The standard model of particle physics says neutrinos should be massless, but experiments have shown that they have a small but nonzero mass—the subject of the 2015 Nobel Prize in Physics. Several methods, such as the seesaw mechanism, that account for neutrino mass require neutrinos to be Majorana fermions serving as their own antiparticle. Majorana neutrinos would allow otherwise forbidden processes such as neutrinoless double-beta decay to occur.
In double-beta decay, two neutrons transform into protons through the emission of two electrons and two antineutrinos. In the neutrinoless process, the two antineutrinos effectively annihilate, which proves they are Majorana fermions. Observing such a process that is not balanced by antimatter would be an important step in understanding why the universe is dominated by matter. (See the article by Rabi Mohapatra, Physics Today, April 2010, page 68.)
Now European researchers in the Germanium Detector Array (GERDA) collaboration based at the Gran Sasso Laboratory in Italy have achieved the best sensitivity yet in the search for double-beta decay. GERDA uses high-purity Ge as the detector medium and enriches it with radioactive 76Ge, which has an unstable nucleus that decays through double-beta decay. This underground experiment consists of several Ge detectors the size of soda cans placed in a 64 000-liter bath of liquid argon inside a 590 000-liter tank of pure water.
Since the GERDA collaborators took their last batch of data in 2013, they have installed more Ge detectors to bring the number to 40. They have also achieved a background-free regime—less than one event for the energy range of interest in the total exposure time—through active background suppression strategies. In particular, they reject any signature that doesn’t look like what they’d expect from double-beta decay. Such an event would lose its energy within a few millimeters of a single detector, as shown in the top of the figure. Multiple interactions in a single detector or an event accompanied by scintillation in the liquid argon (middle of the figure) are discarded because they are likely due to a gamma ray produced by other elements in the liquid argon chamber scattering off particles in the detector.
After running for more than two years and achieving the highest-yet sensitivity of 1.1 × 1026 years, GERDA saw no neutrinoless double-beta decay. The null result provides a lower-limit half-life of 0.9 × 1026 years. The hunt for double-beta decay continues at GERDA and elsewhere. A new worldwide collaboration, the Large Enriched Germanium Experiment for Neutrinoless Double Beta Decay, or LEGEND, is currently preparing an experiment that will reach a sensitivity of 1027 years in the next five years. (M. Agostini et al., Science 365, 1445, 2019.)
