Search for excited leptons in proton-proton collisions at \sqrt{s}=8\,\text{TeV}

A search for compositeness of electrons and muons is presented using a data sample of proton-proton collisions at a center-of-mass energy of collected with the CMS detector at the LHC and corresponding to an integrated luminosity of 19.7. Excited leptons () produced via contact interactions in conjunction with a standard model lepton are considered, and a search is made for their gauge decay modes. The decays considered are and , which give final states of two leptons and a photon or, depending on the -boson decay mode, four leptons or two leptons and two jets. The number of events observed in data is consistent with the standard model prediction. Exclusion limits are set on the excited lepton mass, and the compositeness scale . For the case the existence of excited electrons (muons) is excluded up to masses of 2.45 (2.47) at 95% confidence level. Neutral current decays of excited leptons are considered for the first time, and limits are extended to include the possibility that the weight factors and , which determine the couplings between standard model leptons and excited leptons via gauge mediated interactions, have opposite sign.


CERN-PH-EP/2013-037 2019/\two@digits7/\two@digits26


Search for excited leptons in proton-proton collisions at

The CMS Collaboration111See Appendix LABEL:app:collab for the list of collaboration members


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Published in the Journal of High Energy Physics as doi:10.1007/JHEP03(2016)125.

© 2019 CERN for the benefit of the CMS Collaboration. CC-BY-3.0 license

1 Introduction

The standard model (SM) of particle physics describes the observed phenomena very successfully, however it provides no explanation for the three generations of the fermion families. Attempts to explain the observed hierarchy have led to a class of models postulating that quarks and leptons may be composite objects of fundamental constituents [Pati:1975md, compositeness1, Eichten1982, Eichten:1983hw, harari, ssc-physics, Baur90, Greenberg:1974qb, Greenberg:1980ri]. The fundamental constituents are bound by an asymptotically free gauge interaction that becomes strong at a characteristic scale . Compositeness models predict the existence of excited states of quarks () and leptons () at the characteristic scale of the new binding interaction. Since these excited fermions couple to the ordinary SM fermions, they could be produced via contact interactions (CI) in collider experiments, with subsequent decay to ordinary fermions through the emission of a boson, or via CI to other fermions.

Searches at LEP [Buskulic:1996tw, Abreu:1998jw, Abbiendi:1999sa, Achard:2003hd], HERA [H1estar], and the Tevatron [CDFestar, cdfmu, d0, D0estar] have found no evidence for excited leptons. At the Large Hadron Collider (LHC) at CERN, previous searches performed by the CMS [cms-limit_new] and the ATLAS collaborations [atlas-limit_new] have also found no evidence of excited leptons, obtaining a lower limit on the mass for the case .

In this paper, a search for excited leptons ( and ) is presented, using a data sample of pp collisions at a center-of-mass energy collected with the CMS detector at the LHC in 2012 and corresponding to an integrated luminosity of  [CMS-PAS-LUM-13-001]. We consider the production of an excited lepton in association with an oppositely charged lepton of the same flavor, with subsequent radiative decays () or neutral current decays ().

2 Theory and model assumptions

The composite nature of quarks and leptons, if it exists, will manifest itself, above a characteristic energy scale , as a spectrum of excited states. Such excited fermions, f, may couple to SM leptons and quarks via a four-fermion CI that can be described by the effective Lagrangian


where is the energy scale of the substructure, assumed to be equal to or larger than the excited fermion mass. The quantities , and , defined in Ref. [Baur90], involve only left-handed currents by convention. In addition to the coupling via CI, excited fermions can also interact with SM fermions via gauge interactions. For excited leptons, the corresponding Lagrangian for the gauge-mediated (GM) interaction is given by


where and are the field-strength tensors of the SU(2) and U(1) gauge fields, and . The quantity, represents the electroweak gauge coupling with the Weinberg angle , and and are the generators of the U(1) and SU(2) groups, respectively. The quantities and are the right and left-handed components of the lepton or excited lepton. The weight factors and define the couplings between SM leptons and excited leptons via gauge interactions [Baur90]. The compositeness scales contained in and are assumed to be the same.

The excited lepton, , can decay to a SM lepton via a CI , where is a fermion, or through the mediation of a gauge boson via a gauge interaction. The following gauge-interaction-mediated decays are possible: radiative decay , charged-current decay , and neutral-current decay . All four transitions, the CI and the three gauge interactions, are possible if , while forbids decays via photon emission. Since the exact relationship between the weight factors is unknown, the results are interpreted for two extreme values: and .

Figure 1: Branching fractions for the decay of excited leptons, as a function of the ratio of their mass to their compositeness scale, for the coupling weight factors (left) and (right). The process indicates the decay via CI, while the other processes are gauge mediated decays.

In the present analysis we search for the production of excited electrons and muons, and , through a CI, which is dominant at the LHC for the model considered here. Excited leptons can also be produced via gauge interactions, but those processes involve electroweak couplings and contribute less than to the cross section at the LHC; they have therefore been neglected here. For light , the decay of excited leptons via gauge interactions is dominant, while the decay via a CI becomes dominant at high masses, as shown in Fig. 1. The decay via a CI is not considered in the simulated samples used here.

