Final Results of GERDA on the Search for Neutrinoless Double-${\hat{I}}^2$ Decay

Abstract

The GERmanium Detector Array (GERDA) experiment searched for the lepton-number-violating neutrinoless double-${\hat{I}}^2$ ($0\hat{I}½{\hat{I}}^2{\hat{I}}^2$) decay of ${}^{76}$Ge, whose discovery would have far-reaching implications in cosmology and particle physics. By operating bare germanium diodes, enriched in ${}^{76}$Ge, in an active liquid argon shield, GERDA achieved an unprecedently low background index of $5.2\times 10^{-4}$ counts/(keV$\cdot$kg$\cdot$yr) in the signal region and met the design goal to collect an exposure of 100 kg$\cdot$yr in a background-free regime. When combined with the result of Phase I, no signal is observed after 127.2 kg$\cdot$yr of total exposure. A limit on the half-life of $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$ decay in ${}^{76}$Ge is set at $T_{1/2}>1.8\times 10^{26}$ yr at 90% C.L., which coincides with the sensitivity assuming no signal.

7 pages, 3 figures, submitted to Physical Review Letters