Final Results of GERDA on the Search for Neutrinoless Double-$\beta$ Decay

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

Figure 1

Energy distribution of GERDA Phase II events between 1.0 and 5.3 MeV before and after analysis cuts; the exposure is 103.7 kg yr. The expected distribution of $2\nu \beta \beta$ decay events is shown assuming the half-life measured by GERDA [31]. The prominent $\gamma$ lines and the $\alpha$ population around 5.2 MeV are also labeled.

Figure 2

Top: Enlarged view of the energy distribution of GERDA Phase II events between 1900 and 2650 keV before and after analysis cuts. This energy interval includes the analysis window (edges marked by dashed lines) and the regions of expected $\gamma$ lines (marked by gray areas), among those the prominent $\gamma$ line at 2615 keV. Bottom: Result of the unbinned extended likelihood fit: The blue peak displays the expected $0\nu \beta \beta$ decay signal for $T_{1/2}$ equal to the lower limit, $1.8\times {10}^{26}\mathrm{yr}$. Its width is the resolution ${\sigma }_k$ of the partition which contains the event closest to $Q_{\beta \beta }$. Vertical lines indicate the energies of the events in the analysis window after analysis cuts.

Figure 3

Circles: lower limit (90% C.L.) on the $0\nu \beta \beta$ decay half-life of ${}^{76}\mathrm{Ge}$ set by GERDA as a function of the exposure [3, 18, 33, 34]. Triangles: median expectation in the assumption of no signal.