The MAJORANA DEMONSTRATOR Gives Its Final Answer about a Rare Nuclear Decay

The Science

The neutrino is one of the most abundant elementary particles in the universe. Scientists do not know its exact mass but understand that mass to be minuscule. Neutrinos have no charge and rarely interact with other forms of matter, such as other subatomic particles. However, they may hold the secret to the greater abundance of matter than antimatter in our observable universe. One approach to unlocking this secret is the observation of an extremely rare nuclear process called neutrinoless double-beta decay. Observing this process demands an ultra-sensitive radiation detector operated within a shielded environment.

The Impact

The MAJORANA DEMONSTRATOR is a proof-of-concept experiment. The researchers constructed an ultrasensitive 44-kilogram germanium radiation detector to seek the extremely rare nuclear process. Like a bigger telescope can collect more light to view fainter objects, a greater mass of germanium improves the odds of observing rare decay. The experiment achieved world-leading energy resolutions. It also showed that it is feasible for a larger detector to search for the hypothesized nuclear decay. The experiment did not find the decay’s signature. However, the collaboration advanced germanium-based radiation detector technologies and ultra-pure materials development. It is an important first step toward a larger experiment to potentially discover this rare decay.

Summary

The MAJORANA DEMONSTRATOR experiment operated at the Sanford Underground Research Facility in Lead, South Dakota, from 2016 to 2021. It was designed to search for a hypothesized but never-seen way for an atomic nucleus to fall apart in which two neutrons decay simultaneously to produce two protons, two electrons, and no neutrinos. If scientists observed this rare process, called neutrinoless double-beta decay, the absence of neutrinos would signal that neutrinos are their own antiparticles. This would help explain the excess of matter over antimatter in the early universe. Detection of this process relies on a signature based on released energy that can be masked by ubiquitous natural background radiation. To address this issue, the MAJORANA DEMONSTRATOR used precise energy characterization of decay products, electronic signal processing, and ultrapure materials.

Upon conclusion of the experiment’s primary measurements, the researchers found no evidence of neutrinoless double-beta decay. However, the findings confirm the long lifetime for this type of decay. Thus, it will take a much larger experiment to increase the chances of observing it. Among other neutrinoless double-beta decay experiments, the MAJORANA DEMONSTRATOR achieved the most precise energy measurement of the decay products and the second-lowest background. Moreover, it set the stage for the next-generation Large Enriched Ge Experiment for Neutrinoless ββ Decay (LEGEND) experiment, which has partnered with the complimentary European GERmanium Detector Array (GERDA) experiment to combine the best qualities of both experiments.

Funding

This work was supported by the Department of Energy Office of Science, Office of Nuclear Physics; the Particle Astrophysics Program and Nuclear Physics Program of the National Science Foundation; the Department of Energy through the Laboratory Directed Research & Development programs at Lawrence Berkeley National Laboratory, Los Alamos National Laboratory, Oak Ridge National Laboratory, and Pacific Northwest National Laboratory; the South Dakota Board of Regents; the Natural Sciences and Engineering Research Council of Canada; the Canada Foundation for Innovation John R. Evans Leaders Fund; the Oak Ridge Leadership Computing Facility at Oak Ridge National Laboratory; and the National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory.