A search for the pair-production of heavy leptons (N 0 ,L ± ) predicted by the type-III seesaw theory formulated to explain the origin of small neutrino masses is presented. The decay channels N 0 →W ± l ∓ (ℓ=e,μ,τ ) and L ± →W ± ν (ν=ν e ,ν μ ,ν τ ) are considered. The analysis is performed using the final state that contains two leptons (electrons or muons), two jets from a hadronically decaying W boson, and large missing transverse momentum. The data used in the measurement correspond to an integrated luminosity of 20.3fb⁻¹ of pp collisions at √s=8 TeV collected by the ATLAS detector at the LHC. No evidence of heavy lepton pair-production is observed. Heavy leptons with masses below 325--540 GeV are excluded at the 95% confidence level, depending on the theoretical scenario considered.
