Majorana neutrinos remain at large

21 April 2023
Majorana Demonstrator cryostat

Neutrinoless double-beta decay (0νββ) remains as elusive as ever, following publication of the final results from the Majorana Demonstrator experiment at SURF, South Dakota, in February. Based on six years’ monitoring of ultrapure 76Ge crystals, corresponding to an exposure of 64.5 kg × yr, the collaboration has confirmed that the half-life of 0νββ in this isotope is greater than 8.3 × 1025 years. This translates to an upper limit of an effective neutrino mass mββ of 113–269 meV, and complements a number of other 0νββ experiments that have recently concluded data-taking. 

Whereas double-beta decay is known to occur in several nuclides, its neutrinoless counterpart is forbidden by the Standard Model. That’s because it involves the simultaneous decay of two neutrons into two protons with the emission of two electrons and no neutrinos, which is only possible if neutrinos and antineutrinos are identical “Majorana” particles such that the two neutrinos from the decay cancel each other out. Such a process would violate lepton-number conservation, possibly playing a role in the matter–antimatter asymmetry in the universe, and be a direct sign of new physics. The discovery that neutrinos have mass, which is a necessary condition for them to be Majorana particles, motivated experiments worldwide to search for 0νββ in a variety of candidate nuclei.

Germanium-based detectors have an excellent energy resolution, which is key to be able to resolve the energy of the electrons emitted in potential 0νββ decays. The Majorana Demonstrator is also located 1.5 km underground, with low-noise electronics and ultrapure in-house-grown electroformed copper surrounding the detectors to shield it from background events. Despite a lower exposure, the collaboration was able to achieve similar limits to the GERDA experiment at Gran Sasso National Laboratory, which set a lower limit on the 76Ge 0νββ half-life of 1.8 × 1026 yr. Also among the projects of the collaboration is an ongoing search for the influence of dark-matter particles in the decay of metastable 180mTa – nature’s rarest isotope. Although no hints have been found so far, the search has already improved the sensitivity of dark-matter searches in nuclei significantly. 

The search has already improved the sensitivity of dark-matter searches in nuclei significantly

Other experiments, such as KamLAND- ZEN and EXO-200, use 136Xe to search for 0νββ. While the former recently set the most stringent limit of 2.3 × 1026 yr and is ongoing, the latter arrived at a value of 3.5 × 1025 yr with a total 136Xe exposure of 234.1 kg × yr based on its full dataset. Searches at Gran Sasso with CUORE using 1t × yr exposure of 130Te led to a half-life of 2.2 × 1025 yr and at CUORE’s successor, CUPID-0, which used 82Se with a total exposure of 8.82 kg × yr, of the order 1023 yr.

Having demonstrated the required sensitivity for 0νββ detection in 76Ge, the designs of Majorana Demonstrator and GERDA have been incorporated into the next-generation experiment LEGEND-200, which uses high-purity germanium detectors surrounded by liquid argon. The experiment, based at Gran Sasso, started operations last spring and could have initial results later this year, says co-spokesperson Steven Elliot (LANL): “Once all the detectors are installed, we plan to run for five years, while the next stage, LEGEND-1000, is proceeding through the DOE Critical Decision process. We hope to begin construction in summer 2026, with first data available early next decade.”

Further reading

Majorana Collab. 2023 Phys. Rev. Lett. 130 062501. 

GERDA Collab. 2020 Phys. Rev. Lett. 125 252502.

KamLAND-Zen Collab. 2023 Phys. Rev. Lett. 130 051801.

EXO-200 Collab. 2019 Phys. Rev. Lett. 123 1611802. 

CUORE Collab. 2022 Nature 604 53.

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