Dark matter hunt yields unexplained signal
The Xenon1T detector was installed in the Italian Gran Sasso laboratory from 2016 to 2018
An experiment that looked for signs of elusive dark matter found an unexplained signal.
Scientists working on the Xenon1T experiment have found more activity in their detector than they would otherwise expect.
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This "excess of events" could indicate the existence of a previously undiscovered particle of dark matter called an axion.
Dark matter makes up 85% of the matter in the cosmos, but its nature is unknown.
Whatever it is, it does not reflect or emit detectable light, hence the name.
There are three possible explanations for the new signal from the Xenon1T experiment. Two require new physics to explain, while one of them agrees with the existence of the sun axion.
The results were published on the Arxiv pre-print server.
So far, scientists have only observed indirect evidence of dark matter. A final direct detection of dark matter particles has yet to be made.
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There are different theories to explain what this particle might look like. The most popular was the WIMP or Weakly Interacting Massive Particle.
Physicists working on the xenon test series have spent more than a decade looking for signs of these WIMPs. But the search was unsuccessful.
However, Xenon1T, the most recent iteration, was also sensitive to other candidate particles.
The experiment was carried out deep underground in Gran Sasso in Italy from 2016 to 2018.
Its detector was filled with 3.2 tons of high-purity liquefied xenon, two tons of which served as a "target" for interactions between the xenon atoms and other passing particles.
When a particle crosses the target, it can generate tiny flashes of light and free electrons from a xenon atom.
Most of these interactions - also known as events - take place with particles that we already know, such as muons, cosmic rays and neutrinos. This is what scientists call a background signal.
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A potential signal from an undiscovered particle must be strong enough to rise above this background noise.
Scientists have carefully estimated the number of background events in Xenon1T. They expected about 232, but the experiment saw 285 instead - an excess of 53 events.
An explanation could be a new, previously neglected source of background contamination caused by the presence of tiny amounts of tritium in the Xenon1T detector.
It could also be due to neutrinos, trillions of which run freely through your body every second. One explanation could be that the magnetic moment (a property of all particles) of neutrinos is greater than its value in the standard model that categorizes the elementary particles in physics.
This would be a strong indication that another new physics is needed to explain this.
The excess, however, best matches a signal from sun axes, a very bright, unrecognized particle that is also a candidate for dark matter.
Statistically, the sun axion hypothesis has a significance of 3.5 sigma.
Although this significance is quite high, it is not large enough to indicate the existence of axions. Five sigma is generally the accepted threshold for a discovery.
The meaning of both the tritium and neutrino magnetic moment hypothesis corresponds to 3.2 sigma, which means that they also agree with the data.
Scientists working on Xenon collaboration are currently upgrading to another iteration called Xenon-nT. With better data from this future version, they are confident of finding out soon whether the surplus is a statistical accident, a background contamination or something much more exciting.
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