Search for Type-III Seesaw Heavy Leptons in pp Collisions at √s̅ = 8 TeV with the ATLAS Detector

A search for the pair production of heavy leptons (N⁰,L^±) predicted by the type-III seesaw theory formulated to explain the origin of small neutrino masses is presented. The decay channels N⁰→W^±l^∓ (ℓ=e,μ,τ) and L^±→W^±ν {(ν=νe,νμ,ντ)} 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 20.3 fb⁻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.