Postulating the existence of right-handed neutrinos with Majorana masses below the electroweak scale could be enough to solve the outstanding problems of neutrino masses, matter-
antimatter asymmetry, and dark matter. This talk focuses on the search for heavy neutral leptons (HNLs) produced in W boson decays using a displaced-vertex signature with the ATLAS detector. W boson decays into a muon and a muon neutrino that will mix with a HNL are considered. The prompt muon from the W decay is used for triggering and the requirement of a di-lepton vertex displaced by 4−300 mm from the interaction point helps to reduce the backgrounds to negligible levels.The search is expected to set the best constraints so far on the HNL.