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

TL;DR

The LEGEND-200 experiment searches for neutrinoless double beta decay in germanium-76, with initial results showing no evidence but setting new lower limits on the decay half-life, advancing the search for physics beyond the Standard Model.

Contribution

This paper reports the first results from LEGEND-200, demonstrating its current status, analysis methods, and setting new limits on neutrinoless double beta decay half-life.

Findings

No evidence for 0νββ decay was observed.

A lower limit of 0.5×10^{26} years on the half-life was set.

Combined analysis with other experiments improves the limit to 1.9×10^{26} years.

Abstract

The LEGEND Experiment searches for the neutrinoless double beta (0$\nu\beta\beta$) 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$_{\beta\beta} \sim$ 2039 keV and a final exposure of 1000 kg yr, LEGEND-200 aims to reach a 3$\sigma$ discovery sensitivity for a 0$\nu\beta\beta$ 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 $Q_{\beta\beta}$ will be analyzed. The performed analysis of LEGEND-200 data finds no evidence for a 0$\nu\beta\beta$ signal: a lower limit to its half-life is set instead, $T_{1/2}^{0\nu} > 0.5 \cdot 10^{26}$ yr, at 90% CL. A joint GERDA + MAJORANA Demonstrator + LEGEND-200 analysis provides a limit of $T_{1/2}^{0\nu} > 1.9 \cdot 10^{26}$ 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.