An underground dark matter experiment, about a mile below Italy's Abruzzo mountains, has detected unexplained "excess events," hinting at the possibility researchers have stumbled upon some tantalizing new physics. The Xenon1T experiment, as it's known, has a track record for interesting physics observations. Last year, it observed "the rarest event ever recorded" — but background data gathered by the machine may have gone one better.
In a new study, published on the preprint server arXiv and yet to be peer-reviewed, researchers with the Xenon collaboration report a new, currently unexplainable event within the detector they don't yet fully understand. Now, that doesn't mean they've found dark matter — this event may be explained by contamination within the experiment — but there are two interesting possibilities.
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"This is exciting science for sure, even if it is mostly a cliffhanger of a result that requires more data," says Samuel Hinton, an astrophysicist at the University of Queensland who was not associated with the study.
The Xenon1T experiment consists of a cylindrical tank filled with over 6,000 pounds of liquid xenon cooled to -139 degrees Fahrenheit (-95 degrees Celsius). It's so far underground that it blocks out any radioactive interference that could mess with potential dark matter measurements. Around 85% of the universe is thought to consist of dark matter but it is mysterious and invisible. We know it exists because of its effects on matter we can see — but we have no idea what it is or what particles might make it up.
Researchers looked at measurements taken by Xenon1T during a science run between February 2017 and February 2018. The amount of events observed was much higher than expected. The team saw 285 events, against an expected maximum of around 247.
"This is really a rather surprising result," said Rafael Lang, associate professor of physics and astronomy at Purdue University and co-author on the study, in a release.
That led the team to ask and explore: Where did this "excess" of low-energy events come from? They believe there are three possibilities for the excess:
- Tritium, a rare hydrogen isotope, may be an artifact in the data.
- Solar axions, a theoretical particle produced in the sun which hasn't been detected before.
- New properties detected in neutrinos, subatomic particles that pass through basically everything.
"The excess might be due to small traces of tritium, but the idea that we might be sitting on something more exotic is really exciting for us," said Luca Grandi, a physicist at the University of Chicago and co-author on the study.
Tritium does seem to form in similar dark matter experiments deep underground and this would be the mundane possibility, but the concentration cannot be measured in the experiment. Instead, the researchers write, their calculations suggest tritium would be too small to account for the energy excess.
The best fit for the data, according to the team, is the solar axions hypothesis. A solar axion is a hypothetical particle generated in the core of the sun with a low mass that could help explain dark matter. The energy excess seems to point toward detection of this elusive and mysterious particle — but scientists can't say for sure.
"If we need a summary of the entire result, I would at the moment put almost all my money on tritium being the explanation, but still am hopeful that it's not," Hinton said.
The Xenon1T detector was shut down in December 2018 and is in the process of getting an upgrade that is more sensitive — it will contain more liquid xenon and a lower radioactive background — and the researchers believe this will help pull the results apart a little more and give a more definitive answer. There's a chance it will confirm these results are new physics, a major breakthrough decades in the making. But scientists are cautious, and patience is key.
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