Search for evidence of the type-III seesaw mechanism in multilepton final states in proton-proton collisions at sqrt(s) = 13 TeV

TL;DR

This study searches for evidence of the type-III seesaw mechanism in multilepton final states at 13 TeV proton-proton collisions, finding no excess but setting new limits on heavy fermion masses.

Contribution

It provides the most sensitive search to date for the type-III seesaw mechanism using multilepton events at the LHC.

Findings

Heavy fermions below 840 GeV are excluded for equal branching fractions.

No significant deviation from the standard model was observed.

Limits are set on the production of heavy fermions as a function of lepton flavor branching ratios.

Abstract

A search for a signal consistent with the type-III seesaw mechanism in events with three or more electrons or muons is presented. The data sample consists of proton-proton collisions at sqrt(s) = 13 TeV collected by the CMS experiment at the LHC in 2016 and corresponds to an integrated luminosity of 35.9 inverse femtobarns. Selection criteria based on the number of leptons and the invariant mass of opposite-sign lepton pairs are used to distinguish the signal from the standard model background. The observations are consistent with the expectations from standard model processes. The results are used to place limits on the production of heavy fermions of the type-III seesaw model as a function of the branching ratio to each lepton flavor. In the scenario of equal branching fractions to each lepton flavor, heavy fermions with masses below 840 GeV are excluded. This is the most sensitive probe to date of the type-III seesaw mechanism.