Neutrinos are curious particles. According to the standard model of particle physics they should be massless. Yet experiments show that they do have a mass, which can be explained naturally if neutrinos are assumed to be Majorana fermions — particles that are their own anti-particles. One way to prove this conjecture would be the observation of neutrinoless double-β decay, a nuclear process that can occur in several different nuclei. Here, the authors set a half-life limit of 1.1 × 1025 years for this process in the noble-gas isotope xenon-136 based on new high-exposure data from the EXO-200 detector. Their limit establishes xenon-136 as an important test bed for the search for neutrinoless double-β decay.
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