Search for type-III seesaw heavy leptons in proton-proton collisions at $\sqrt{s}=13$ TeV

A search for the pair production of heavy leptons ($N^0$, $L^\pm$) as predicted by the type-III seesaw mechanism is presented. The search uses proton-proton collision data at a centre-of-mass energy of 13 TeV corresponding to 79.8 fb$^{-1}$ of integrated luminosity recorded in 2015, 2016 and 2017 by the ATLAS detector at the Large Hadron Collider. The analysis focuses on the $N^0$ and $L^\pm$ decays with intermediate $W$ bosons and yielding two final-state light leptons (electrons or muons) of different flavor and charge combinations, with at least two jets and large missing transverse momentum in the final state. The search is optimized in six channels distinguished by the flavor combination and charge product of the final state lepton pair, where same charge or opposite charge final states are considered. Good agreement is observed between the number of events in data and Standard Model predictions. The results are translated into exclusion limits on heavy lepton $m(N^0, L^\pm)$ masses, where $N^0$ and $L^\pm$ are considered mass-degenerate. The observed lower 95 % confidence level limit on the mass of the type-III seesaw heavy leptons, where the branching fractions to all lepton flavors are assumed to be equal, is 560 GeV.