A search for the pair-production of heavy leptons ($N^0,L^{ \pm}$) predicted by the type-III seesaw theory formulated to explain the origin of small neutrino masses is presented. The decay channels $N^0\rightarrow W^{ \pm}l^{ \mp}$ ($\ell = e, \mu, \tau$) and $L^{ \pm}\rightarrow W^\pm \nu$ ($\nu=\nu_e, \nu_\mu, \nu_\tau$) 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 $\sqrt{ s} = 8$ 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.
Categories: hep-ex