Several outstanding problems currently not addressed by the Standard Model (SM) of particle physics including neutrino masses, matter-antimatter asymmetry
and dark matter could be succesfully addressed by the addition of righthanded neutrinos with Majorana masses below the electroweak scale. In this thesis leptonic decays of the W boson produced in 36:1 fb􀀀1 of p s = 13TeV proton-proton collisions at the Large Hadron Collider (LHC) are searched for Heavy Neutral Leptons (HNLs). The HNLs are produced through mixing with muon or electron neutrinos, and detected in the ATLAS detector through their leptonic decays. The HNL Majorana nature is used to find the smoking-gun signature of three leptons ```0, `; `0 = e; otherwise not observed in decays of SM particles. This signature allows a search for HNLs in mass ranges of 5 to 50 GeV that decay close to the beamline. This search sets best constraints on the mixing angle in the HNL mass range 15 to 42 GeV for the mixing of HNLs with muon neutrinos and for 25 to 50 GeV for the mixing with electron neutrinos and is the first result in the mass range 5 to 50 GeV using data measured by ATLAS.