Search for Type III Seesaw Model Heavy Fermions in Events with Four Charged Leptons using 5.8 fb$^{-1}$ of $\sqrt{s} =$ 8 TeV data with the ATLAS Detector

A search for Type III Seesaw model heavy fermions is presented. The search is performed in a data sample corresponding to 5.8 fb${}^{-1}$ of integrated luminosity collected in 2012 in $pp$ collisions at $\sqrt{s} = 8$~TeV by the ATLAS detector at the LHC. Charged heavy fermions, $N^{\pm}$, are reconstructed in the channel $Z(\ell\ell)+\ell^{\pm}$ together with an additional charged lepton from the decay of a partner heavy fermion, $N^{0/\mp} \to \ell X$, where $\ell=e,~\mu$. No evidence of the $N^{\pm}$ is observed. Upper limits at 95\% confidence level are set on the production cross section of $N^+N^0$ times the branching fraction to the examined four lepton final states. When interpreted in the context of the Type III Seesaw model, the results translate into a lower limit on the mass of the $N$ states of 245 GeV at 95\% confidence level.