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

Area of Science:

• Particle Physics
• Nuclear Physics
• Astroparticle Physics

Background:

• Neutrinoless double-beta decay (0νββ) is a hypothetical process that could reveal information about neutrino mass and the violation of lepton number conservation.
• Searches for 0νββ decay are crucial for understanding the fundamental properties of neutrinos and their role in the universe.
• The LEGEND experiment builds upon the technological advancements of the GERDA and MAJORANA DEMONSTRATOR experiments.

Purpose of the Study:

• To conduct a sensitive search for neutrinoless double-beta decay using high-purity germanium detectors.
• To establish new limits on the half-life of 0νββ decay and constrain the effective Majorana neutrino mass.
• To characterize the background levels in the LEGEND-200 detector for future searches.

Main Methods:

• Operation of high-purity germanium detectors enriched in ^{76}Ge within a low-background liquid argon environment.
• Collection and analysis of 61.0 kg·yr of data from the LEGEND-200 experiment.
• Combined analysis with data from GERDA and MAJORANA DEMONSTRATOR experiments.

Main Results:

• An estimated background level of 0.5_{-0.2}^{+0.3} cts/(keV·ton·yr) in the 0νββ decay signal region.
• No evidence for a 0νββ decay signal was observed.
• A new observed lower limit on the 0νββ decay half-life of T_{1/2}^{0ν} > 1.9 × 10^{26} yr (90% confidence level).

Conclusions:

• The search for neutrinoless double-beta decay by the LEGEND Collaboration has set stringent new limits.
• The results provide an upper limit on the effective Majorana neutrino mass, m_{ββ} < 75-200 meV, depending on nuclear matrix element calculations.
• These findings advance the quest to understand neutrino nature and fundamental symmetries.