Dark matter detector picks up unexplained and 'unexpected' signals

lang-xepr1 — The detector at Gran Sasso
Xenon collaboration

An underground experiment with dark matter, about a mile below Abruzzo in Italy, has uncovered unexplained “over events”, suggesting the possibility that researchers have come across enticing new physics. The so-called Xenon1T experiment has a track record for interesting physical observations. It was like that last year observed “the rarest event ever recorded” – but the background data collected by the machine may have improved.

In a new study that was published on the arXiv preprint server and has not yet been reviewed by experts, xenon collaboration researchers report a new, currently inexplicable event within the detector that they do not yet fully understand. That doesn’t mean they did it found Dark matter – this event can be explained by contamination within the experiment – but there are two interesting options.

“This is certainly an exciting science, even if it is mostly a cliffhanger of a result that requires more data,” said Samuel Hinton, astrophysicist at the University of Queensland, who was not involved in the study.

The Xenon1T experiment consists of a cylindrical tank filled with over 6,000 pounds of liquid xenon and cooled to -95 degrees Celsius (-139 degrees Fahrenheit). It is so far underground that it hides any radioactive interference that could interfere with potential dark matter measurements. It is believed that about 85% of the universe is dark matter, but it is mysterious and invisible. We know that it exists because of its effects on the matter that we have can see – but we have no idea what it is or what particles it could form.

The researchers examined measurements that Xenon1T took during a science run between February 2017 and February 2018. The number of events observed was much higher than expected. The team saw 285 events, against an expected maximum of around 247.

“This is really a pretty surprising result,” said Rafael Lang, associate professor of physics and astronomy at Purdue University and co-author of the study, in a press release.

Think of it as a bag of cones. The team knows that each bag should contain 60 cones, each with 10 colors. But they found that there are 15 of a particular color – and they’re not sure how those extra cones got in there. The strange result prompted the team to ask and explore: where did this “surplus” come from? They believe there are three options:

Tritium, a rare hydrogen isotope, can be an artifact in the data.
Sun axes, a theoretical particle that is generated in the sun and has not yet been discovered.
New properties in neutrinos, subatomic particles that basically happen to everything.

“The excess may 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, a physicist at the University of Chicago and co-author of the study.

Tritium appears to form deep underground in similar dark matter experiments, and this would be the worldly possibility, but the concentration cannot be measured in the experiment. Instead, the researchers write, their calculations suggest that tritium is too small to explain the excess energy.

According to the team, the best fit for the data is the sun axis hypothesis. A sun axion is a hypothetical particle that is generated in the core of the sun with a small mass and could help explain dark matter. The excess of energy seems to indicate the discovery of this elusive and mysterious particle – but scientists cannot say for sure.

“If we needed a summary of the overall result, I would put almost all my money on tritium as an explanation at the moment, but I am confident that it will not,” said Hinton.

The Xenon1T detector was shut down in December 2018 and is currently being updated. It contains more liquid xenon and less radioactive background to improve sensitivity. The researchers believe that this will help pull the results apart a bit and give a definitive answer. There is little chance of confirming that these results are new physics, a major breakthrough that has been under development for decades. But scientists are careful and patience is key.