CERN Accelerating science

ATLAS Note
Report number	ATL-PHYS-PROC-2021-100
Title	Search for Type-III SeeSaw heavy leptons in leptonic final states in $pp$ collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector
Author(s)	Carratta, Giuseppe (Universita e INFN, Bologna (IT))
Corporate Author(s)	The ATLAS collaboration
Collaboration	ATLAS Collaboration
Publication	2021
Imprint	12 Nov 2021
Number of pages	4
In:	22nd Particles and Nuclei International Conference (PANIC 2021), Lisbon, Portugal, 5 - 10 Sep 2021
Subject category	Particle Physics - Experiment
Accelerator/Facility, Experiment	CERN LHC ; ATLAS
Free keywords	EXOTICS
Abstract	Heavy leptons with masses ranging from GeV to TeV energies appear in several Beyond the Standard Model (BSM) mechanisms, proposed to explain the neutrino mass generation. The SeeSaw mechanism provides an elegant extension of the Standard Model (SM) explaining the smallness of neutrino masses. In particular, it introduces at least one extra fermionic triplet field with zero hypercharge in the adjoint representation of $SU(2)_L$ which couples to the electroweak gauge bosons. These new charged and neutral heavy leptons could be produced via EW processes at the Large Hadron Collider (LHC). This search is performed using data collected by the ATLAS detector at $\sqrt{s}$ = 13 TeV with an integrated luminosity of 139 fb$^{-1}$ corresponding to the full Run-2 dataset recorded in LHC Run 2 (2015-2018). The analysis is focused on final states with large lepton multiplicity, which allows to reject a significant part of background providing a large discovery power. For the first time, a result considering a combination of the most important Type-III SeeSaw heavy leptons decay modes is presented