Collaboration of dark matter observes the rarest event ever recorded

How do you observe a process that takes more than one trillion times longer than the age of the universe? The XENON Collaboration research team did it with an instrument designed to search for the most elusive particle of the universe – dark matter. In an article to be published tomorrow in the journal Nature, researchers have reported observing the radioactive decay of xenon-124, whose half-life is 1.8 x 1022 years.

"We have actually seen this degradation happen. This is the longest and slowest process ever observed directly, and our dark matter detector was sensitive enough to measure it, "said Ethan Brown, an assistant professor of physics at Rensselaer and co-author of the study. "It's great to witness this process and it is said that our detector can measure the rarest thing ever recorded."

The XENON collaboration exploits XENON1T, a 1,300-kilogram vessel of ultra-pure high-purity liquid xenon protected from cosmic rays in a cryostat submerged in deep water at 1,500 meters under the Gran Sasso Mountains in Italy. Researchers are looking for dark matter (five times more abundant than ordinary matter, but rarely interact with ordinary matter) by recording tiny flashes of light created when particles interact with xenon in the detector. And while XENON1T was built to capture the interaction between a dark matter particle and the nucleus of a xenon atom, the detector actually captures the signals of any interaction with xenon.

Evidence for the decay of xenon has been produced as a proton inside the nucleus of a xenon atom converted to a neutron. In most elements subject to disintegration, this occurs when an electron is pulled into the nucleus. But a proton in a xenon atom must absorb two electrons to turn into a neutron, an event called "double electron capture".

Double electron capture only happens when two of the electrons are right next to the nucleus at the right time, said Brown, which is "a rare thing multiplied by another rare thing, making it ultra-rare."

When the ultra rare occurred and a double electron capture took place inside the detector, the instruments picked up the signal from the electrons in the atom in reorganizing to fill the two that were absorbed by the nucleus.

"The double-capture electrons are removed from the innermost shell around the nucleus, which creates room in this shell," Brown said. "The remaining electrons have collapsed in the ground state, and we have seen this process of collapse in our detector."

This is the first time scientists have measured the half-life of this xenon isotope on the basis of direct observation of its radioactive decay.

"This is a fascinating discovery that advances the frontiers of knowledge on the most fundamental characteristics of matter," said Curt Breneman, Dean of the School of Science. "Dr. Brown's work to calibrate the detector and ensure that xenon was washed to the highest purity possible was critical to achieving this important observation. "

The XENON collaboration brings together more than 160 scientists from Europe, the United States and the Middle East. Since 2002, it has been operating three increasingly sensitive liquid xenon detectors at the Gran Sasso National Laboratory in Italy. XENON1T, the largest detector of its kind ever built, acquired data from 2016 to December 2018, when it was shut down. Scientists are currently upgrading the experiment for the new XENONnT phase, which will feature an active detector mass three times that of XENON1T. Associated with a reduced background level, the sensitivity of the detector will be amplified by an order of magnitude.

Brown's participation in the project is funded by a grant from the National Science Foundation.

Publication: XENON Collaboration, "Observation of Double Neutrino Electron Capture in ¹²⁴Xe with XENON1T, "Nature, volume 568, pages 532-535 (2019)