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

Journal of High Energy Physics, Dec 2014

Abstract 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.

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

Radovan Dermsek 0 1 Jonathan P. Hall 0 1 Enrico Lunghi 0 1 Seodong Shin 0 1 Open Access 0 1 c The Authors. 0 1 0 727 E. Third St. , Bloomington, IN 47405 , U.S.A 1 Physics Department, Indiana University limits on a new charged lepton, e4, 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 e4 W , the general limits in two scenarios with e4 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 e44 and 44 in addition to e4+e, where 4 is a new neutral state accompanying e4. We find that some combinations 4 of branching ratios are poorly constrained, whereas some are constrained up to masses of more than 500 GeV. In the doublet case, assuming BR(4 below about 300 GeV are ruled out. Even if this condition is relaxed and additional decay modes, 4 Z and 4 h, 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. - e Z, and e h. 4 4 We derive generally applicable limits on the new lepton Contents 1 Introduction 2 3 4 Model and strategy Event generation Extraction of the limits Five-lepton cut Discussion and conclusions Introduction Generally applicable limits compared to predicted cross-sections Combined results under different assumptions to the Higgs boson and thus they are much less constrained. also ref. [20] and references therein. somewhat stronger than ours are obtained. and a different algorithm for identifying hadronic taus [22]. hadronic taus; leptonically decaying taus show up as electrons and muons.) They set energy or just generally. Model and strategy most general Yukawa and mass terms for the muon and new leptons are MLLLLR ME ELER + c.c., are generated (see ref. [16]). should not affect the results. W +)BR(e4 Z)). BR(4 W +) = BR(4 W +). below Higgs threshold above Higgs threshold below Higgs threshold above Higgs threshold results under different assumptions. does not couple to heavy neutrinos. variables in each case is summarised in table 1. The strategy is to produce event samples of 20 possible processes (pp that the observed number of events for this cut is zero. not really constraining for typical sizes of heavy lepton production. five-lepton cut. Event generation e. Kinematically, the only differences between these processes are the different proportions 4 if the relevant branching ratios are actually constrained. of events. We generate enough events in each case such that the best cut efficiency is calculated with sufficient accuracy (at least 100K events per final state). using FastJet [39, 40], and proceeds as explained in the following subsection.3 this is the final event selection requirement. are then: communication with the main author of that Rivet analysis. classified as off-Z if there are leptons from other sources (or more than one Z). are defined in terms of the following variables: HTl the scalar sum of pT s of the three leptons that define the event, min plT the minimum pT of the three leptons, Tj the magnitude of the vector sum of all jet pT s, meff the scalar sum E/ T + H E/ T the missing transverse energy. summarised in table 1 of the ATLAS paper [21]. Extraction of the limits electrons and muons. expected number of events Here the likelihoods p are given by the Poisson distribution P 0.05 = ratios that these general limits imply under various assumptions. Generally applicable limits compared to predicted cross-sections future analyses could be useful. than 500 GeV. singlet case the entry is coloured blue. Combined results under different assumptions from 300 GeV upwards).5 independent parameters. straining in different directions. The ruled out parameter space at 105 and 125 GeV lepton masses. 105 125 150 200 mass / GeV 105 125 150 200 mass / GeV (e4+e4)BR(e4 Z)BR(e4 W ) limit 101 102 b f it/ 101 101 102 101 102 105 125 150 200 mass / GeV 105 125 150 200 105 125 150 200 mass / GeV mass / GeV right plots these production cross-sections are multiplied by 14 . Thus when a black point is below a charged leptons in the final state. HTj < 150 GeV HTj < 150 GeV, E/ T > 100 GeV HTj < 150 GeV, E/ T > 200 GeV HTj < 150 GeV, E/ T > 300 GeV min plT > 50 GeV HTl > 200 GeV HTl > 500 GeV meff > 1000 GeV HTj > 150 GeV, E/ T > 200 GeV / ET > 100 GeV, meff > 600 GeV / ET > 100 GeV, meff > 1200 GeV to too difficult-to-observe processes) to be useful with current data. trivial limit. Discussion and conclusions 105 125 150 200 300 400 500 105 125 150 200 300 400 500 mass / GeV mass / GeV 101 102 101 102 102 101 102 105 125 150 200 300 400 500 105 125 150 200 300 400 500 mass / GeV mass / GeV 101 102 101 102 mass / GeV mass / GeV 105 125 150 105 125 150 105 125 150 105 125 150 mass / GeV mass / GeV 101 102 101 102 101 102 105 125 150 mass / GeV 102 101 102 101 102 105 125 150 105 125 150 mass / GeV mass / GeV 105 125 150 105 125 150 mass / GeV mass / GeV 101 102 101 102 101 102 105 125 150 105 125 150 mass / GeV mass / GeV the right plots these production cross-sections are multiplied by 14 105200 GeV ruled out 105300 GeV ruled out 105400 GeV ruled out 105500 GeV ruled out push in from the bottom-left. doublet case, below Higgs threshold, 105 GeV doublet case, below Higgs threshold, 125 GeV 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Figure 9. The general doublet case for 200 GeV above the Higgs threshold. Blue points are These are the six combinations of the four independent branching ratios. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 third independent branching ratio. The general doublet case for different masses. Blue points are ruled out for all values two independent branching ratios; other points are shown in The orange/green gradient shows the upper limit on the a third branching ratio divided value BR(e4 cuts with the pT cuts in future analyses could be useful. of) heavy leptons. Acknowledgments of Energy under grant number DE-FG02-13ER42002. Open Access. This article is distributed under the terms of the Creative Commons any medium, provided the original author(s) and source are credited. [arXiv:1201.0315] [INSPIRE]. new physics, Phys. Rev. D 64 (2001) 013014 [hep-ph/0102122] [INSPIRE]. [arXiv:1111.2551] [INSPIRE]. 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Radovan Dermíšek, Jonathan P. Hall, Enrico Lunghi, Seodong Shin. Limits on vectorlike leptons from searches for anomalous production of multi-lepton events, Journal of High Energy Physics, 2014, 13, DOI: 10.1007/JHEP12(2014)013