淭his 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,鈥� says at Brown University in Rhode Island, who wasn鈥檛 part of the analysis.
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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 , 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. 鈥淵ou must never make an assumption that nature is going to do something the easy way,鈥� says Gaitskell. 鈥淣obody can accuse our universe of making the simplest and most natural-seeming choices.鈥�
Such a high-energy event can鈥檛 have come from the most simple type of interaction between a WIMP and a nucleon 鈥� instead, it must have happened through a more complicated coupling between the WIMP and the entire xenon nucleus. If that is the case, the WIMP must have a mass more than 200 times the mass of a proton.
鈥淚t鈥檚 not just that they鈥檙e seeing something, but that there鈥檚 a hint at what the underlying mechanism might be,鈥� says at the University of California, Berkeley, who wasn鈥檛 involved in the analysis. 鈥淚f this all stands up, our learning curve is going to be pretty steep from now on.鈥�
Actually detecting a dark matter particle could reveal some of the universe鈥檚 most closely-held secrets, including information about the early universe that is otherwise nearly impossible to determine, and drastically shake up our standard model of particle physics. But as Haxton says, this detection is not yet certain.
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. 鈥淚n science we sit down and we do so many damned experiments, I鈥檓 afraid you come across a 1-in-100 event fairly often,鈥� says Gaitskell.
鈥淏efore one can declare victory, you need a few more data points, but now they have something to aim for, they know where to look,鈥� says Haxton. The good news is that this analysis only covered about one-third of the data that LZ has already taken, and other dark matter detectors around the world have their own data, as well. A few more events in the same energy range could bump the discovery of WIMPs up to 5 sigma 鈥� and change our understanding of physics and the universe forever.
