Ultracold detector probes neutrinoless double beta decay

Proposed by Maria Goeppert Mayer in 1935, double beta decay is a nuclear weak process in which two neutrons inside a nucleus convert into protons by ejecting a pair of electrons and a pair of antineutrinos. Although particle physicists have spotted some of those rare decays, they are fixated on a variation that has never been observed: one in which the antineutrinos annihilate each other. The detection of neutrinoless double beta decay would confirm Ettore Majorana’s 1937 theory that neutrinos are their own antiparticles and would address some of the biggest fundamental mysteries in physics, particularly the dominance of matter over antimatter and the masses of neutrinos (see Physics Today, January 2010, page 20 ).

Among the experiments searching for the neutrinoless decay, which has an expected half-life of at least 10²⁵ years, is the Cryogenic Underground Observatory for Rare Events. The CUORE researchers went to great lengths to track down the elusive process. First, they installed their experiment at the underground Gran Sasso National Laboratory in Italy. By virtue of the 1.4 km layer of rock above, the lab is shielded from many of the cosmic rays and atmospheric muons that are present on Earth’s surface. To further avoid the intrusion of outside particles, the researchers enclosed the experiment in radiation-shielding lead, some of which has especially low radioactivity of its own because it dates back to ancient Rome. For their source of the hoped-for decays, the researchers chose tellurium-130, in the form of 19 towers of tellurium dioxide crystals. Among the benefits of ¹³⁰Te is that the energy emitted in the event of a double beta decay should be considerably higher than that of most of the background events that need to be filtered out during data analysis.

Setting up an ultrasensitive physics experiment often requires engineering ingenuity, and for CUORE it comes in the cooling system. Since 2017 a custom-made dilution refrigerator, which achieves temperatures near absolute zero through the cycling of both helium-3 and helium-4, has kept the 1.5-metric-ton CUORE apparatus at a nearly constant 10 mK. (The researchers say that the previous lowest temperature achieved for equipment of similar mass was six times as high.) At that low, stable temperature, the device is sensitive enough to detect the absorption of the 2527.5 keV of energy that would be transferred by the two electrons generated in the neutrinoless process.

The CUORE team reports that, in nearly four years’ worth of collected data, it did not find evidence of neutrinoless double beta decay events. The researchers set a lower limit for the half-life of the process in ¹³⁰Te and found that the mass of a hypothetical Majorana neutrino is no more than 90–305 meV. Other experiments that probe xenon-136 , germanium-76 , and other candidate nuclei for the rare decay have also reported null results in recent years. But the experiments have yet to rule out some popular models, so the searches will continue. For a next-generation experiment, the CUORE collaboration is repurposing its cooling and shielding apparatus to enclose crystals that contain another double beta emitter, molybdenum-100.

The benefits of CUORE may extend beyond particle physics. The ability to reliably maintain such massive equipment at millikelvin temperatures, the researchers say, could prove useful for quantum computing, which depends on keeping qubits cold to reduce decoherence. (CUORE collaboration, Nature 604, 53, 2022 .)