First Results on the Search for Lepton Number Violating Neutrinoless Double Beta Decay with the LEGEND-200 Experiment

Abstract

The LEGEND collaboration is searching for neutrinoless double beta ($0νββ$) decay by operating high-purity germanium detectors enriched in $^{76}$Ge in a low-background liquid argon environment. Building on key technological innovations from GERDA and the MAJORANA DEMONSTRATOR, LEGEND-200 has performed a first $0νββ$ decay search based on 61.0 kg yr of data. Over half of this exposure comes from our highest performing detectors, including newly developed inverted-coaxial detectors, and is characterized by an estimated background level of $0.5^{+0.3}_{-0.2}$ cts/(keV kg yr) in the $0νββ$ decay signal region. A combined analysis of data from GERDA, the MAJORANA DEMONSTRATOR, and LEGEND-200, characterized by a 90% confidence level exclusion sensitivity of $2.8 \times 10^{26}$ yr on the half-life of $0νββ$ decay, reveals no evidence for a signal and sets a new observed lower limit at $T^{0ν}_{1/2} > 1.9 \times 10^{26}$ yr (90% confidence level). Assuming the decay is mediated by Majorana neutrinos, this corresponds to an upper limit on the effective Majorana mass in the range $m_{ββ} < 75-200$ meV, depending on the adopted nuclear matrix element.

Figures (2)

• Figure 1: The energy spectrum of the first LEGEND-200 data set, corresponding to 61.0 kg yr of germanium exposure, above the ^39Ar $Q_{\textnormal{\beta}} = 565$ keV. The white histogram shows events passing quality and muon anti-coincidence cuts and with energy deposited in one single hpge detector. The main radioactive background contributors are indicated in green. The grey and red histograms show the subsets of events passing the psd cut and additionally the lar anti-coincidence cut, respectively. The inset shows a close-up around the region of interest for 0$\textnormal{\nu}^{}\textnormal{\beta}\textnormal{\beta}$ decay with finer binning. The expected contribution from the 2$\textnormal{\nu}^{}\textnormal{\beta}\textnormal{\beta}$ decay of ^76Ge ($T^{2\textnormal{\nu}^{}}_{1/2} = 2.022 \times 10^{21}\;\text{yr}$GERDA:2023wbr) corresponds to the solid blue line. The events used to set a constraint on the 0$\textnormal{\nu}^{}\textnormal{\beta}\textnormal{\beta}$ decay rate are contained by the analysis window, marked in green. Only two significant γ peaks (shaded areas) are expected within this window, and the corresponding 10 keV wide energy regions are excluded from the statistical analysis.
• Figure 2: The energy spectrum of the first LEGEND-200 data set (61.0 kg yr) after all analysis cuts in the $[1930, 2190]$ keV window. Events in the gray regions, corresponding to $\pm5$ keV around known γ lines, are excluded from the analysis. Confidence intervals from the frequentist analysis are visualized for the exposure-weighted combined background index (68% *CL, in green) and for the signal strength (90% *CL, in blue). The 0$\textnormal{\nu}^{}\textnormal{\beta}\textnormal{\beta}$ decay rate upper limit derived by including data from other ^76Ge experiments is shown in dark blue.