Inside the XENON1T experiment, over a hundred nuclei of xenon-124 were caught capturing two of their orbiting electrons simultaneously, and transforming into tellurium-124. Do not worry about the xenon in your car lights turning to tellurium though -- just one in a thousand xenon nuclei is xenon-124, and waiting until half of it has decayed takes a trillion times longer than the age of the universe. To observe this ultra-rare process, XENON1T watched a tonne of ultra-pure liquid xenon for a year.
The observation, featured on the cover of the scientific journal 'Nature', shows detectors such as XENON1T are probing new extremes in the search for rare signals. No slower process has ever been observed in a detector.
(Un)detecting Dark Matter
XENON1T was built to search for even more exotic signals: collisions between 'dark matter' particles and xenon atoms. While astronomers and cosmologists found overwhelming evidence that ~80% of matter in our universe is dark matter, nobody knows what particles dark matter is made of, and particle physicists have been unable to detect it.
To detect dark matter, the xenon inside XENON1T was shielded from natural radioactivity to a level never previously achieved, deep underground in the LNGS laboratory under the Italian Apennines. If dark matter would interact just a tiny bit with regular matter, XENON1T should have seen signals from collisions between dark matter and xenon nuclei.
Only it didn't. This proved that dark matter interacts even more feebly with regular matter than many theorists assumed. But rare-signal detectors such as XENON1T are sensitive to other signals too, as this latest result shows.
The nature of the neutrino
Besides dark matter, XENON1T and its even more sensitive successors will search for rare elastic neutrino-nucleus collisions, and other exotic radioactive decays. This could provide insight into the nature of the neutrino. If slight variations on the process XENON1T observed -- neutrinoless double electron capture and neutrinoless double-beta decay -- are also observed, it would show the neutrino and antineutrino are the same particle (known as a "Majorana particle").
XENOnT, a larger and even better shielded upgrade of XENON1T, is currently under construction, and will resume the hunt for dark matter and other rare signals next year. Besides further analysis of the XENON1T data, the OKC's XENON team is testing ultrasensitive photomultipliers that will be the "eyes" of XENONnT, using a liquid xenon setup in the AlbaNova basement.
Reference:
Caught in the act - Nature Volume 568 Issue 7753, 25 April 2019
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