Search for Type-III Seesaw heavy leptons using proton-proton collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

The discovery of neutrino flavour oscillations implies non-null masses for these particles. The smallness of neutrino masses is difficult to accommodate in a natural way through a pure Standard Model Yukawa coupling to the Higgs field. Type-III SeeSaw mechanism extends the Standard Model, introducing at least two new triplets of fermionic fields with zero hypercharge in the adjoint representation of SU(2)$_\textrm{L}$, resulting in two heavy Dirac charged leptons and a heavy Majorana neutral lepton. In this work, the search for heavy leptons based on the $pp\rightarrow W^*\rightarrow L^{\pm}N^0\rightarrow W^{\pm}\nu W^{\pm}\ell^{\mp}$ process and using the data collected by ATLAS detector at $\sqrt{s}=13$ TeV with an integrated luminosity of 79.8 fb$^{-1}$, is performed. The power of the fully leptonic channel lies in the low expected background from Standard Model processes. No excess over the Standard Model predictions is found and a lower limit on the heavy leptons mass is set at 419 GeV at 95\% confidence level.