Search for Evidence of the Type-III Seesaw Mechanism in Multilepton Final States in Proton-Proton Collisions at s=13  TeV

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 fb^{-1}. Selection criteria based on the number of leptons and the invariant mass of oppositely charged 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.

A call for new physics: The muon anomalous magnetic moment and lepton flavor violation

“…A recent analysis has been conducted by the CMS collaboration [521] with 13 TeV of center-of-mass energy and 35.9f b −1 of integrated luminosity. We have implemented the model in Madgraph and assumed that all exotic fermions are degenerate in mass, m Σ , and feature 100% branching ratio into charged leptons, similarly to what has been done in [521].…”

“…We have implemented the model in Madgraph and assumed that all exotic fermions are degenerate in mass, m Σ , and feature 100% branching ratio into charged leptons, similarly to what has been done in [521]. We simulated the events using similar selection criteria as in [521].…”

“…We have implemented the model in Madgraph and assumed that all exotic fermions are degenerate in mass, m Σ , and feature 100% branching ratio into charged leptons, similarly to what has been done in [521]. We simulated the events using similar selection criteria as in [521]. Our lower mass bound on m Σ is slightly overoptimistic compared to the one found by CMS collaboration: m Σ > 900 GeV (ours), m Σ > 840 GeV (CMS) for the benchmark model where the charged fermions decay democratically into charged leptons.…”

“…We analysed only the Σ + Σ − production channel with the possible decay chains containing at least three charged leptons as enforced in [521]:…”

Lepton number violating phenomenology of d = 7 neutrino mass models

“…The same-sign dilepton searches [25][26][27][28], discussed above, can be recasted into lower mass limits valid for our models. In addtion, also multi-lepton searches [31], motivated by the seesaw type-III, can be used to obtain interesting limits. We note in passing that we have also checked that the LNV searches for lljj [15,19] are currently not competitive for the models we consider in this paper.…”