Modelling Heavy Neutral Leptons in Accelerator Beamlines

Abstract: Heavy Neutral Leptons (HNLs) with masses $\mathcal{O}(0.1 - 1\,\,\text{GeV}/c^{2})$ are promising candidates for the simultaneous explanation of the smallness of the observed neutrino masses as well as the matter-antimatter asymmetry in the observable Universe. These particles can be produced in the decay of hadrons typically produced in a neutrino beamline used for oscillation experiments, and have sufficient lifetime to propagate to a near detector, where they decay to observable particles. For the approximation of a single new mass eigenstate mixing with the Standard Model via the lepton mixing matrix, a simulation framework based on the GENIE event generator has been developed. This module is designed to facilitate searches for HNL through a unified, minimal interface employing a detailed treatment of the kinematics and dynamics of massive unstable neutrinos, with a transparently organised suite of physics effects tracking the HNL from its production to its decay. These mechanisms are expounded on in the current work, underlining the rich landscape for novel, non-trivial physics that has already been identified in previous literature. This framework is an ongoing effort to provide a consistent and comprehensive description of heavy neutrinos from particle decays. We highlight use cases and future applications of interest to the accelerator neutrino community.