Dark matter sensor picks up an “unexpected” and unexplained signal

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The Gran Sasso detector Xenon collaboration

An underground dark matter experiment, about a mile below the Abruzzo mountains in Italy, detected unexplained “excessive events”, hinting at the possibility that the researchers had come across some exciting new physics. The Xenon1T experiment, as it is called, has a proven track record of interesting physical observations. Last year, he observed “the rarest event ever recorded” – but the background data collected by the machine may have been improved.

In a new study, published on the arXiv preprint server and not yet peer-reviewed, researchers from the Xenon collaboration report a new, currently inexplicable event in the detector that they do not yet fully understand. Now, that doesn’t mean they have found dark matter – this event can be explained by a contamination within the experiment – but there are two interesting possibilities.

“It’s an exciting science, even if it’s mostly a cliffhanger of an outcome that requires more data,” said 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 6000 pounds of liquid xenon cooled to -139 degrees Fahrenheit (-95 degrees Celsius). It is so far underground that it blocks any radioactive interference that could interfere with potential dark matter measurements. It is believed that about 85% of the universe is made up of dark matter, but it is mysterious and invisible. We know it exists due to its effects on matter that we can see – but we have no idea what it is or what particles might make it up.

The researchers examined the measures taken by Xenon1T during a scientific campaign between February 2017 and February 2018. The number of events observed was much higher than expected. The team attended 285 events, compared to 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 of the study, in a statement.

This led the team to wonder and explore: where does this “excess” of low-energy events come from? They believe there are three possibilities for the excess:

  • Tritium, a rare isotope of hydrogen, can be an artifact in the data.
  • Solar axions, a theoretical particle produced in the sun that has not been detected before.
  • New properties detected in neutrinos, subatomic particles which cross almost everything.

“The excess could be due to small traces of tritium, but the idea that we could be sitting on something more exotic is really exciting for us,” said Luca Grandi, physicist at the University of Chicago and co -author of the study.

Tritium seems to form in similar experiments on dark matter deep underground and that would be the common possibility, but the concentration cannot be measured in the experiment. Instead, the researchers write, their calculations suggest that the tritium is too small to account for excess energy.

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 heart of the sun with a low mass that could help explain dark matter. The excess energy seems to point to the detection of this elusive and mysterious particle – but scientists cannot say for sure.

“If we need a summary of the overall result, I would put almost all of my money on tritium at the moment, but I still hope that is not the case,” said Hinton.

The Xenon1T detector was discontinued in December 2018 and is in the process of obtaining a more sensitive upgrade – it will contain more liquid xenon and a lower radioactive background – and the researchers believe this will help to further separate the results further and give a more definitive answer. This may confirm that these results are new physics, a major breakthrough of several decades. But scientists are cautious and patience is the key.


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