Search for heavy lepton resonances decaying to a $Z$ boson and a lepton in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

TL;DR

This paper reports a search for heavy leptons decaying into a Z boson and a lepton using 8 TeV proton-proton collision data from the ATLAS detector, setting limits on new particle masses in specific theoretical models.

Contribution

It presents the first ATLAS search for heavy leptons in the Z plus lepton decay channel at 8 TeV, providing new exclusion limits on vector-like and type-III seesaw models.

Findings

No significant excess observed above Standard Model background.

Most heavy lepton masses in the range 114-176 GeV are excluded for vector-like leptons.

Most masses in the range 100-468 GeV are excluded for type-III seesaw leptons.

Abstract

A search for heavy leptons decaying to a $Z$ boson and an electron or a muon is presented. The search is based on $pp$ collision data taken at $\sqrt{s}=8$ TeV by the ATLAS experiment at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb$^{-1}$. Three high-transverse-momentum electrons or muons are selected, with two of them required to be consistent with originating from a $Z$ boson decay. No significant excess above Standard Model background predictions is observed, and 95% confidence level limits on the production cross section of high-mass trilepton resonances are derived. The results are interpreted in the context of vector-like lepton and type-III seesaw models. For the vector-like lepton model, most heavy lepton mass values in the range 114-176 GeV are excluded. For the type-III seesaw model, most mass values in the range 100-468 GeV are excluded.