Search for Type-III SeeSaw heavy lepton with the ATLAS Detector at the LHC using $\sqrt {s}$=13 TeV up to 140 fb$^{–1}$

A search of heavy leptons $N^0$ and $L^±$ pair production predicted by the Type-III SeeSaw mechanism is presented. This 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. This search uses data collected by the ATLAS detector at the Large Hadron Collider in protonproton collision at a centre-of-mass energy of 13 TeV, with an integrated luminosity up to 140 fb$^{−1}$ corresponding to the full Run-2 dataset recorded between 2015-2018.The analysis focuses on all the possible production and boson decay channels of these heavy leptons, which are assumed to be degenerate in mass. The search is based on the separate optimization of each lepton multiplicity final state, considering 2, 3, 4 or more than 5 leptons. The power of the leptonic channels lies in the low expected background from Standard Model processes. Different control and signal regions are defined for each final state, designed to be orthogonal one each other. Good agreement between the number of events in data and Standard Model predictions is observed.The results are translated into exclusion limits on heavy lepton masses with a 95% confidence level using statistical uncertainties only. Assuming the branching ratios to all lepton flavours to be equal, a lower limit of 560 GeV is obtained.