Search for type-III Seesaw heavy leptons in $pp$ collisions at $\sqrt{s}= 8$ TeV with the ATLAS Detector

TL;DR

This paper reports a search for heavy leptons predicted by the type-III seesaw model using 8 TeV proton-proton collision data from the ATLAS detector, finding no evidence and setting mass exclusion limits.

Contribution

First search for type-III seesaw heavy leptons in 8 TeV data, establishing new mass exclusion limits based on the ATLAS detector analysis.

Findings

No evidence of heavy lepton pair-production was observed.

Heavy leptons with masses below 325--540 GeV are excluded at 95% confidence level.

The analysis constrains the parameter space of the type-III seesaw model.

Abstract

A search for the pair-production of heavy leptons ($N^0,L^{\pm}$) predicted by the type-III seesaw theory formulated to explain the origin of small neutrino masses is presented. The decay channels $N^0\rightarrow W^{\pm}l^{\mp}$ ($\ell = e, \mu, \tau$) and $L^{\pm}\rightarrow W^\pm \nu$ ($\nu=\nu_e, \nu_\mu, \nu_\tau$) are considered. The analysis is performed using the final state that contains two leptons (electrons or muons), two jets from a hadronically decaying $W$ boson, and large missing transverse momentum. The data used in the measurement correspond to an integrated luminosity of $20.3\,fb^{-1}$ of $pp$ collisions at $\sqrt{s} = 8$ TeV collected by the ATLAS detector at the LHC. No evidence of heavy lepton pair-production is observed. Heavy leptons with masses below 325--540 GeV are excluded at the 95% confidence level, depending on the theoretical scenario considered.