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 $N^0\to W^{\pm }{l}^{\mp }$ ($\ell =e,\mu ,\tau$) and $L^{\pm }\to W^{\pm }\nu$ ($\nu ={\nu }_e,{\nu }_{\mu },{\nu }_{\tau }$) 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 $20.3{\mathrm{fb}}^{-1}$ of $pp$ collisions at $\sqrt{s}=8\mathrm{TeV}$ 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.

• Received 8 June 2015

DOI:https://doi.org/10.1103/PhysRevD.92.032001

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© 2015 CERN, for the ATLAS Collaboration