Scientists working on the world's most sensitive dark matter detector have announced they have detected a surprising excess of background signal, the source of which is not fully understood. While, not dark matter, this is still exciting.
Between 2016 and 2018, the XENON1T experiment was working to detect hints of dark matter, the hypothetical substance that keeps galaxies in shape. As yet, there have been no confirmed detections of dark matter, but an international team of researchers has reported seeing a hint of "something" unexpected. Whether that something is regular matter, hypothetical particles, new physics, or just a fluke is too early to tell, but it's an interesting insight into what we should look for in the future.
The detector contains 3.2 tonnes of ultra-pure liquefied xenon, a noble gas found in trace amounts in Earth's atmosphere, 2 tonnes of which act as a target for particle interactions. When particles hit the xenon atoms light and electrons are liberated. The experiment is located deep inside a mountain in Italy to reduce possible background contamination from other sources such as cosmic rays. Given that dark matter doesn’t interact much with regular matter the mountain shouldn't be a problem for it to be detected through.
The researchers report in a preprint paper that all the background signals anticipated to be detected, when considered together, should total an expected 232 events. And this is where it gets exciting. The researchers observed 53 more than that. This "excess of events" was announced in an online seminar by the XENON collaboration.
So the question is what could it be?
First of all, it might be a statistical fluke. This wouldn’t be the first time an experiment detected something that disappeared after more observations. It is an unfortunate rule of the game, and this is why the discovery of new particles requires multiple experiments and independent confirmation.
Another possibility is that there is a background source that has not been considered: tritium. Tritium is a radioactive type of hydrogen with two neutrons and one proton at its center. It decays by releasing electrons at similar energies as the one observed in the excess, and you would need contamination of only a few tritium atoms in the experiment to explain it.
Neutrinos are another possibility. Neutrinos are subatomic particles similar to electrons, extremely light, and do not interact much with regular matter so they can easily traverse our planet without being hindered, or easily detected. The occasional interaction does happen and can be detected, but to explain this signal, the magnetic properties of neutrinos would have to be different than expected, meaning new physics will be necessary to explain it.
However, there is a new physics explanation that fits the data: a solar axion. Axions are hypothetical particles, some of which are considered dark matter candidates. Unfortunately, not the one supposed to have been detected here though. The axion explanation is only slightly more statistically likely than the others but none of them amount to confirmation so far.
XENON1T was responsible for the recording of the rarest event in the universe, and the detector is currently being upgraded to the XENONnT that has three times as much active xenon mass to act as target and a lower background, so the researchers are confident the true nature of this signal might be discovered soon.