Probing Majorana neutrinos with double-$\beta$ decay

TL;DR

This paper reports on the search for neutrinoless double-beta decay using germanium detectors, setting new lower limits on the decay half-life and constraining the Majorana neutrino mass, which has implications for particle physics and cosmology.

Contribution

The study provides the most stringent lower limit on the neutrinoless double-beta decay half-life of $^{76}$Ge and constrains the effective Majorana neutrino mass.

Findings

No evidence of neutrinoless double-beta decay was observed.

Lower half-life limit of T$_{1/2}$ > 0.9×10^{26} yr at 90% C.L.

Constraints on the effective Majorana neutrino mass of 0.07 - 0.16 eV.

Abstract

A discovery that neutrinos are not the usual Dirac but Majorana fermions, i.e. identical to their antiparticles, would be a manifestation of new physics with profound implications for particle physics and cosmology. Majorana neutrinos would generate neutrinoless double-$\beta$ ($0\nu\beta\beta$) decay, a matter-creating process without the balancing emission of antimatter. So far, 0$\nu\beta\beta$ decay has eluded detection. The GERDA collaboration searches for the $0\nu\beta\beta$ decay of $^{76}$Ge by operating bare germanium detectors in an active liquid argon shield. With a total exposure of 82.4 kg$\cdot$yr, we observe no signal and derive a lower half-life limit of T$_{1/2}$ > 0.9$\cdot$10$^{26}$ yr (90% C.L.). Our T$_{1/2}$ sensitivity assuming no signal is 1.1$\cdot$10$^{26}$ yr. Combining the latter with those from other $0{\nu}\beta\beta$ decay searches yields a sensitivity to the effective Majorana neutrino mass of 0.07 - 0.16 eV, with corresponding sensitivities to the absolute mass scale in $\beta$ decay of 0.15 - 0.44 eV, and to the cosmological relevant sum of neutrino masses of 0.46 - 1.3 eV.