Search for Neutrinoless Double-𝛽 Decay with the LEGEND-200 Experiment

Neutrinoless double-beta (0νββ) decay is a hypothetical lepton-number-violating process whose observation would establish the neutrino as a Majorana particle — i.e., its own antiparticle — and would provide a unique window onto the absolute neutrino mass scale. The Large Enriched Germanium Experiment for Neutrinoless ββ Decay (LEGEND) collaboration searches for this decay in ⁷⁶Ge using high-purity germanium detectors. LEGEND-200, currently operating at the Laboratori Nazionali del Gran Sasso, deploys approximately 140 kg of enriched germanium detectors immersed in liquid argon as an active veto, combining the low-background heritage of GERDA and the MAJORANA DEMONSTRATOR with newly developed inverted-coaxial detectors and low-radioactivity front-end electronics. With a target background index of 2×10⁻⁴ cts/(keV·kg·yr) at the Q-value of 2039 keV, the experiment is projected to reach a 3σ discovery sensitivity to 0νββ half-lives of order 10²⁷ years after an exposure of 1 ton·yr. Based on a first 61.0 kg·yr dataset from its ongoing physics run, LEGEND-200 has achieved a background level of 0.5⁺⁰·³₋₀.₂ cts/(keV·ton·yr) in its highest-performing detectors. A combined frequentist analysis of LEGEND-200, GERDA, and MAJORANA DEMONSTRATOR data finds no evidence for 0νββ decay and sets a new lower limit of T₁/₂⁰ν > 1.9 × 10²⁶ yr at 90% C.L., with a median exclusion sensitivity of 2.8 × 10²⁶ yr. In this talk, I will present the LEGEND-200 detector design, analysis strategy, and these first physics results.