Limits on Vectorlike Leptons from Searches for Anomalous Production of Multi-Lepton Events

TL;DR

This paper sets limits on new vectorlike leptons using multi-lepton event searches at ATLAS, constraining their masses and decay modes, especially in doublet and singlet scenarios, with some masses ruled out up to over 500 GeV.

Contribution

It provides model-independent limits on vectorlike lepton masses and decay branching ratios, interpreting results in both doublet and singlet cases, and suggests improvements for experimental searches.

Findings

Masses below 300 GeV are ruled out in the doublet scenario.

Certain branching ratio combinations are poorly constrained.

Almost all parameter space is excluded below the Higgs threshold.

Abstract

We consider extensions of the Standard Model by vectorlike leptons and set limits on a new charged lepton, $e_4^\pm$, using the ATLAS search for anomalous production of multi-lepton events. It is assumed that only one Standard Model lepton, namely the muon, dominantly mixes with vectorlike leptons resulting in possible decays $e_4^\pm \to W^\pm \nu_\mu$, $e_4^\pm \to Z\mu^\pm$, and $e_4^\pm \to h \mu^\pm$. We derive generally applicable limits on the new lepton treating the branching ratios for these processes as free variables. We further interpret the general limits in two scenarios with $e_4^\pm$ originating predominantly from either the $SU(2)$ doublet or the $SU(2)$ singlet. The doublet case is more constrained as a result of larger production cross-section and extra production processes $e_4^\pm \nu_4$ and $\nu_4\nu_4$ in addition to $e_4^+ e_4^-$, where $\nu_4$ is a new neutral state accompanying $e_4$. We find that some combinations of branching ratios are poorly constrained, whereas some are constrained up to masses of more than 500 GeV. In the doublet case, assuming BR$(\nu_4\rightarrow W\mu) = 1$, all masses below about 300 GeV are ruled out. Even if this condition is relaxed and additional decay modes, $\nu_4 \to Z \nu_\mu$ and $\nu_4 \to h \nu_\mu$, are allowed, below the Higgs threshold still almost all of the parameter space (of independent branching ratios) is ruled out. Nevertheless, even assuming the maximal production cross-section, which coincides with the doublet case, the new charged lepton can still be as light as the LEP-II limit allows. We discuss several possible improvements of current experimental analyses that would dramatically reduce the allowed parameter space, even with current data.