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A decay with an extraordinary half-life

3 May 2019

An experiment designed to hunt dark matter catches an ultrarare process in which a nucleus captures two electrons and emits two neutrinos.

Dark-matter detectors are designed to pick out electroweak interactions that occur extremely rarely. Besides seeking to identify what most of the universe is made of, the detectors can also probe exotic weak-force processes that take place in ordinary, baryonic matter. Using the XENON1T detector, researchers have identified an elusive weak decay—two-neutrino double-electron capture—in a xenon isotope and pinned its half-life at 1.8 × 1022 years, the longest half-life ever measured directly.

Two-neutrino double-electron capture

The evidence comes from a yearlong dark-matter search at the detector in Italy. Sensors monitoring 2 metric tons of ultrapure liquid Xe clock flashes of light that are triggered by particle interactions with the Xe, a thousandth of which is in the form of 124Xe. (Though other Xe isotopes don’t undergo the exotic decay, they should be just as likely to interact with dark matter.) The standard model predicts that in a second-order weak-interaction process, the nucleus of 124Xe can convert two of its protons into neutrons after capturing the pair of electrons in the atom’s innermost shell (left in the figure). The newly formed tellurium-124 then emits two electron neutrinos and, as it relaxes from its excited state, also releases x rays and Auger electrons (right). Though most of the energy in the process gets whisked away by the hard-to-detect neutrinos, the electrons and x rays carry a small but known amount, about 64 keV. Sure enough, the researchers found a clear peak centered at 64.2 keV in the energy spectrum, with 97–155 events consistent with two-neutrino double-electron capture.

Based on the total mass of 124Xe and the duration of the experiment, the researchers calculated that it would take 1.2–2.4 × 1022 years, or 12 orders of magnitude greater than the age of the universe, for half a sample of 124Xe to decay into 124Te. The new measurement demonstrates the ability of the latest dark-matter detectors to probe ultrarare processes predicted by the standard model—and perhaps to find some surprises as well. The most enticing possibility is that a current or future experiment finds evidence of double-electron-capture decays that don’t involve the emission of neutrinos. The researchers hope that observing such a process, the half-life of which likely would be orders of magnitude greater than 1022 years, would reveal that neutrinos are their own antiparticles and provide a means of determining the particles’ absolute masses. (XENON collaboration, Nature 568, 532, 2019.)

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