The search channels considered in this analysis are summarized in Table 2. The final state is represented by the Feynman diagram in Fig. 2 left. A second class of searches seeks decays via the emission of a boson (Fig. 2 right), with the boson decaying to either a pair of electrons, a pair of muons, or a pair of jets. This decay mode allows the phase space where , unexplored by previous LHC searches, to be investigated. The transverse momentum () of the boson coming from the decay of the excited lepton is larger for heavier excited-lepton masses, and at high the final-state particles are highly collimated. This characteristic is exploited in the decay mode, in which jet substructure techniques are used to reconstruct a “fat jet” corresponding to the boson, and in the leptonic channels where the lepton isolation is modified.

Signal samples for both and are produced using pythia8.153 [Sjostrand:pythia8, Sjostrand:2006za], which uses the leading order (LO) compositeness model described in Ref. [Baur90]. Thirteen mass points from 200 to 2600 have been simulated for all channels except the channels, which starts at 600 GeV because of the analysis thresholds. Masses below 200 GeV are excluded by previous searches at 95% confidence level. All simulated events have been passed through the detailed simulation of the CMS detector based on Geant4 [geant4] and have been re-weighted so that the distribution of pileup events (contributions from additional pp interactions in the same bunch crossing) matches that measured in data. The signal cross sections are calculated with pythia8, and are corrected using the branching fraction to the 3-body decays via CI as predicted in Ref. [Baur90], as this decay mode is not implemented in pythia. The factorization and renormalization scales are set to the mass square of the excited lepton (), is set to 10 , and the CTEQ6L1 [cteq] parametrization for the parton distribution functions (PDF) is used. This particular choice of the value of has no impact on the resulting kinematic distributions. Only the width of the resonance and the production cross section depend on . As long as the width of the is small compared to the mass resolution of the detector, the signal efficiency is independent of . Mass-dependent next-to-leading order (NLO) k-factors ranging from 1.2 to 1.35 [kfactor] are applied on the signal event yields. Production cross sections for the signals, as well as those of the different decay modes including the corresponding branching fractions are given in Table 4.

Decay mode Search channel Notation
Radiative decay
Neutral current
Table 2: Final states for excited lepton searches considered in this analysis, where , . The notation for a specific channel is provided in the right most column. For neutral currents, the last two characters in this notation refer to particles from the decay of the boson.
Figure 2: Illustrative diagrams for (left) and (right), where , . Decays of the boson to a pair of electrons, muons or jets are considered.
Production cross sections for excited leptons
(GeV) LO (pb) NLO k-factor
200 0.84 1.30
1000 25.1 1.27
1800 0.28 1.28
2600 1.35
() (pb)
200 0.36
1000 0.70
() (pb)
200 772 0.28
1000 0.20 0.68
(, )) (pb)
200 73.5 256
Table 4: Excited lepton production cross section, and product of cross section and branching fraction for each of the three processes investigated, as a function of the mass of the excited lepton. The values of the k-factors are taken from Ref. [kfactor]. The case does not apply to the channel.

3 The CMS detector

The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal diameter, providing a magnetic field of 3.8 T. Within the superconducting solenoid volume are a silicon pixel and strip tracker, a lead tungstate crystal electromagnetic calorimeter (ECAL), and a brass and scintillator hadron calorimeter (HCAL), each composed of a barrel and two endcap sections. Forward calorimeters extend the pseudorapidity ([JINST] coverage provided by the barrel and endcap detectors. Muons are measured in gas-ionization detectors embedded in the steel flux-return yoke outside the solenoid. In the barrel section of the ECAL, an energy resolution of about 1% is achieved for unconverted or late-converting photons in the tens of GeV energy range. The remaining barrel photons have a resolution of about 1.3% up to , rising to about 2.5% at . In the endcaps, the resolution of unconverted or late-converting photons is about 2.5%, while the remaining endcap photons have a resolution between 3 and 4% [PhoRec]. When combining information from the entire detector, the jet energy resolution amounts typically to 15% at 10, 8% at 100, and 4% at 1, to be compared to about 40%, 12%, and 5% obtained when the ECAL and HCAL calorimeters alone are used. The electron momentum is determined by combining the energy measurement in the ECAL with the momentum measurement in the tracker. The momentum resolution for electrons with from decays ranges from 1.7% for non-showering electrons in the barrel region to 4.5% for showering electrons in the endcaps [Khachatryan:2015hwa]. Muons are identified in the range , with detection planes made using three technologies: drift tubes, cathode strip chambers, and resistive plate chambers. Matching muons to tracks measured in the silicon tracker results in a relative resolution for muons with of 1.3–2.0% in the barrel and better than 6% in the endcaps. The resolution in the barrel is better than 10% for muons with up to 1 [Chatrchyan:2012xi]. A more detailed description of the CMS detector, together with a definition of the coordinate system used and the relevant kinematic variables, can be found in Ref. [JINST].

4 Event selections

4.1 Triggers

The selected trigger for each channel is summarized in Table 4.1. For all channels, except those with a final state, dilepton triggers are exploited: the double electron trigger is used for events with electrons in the final state, while muon events are selected by the dimuon trigger. Both triggers have identical thresholds, of 17 (8) for the leading (subleading) lepton.

Table 6: Trigger requirement, offline and -selection criteria, and event signature for all final state channels of the production and decay.
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