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T: Fachverband Teilchenphysik

T 15: Searches/BSM I

T 15.3: Vortrag

Montag, 16. März 2026, 16:45–17:00, KH 02.018

Reconstruction of Heavy Neutral Leptons decaying into l + ρ with ATLAS using mass-constraint techniques — •Maximilian Ruhl — Institut für Physik, Humboldt-Universität zu Berlin

Heavy Neutral Leptons (HNLs) are well-motivated candidates for new physics in the GeV mass range and are investigated at the LHC, in particular with the ATLAS experiment, through displaced semileptonic decays. The channel N → ℓ ρ → ℓ π^±π⁰ might increase the search sensitivity at low HNL masses because its branching fraction is comparatively large and the visible decay products form a clean displaced-vertex signature. If the π⁰ is not explicitly reconstructed, essential kinematic information is missing. In this study, the three-momentum vector of the neutral pion is estimated using mass-constraint techniques. The photons from the π⁰ decay have a direction very close to the π⁰ momentum, hence Δ R (γ, π_reco⁰) is small. This characteristic pattern can help to reduce the background without requiring full reconstruction of the π⁰. The analysis investigates these photon-based observables and evaluates their potential relevance for future searches for semileptonic HNL decays with ATLAS.

Keywords: Heavy Neutral Leptons