The existence of right-handed neutrinos with Majorana masses below the electroweak scale could help address the origins of neutrino masses, the matter-antimatter asymmetry, and dark matter. In this paper, leptonic decays of W bosons from 140 fb^-1 of 13 TeV proton-proton collisions at the LHC, reconstructed in the ATLAS experiment, are used to search for heavy neutral leptons produced through their mixing with muon or electron neutrinos in a scenario with lepton number violation. The search is conducted using prompt leptonic decay signatures. The considered final states require two same-charge leptons or three leptons, while vetoing three-lepton same-flavour topologies. No significant excess over the expected Standard Model backgrounds is found, leading to constraints on the heavy neutral lepton's mixing with muon and electron neutrinos for heavy-neutral-lepton masses. The analysis excludes |U₄|² values above 8 10^-5 and |U_μ|² values above 5. 0 10^-5 in the full mass range of 8-65 GeV. The strongest constraints are placed on heavy-neutral-lepton masses in the range 15--30 GeV of |U₄|² < 1. 1 10^-5 and |U_μ|² < 5 10^-6.
ATLAS Collaboration (Thu,) studied this question.