Deep beneath the mountains of Gran Sasso in Italy, the world’s most sensitive dark matter experiment has made a surprise detection. No, it’s not dark matter. Instead, the experiment has detected significantly more particle interaction events than predicted by the standard model of particle physics.
Instead of the 232 ± 15 low-energy events expected in a year’s worth of data, from February 2017 to February 2018, the XENON1T Dark Matter Experiment detected 285 – a whopping 53 more than the prediction, and well outside the error margin.
Excitingly, the large international team of physicists involved in the collaboration doesn’t know what’s causing the excess, even though they have been working on the results since 2018.
After careful consideration, they have boiled their options down to three possibilities: one fairly mundane… and two others that would have a huge impact on our understanding of fundamental physics.
The researchers presented their findings in an online seminar on June 17, and have prepared a paper that is currently in pre-print ahead of peer review.
“We observe an excess that’s greater than three sigma, and we don’t know what it is,” said physicist Evan Shockley of the University of Chicago.
XENON1T is a tank filled with 3.2 metric tons of ultra-pure liquid xenon, and fitted with arrays of photomultiplier tubes. It’s completely sealed and completely dark, in order to detect the scintillation and electroluminescence produced when two particles interact with each other, producing tiny flashes of light and a tiny shower of electrons ejected from a xenon atom – what is known as electron recoil.
Since most of these interactions occur from known particles, it’s a relatively straightforward matter to estimate the number of background events that should be occurring. This is how the number 232 for low-energy electron recoil events was derived.
So, “whence the additional 53 events” is the big question.
The first, and most mundane, of the three scenarios that could have produced additional particle interactions is a previously unconsidered source of background events, caused by very small amounts of a rare radioactive isotope of hydrogen called tritium.
Tritium, the researchers noted, could have been introduced into the detector through the cosmogenic activation of xenon, and hydrogen in the detector materials themselves. It would only take a minute amount of tritium – just a few atoms for every 1025 atoms of xenon, way too small to be detected. Attempts to detect tritium by other means were fruitless, so the tritium hypothesis could be neither confirmed nor ruled out.
The second, more intriguing possibility is that the signal could be caused by neutrinos. These particles are similar to electrons but have almost no mass and no charge, and they interact with other particles very infrequently. This is just as well, since neutrinos are the most abundant particle in the Universe.
According to the team’s calculations, neutrinos could be responsible for the excess signal if they had a stronger magnetic moment – that is, magnetic strength and orientation – than we thought. If these stronger magnetic moment neutrinos are responsible for the…