Search for long-lived heavy neutral leptons with lepton flavour conserving or violating decays to a jet and an electron, muon, or tau lepton

A search for long-lived heavy neutral leptons (HNLs) is presented, which considers coupling scenarios involving all three lepton generations. Events containing two leptons (electron or muon) and jets are analysed in a data sample of proton-proton collisions corresponding to an integrated luminosity of $138~\textrm{fb}^{-1}$. A novel jet tagger, based on a deep neural network, has been developed to identify jets from a HNL decay using various features of the jet and its constituent particles. The network can be used as a powerful discriminant to probe a broad range of HNL lifetimes and masses. Contributions from background processes are determined from data in sideband regions. No excess of events in data over the expected background is observed. Limits on the HNL production cross section are derived as a function of the HNL mass and the three coupling strengths $V_{l\textrm{N}}$ to each lepton generation $l$. The best limit on the coupling strength is obtained for pure muon coupling scenarios excluding values of $|V_{\mu\textrm{N}}|^{2}>5 (4)\times10^{-7}$ for Dirac (Majorana) HNLs with a mass of $10~\textrm{GeV}$ at $95\%$ CL.