Displaced Vertex Search for Heavy Neutral Leptons with the ATLAS Detector

The results of a search for long-lived heavy neutral leptons (HNLs) in proton-proton collisions at the Large Hadron Collider are presented. The Standard Model (SM) of particle physics is an extremely successful theory and its major predictions have been precisely confirmed. However, the existence of neutrinos, with small nonzero masses, provides evidence that the SM is incomplete. Introducing HNLs into the SM is a natural way to generate the light neutrino masses through a seesaw mechanism. This search uses 139 fb$^{-1}$ of ATLAS experimental data collected between 2015 and 2018 at a centre-of-mass energy of 13 TeV. A non-standard technique is used to search for a displaced vertex from particle trajectories produced in the HNL decay to leptons. The dominant background from uncorrelated leptons crossing in the ATLAS detector is estimated using an object shuffling method. The reconstructed HNL mass is used to discriminate between signal and background. No excess of events is observed and constraints on the strength of the interactions between HNLs and neutrinos are imposed in various scenarios.