Erice: INTERNATIONAL SCHOOL OF NUCLEAR PHYSICS, 47th COURSE

Located at LNGS, LEGEND (Large Enriched Germanium Experiment for Neutrinoless ββ Decay) aims to probe the Majorana nature of the neutrino by observing the neutrinoless double beta (0νββ) decay in high-purity germanium crystals (HPGe) enriched in 76Ge and immersed in an ultra-pure instrumented liquid argon (LAr) volume. The first phase, LEGEND-200, has collected data since spring 2023. Beyond its primary physics goal, the excellent energy resolution (FWHM ∼0.1% at 2 MeV) and ultra-low background achieved by LEGEND enable searches for other rare nuclear processes.
One such process is the neutrinoless double electron capture (0νDEC), a lepton-number-violating process that can occur only if neutrinos are Majorana particles. Searches for the 0νDEC are complementary to the 0νββ study and probe the same fundamental physics.

In this contribution, we present a search for the 0νDEC of 36Ar, a naturally occurring isotope in the LAr volume of LEGEND-200. The signature is a 430 keV γ-ray from the de-excitation of the daughter nucleus 36S, detected by the HPGe array. From the γ-ray mean free path in LAr, we estimate that approximately 24 t of LAr contribute to the observable signal. For the exposure accumulated during the first LEGEND-200 science run, Monte Carlo simulations indicate a detection efficiency ∼1.8 times higher than that achieved by GERDA, the predecessor experiment that first conducted this search.

We are also investigating signal-background discrimination techniques not exploited in GERDA. In particular, the predominantly multi-site topology of the 430 keV γ-ray can be distinguished from the single-site backgrounds arising from 39Ar β decay and 76Ge 2νββ decay. Using the newly developed LEGEND pulse shape simulation framework, we are studying dedicated pulse shape discrimination cuts optimized with machine-learning approaches based on transformer architectures.

In the absence of a signal, thanks to the improved detection efficiency and enhanced background rejection, LEGEND-200 has the potential to extend the current GERDA lower limit of $1.5×10^{22}$ yr (90% C.L.).