Speaker
Description
The LEGEND experiment is designed to search for the neutrinoless double beta ($0\nu\beta\beta$) decay of $^{76}$Ge using an array of enriched High-Purity Germanium (HPGe) detectors operated as fully depleted diodes. A key feature of the point-contact detector geometries employed in LEGEND is their excellent Pulse Shape Discrimination (PSD) capability, which exploits the correlation between waveform shape and the topology of energy depositions inside the detector to discriminate signal-like bulk single-site events from background multi-site and surface events. As LEGEND moves toward its tonne-scale program, a realistic and scalable modelling of PSD effects in Monte Carlo simulations becomes an essential requirement for physics analyses.
In this poster we present a new HPGe pulse shape modelling framework that enables realistic waveform simulation and scalable PSD observables generation across the full array. The framework combines a full pulse shape simulation including charge transport, detector response, effects of the electronics, and noise, with a computationally efficient, position-dependent template-based method for extracting PSD parameters directly at the Monte Carlo level. This approach relies on the construction of a Pulse Shape Library scanning the detector volume and providing realistic template waveforms at each position. Monte Carlo interactions are then associated with these templates to infer PSD observables without performing full pulse shape simulation for every event. This method accurately reproduces the PSD observables distribution observed in physics data and represents a substantial improvement over previous HPGe pulse shape modelling techniques.
This development provides, for the first time in germanium-based double beta decay experiments, the technical foundation for the construction of background models in which PSD selections can be applied consistently to both data and simulations. Such PSD-informed background modelling constitutes a major methodological advance for LEGEND and enables a more accurate characterization of backgrounds in the energy region of interest around $Q_{\beta\beta}$, directly enhancing the $0\nu\beta\beta$ discovery sensitivity. Furthermore, the possibility to model the energy spectrum after PSD cuts over a broad energy range finally enables a wide range of Beyond Standard Model physics searches, such as exotic double beta decay modes, where high sensitivity to spectral deformations are essential.
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.