Limits on vectorlike leptons from searches for anomalous production of multi-lepton events

摘要

We consider extensions of the Standard Model by vectorlike leptons and set limits on a new charged lepton, e 4 ± , 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 ±  → W ± ν μ , e 4 ±  → Zμ ±, and e 4 ±  → hμ ±. 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 ± 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 ± ν 4 and ν 4 ν 4 in addition to e 4 ± e 4 − , where ν 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(ν 4 → Wμ) = 1, all masses below about 300 GeV are ruled out. Even if this condition is relaxed and additional decay modes, ν 4 → Zν μ and ν 4 → hν μ , 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