A search is performed for long-lived heavy neutral leptons (HNLs), produced through the decay of a 𝑊 boson along with a muon or electron. Two channels are explored; a leptonic channel, in which the HNL decays into two leptons and a neutrino, and a semi-leptonic channel, in which the HNL decays into a lepton and a charged pion. The search is performed with 140 $fb^{-1}$ of $\sqrt{s}$= 13 TeV proton–proton collision data collected by ATLAS during Run 2 of the Large Hadron Collider. No excess of events is observed; Dirac-like and Majorana-like HNLs with masses below 14.5 GeV and mixing coefficients as small as $10^{−7}$ are excluded at the $95\%$ confidence level. The results are interpreted under different assumptions on the flavour of the leptons from the HNL decays.

Observed 95% CL for the 1SFH $e$ Dirac model on the $N$ mean proper lifetime $c\tau_N$ vs. $m_N$.