First results from LEGEND-200: searching for $0νββ$ decay in $^{76}$Ge

Abstract

The LEGEND Experiment searches for the neutrinoless double beta (0$νββ$) decay of $^{76}$Ge employing active high purity germanium detectors enriched in $^{76}$Ge beyond 86%. LEGEND's experimental program is articulated in two phases: LEGEND-200, currently ongoing, and LEGEND-1000, the next generation development. LEGEND-200 started operating in 2023 at Laboratori Nazionali del Gran Sasso (LNGS) and ran in a stable physics data taking regime for about one year with 142.5 kg of detectors installed. With a target background index of $2 \cdot 10^{-4} $ counts/(keV kg yr) at Q$_{ββ} \sim$ 2039 keV and a final exposure of 1000 kg yr, LEGEND-200 aims to reach a 3$σ$ discovery sensitivity for a 0$νββ$ half-life of $10^{27}$ yr. In this contribution, the LEGEND-200 experiment will be presented, with a focus on its current status and on the results obtained with the first year of data. In particular, the employed analysis routines will be introduced, the signal identification and background suppression performance will be discussed, and the background appearing in the region of interest around $Q_{ββ}$ will be analyzed. The performed analysis of LEGEND-200 data finds no evidence for a 0$νββ$ signal: a lower limit to its half-life is set instead, $T_{1/2}^{0ν} > 0.5 \cdot 10^{26}$ yr, at 90% CL. A joint GERDA + MAJORANA Demonstrator + LEGEND-200 analysis provides a limit of $T_{1/2}^{0ν} > 1.9 \cdot 10^{26}$ yr, at 90% CL. This work is supported by the U.S. DOE and the NSF, the LANL, ORNL and LBNL LDRD programs; the European ERC and Horizon programs; the German DFG, BMBF, and MPG; the Italian INFN; the Polish NCN and MNiSW; the Czech MEYS; the Slovak RDA; the Swiss SNF; the UK STFC; the Canadian NSERC and CFI; the LNGS and SURF facilities.

Figures (4)

• Figure 1: $3\sigma$ discovery sensitivity as a function of the background level. The red dashed line corresponds to the LEGEND-1000 goal of 10$^{-5}$ cts/(keV kg yr) = 0.025 cts/(FWHM t yr) with FWHM(Q$_{\beta\beta}$) $\sim$ 2.5 keV.
• Figure 2: The leftmost plot shows the expected topology of a signal-like event, with all the energy deposited in a single location (SSE). The following three plots show different topologies -and the corresponding suppression strategies- for background events: the multiplicity cut exploits Ge-Ge anticoincidences; the PSD cut exploits the correlation between the topology of the energy deposition inside a single Ge detector and the shape of the produced signal; the LAr veto cut exploits Ge-LAr anticoincidences.
• Figure 3: Left: $^{228}$Th calibration spectrum; Right: energy resolution curves.
• Figure 4: Energy spectrum acquired by LEGEND-200 (61 kg yr exposure). The efficiencies of the applied cuts are: $\epsilon_{muon}(Q_{\beta\beta}),\epsilon_{mult}(Q_{\beta\beta}) > 99.9$%, $\epsilon_{LAr}(Q_{\beta\beta}) \sim 93$%, $\epsilon_{PSD}(Q_{\beta\beta}) \sim 76-85$%. Inset: unblinded $0\nu\beta\beta$ analysis window.