New particle searches are ongoing at the large electron positron collider (LEP) running yearly at increasing centre-of-mass energies. The searches for heavy stable and unstable leptons performed by the four LEP experiments (ALEPH, DELPHI, L3 and OPAL) are summarised, followed by a description of the searches for excited leptons. Experimental signatures and results, including the most recent data analysed, are presented. (17 refs)

Vachon, B

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1Search for Heavy and Excited Leptons at LEPB. VachonaaDepartment of Physics and Astronomy, University of VictoriaP.O. Box 3055, Victoria, B.C., V8W 3P6, Canada.New particle searches are ongoing at the Large Electron Positron Collider (LEP) running yearly at increasingcentre-of-mass energies. The searches for heavy stable and unstable leptons performed by the four LEP experi-ments (ALEPH, DELPHI, L3 and OPAL) are summarised, followed by a description of the searches for excitedleptons. Experimental signatures and results, including the most recent data analysed, are presented.1. Physics Beyond the Standard ModelAlthough very successful in describing experi-mental observations, the Standard Model (SM) ofparticle physics is however incomplete. It relieson many assumptions that cannot be explainedwithin its framework. The search for physics be-yond the SM is therefore an active field whichtakes on two different approaches.One approach includes models such as GrandUnified Theories (E6,SO(10)...) and super-symmetry (SUSY). Within this frame work, theknown SM particles are regarded as being fun-damental and point-like. Some of these modelspredict the existence of new fundamental parti-cles such as heavy charged (L±) and neutral (N)leptons.The second approach to physics beyond the SMassumes that known particles have substructureat a scale Λ >∼ 1 TeV. Composite models could ex-plain the number of fermion generations (or fam-ilies) and the mass spectrum observed within theSM. Under the assumption of compositeness, it isnatural to expect the presence of excited states ofthe SM fermions.With the ongoing increase in energy and lumi-nosity at the LEP collider, new particles could bewithin experimental reach. Various searches areperformed by the four LEP experiments. Thisnote reviews the searches for heavy and excitedleptons. Experimental signatures and results ofthe search for heavy stable/long-lived and unsta-ble leptons are presented. Then, a description ofe–e+γ/Z0 L–L+e–e+Z0 NN–Figure 1. Feynman diagrams representing thepair production of heavy charged (L±) and neu-tral (N) leptons in e+e− collisions.the searches for single and pair production of ex-cited leptons follows. Unless stated, the resultspresented here include data taken in 1999 at acentre-of-mass energy up to 202 GeV, correspond-ing to an integrated luminosity of about 220 pb−1per experiment.2. Searches for Heavy LeptonsHeavy leptons can be sequential, mirror or vec-tor leptons[4]. Neutral heavy leptons could, inaddition, be Dirac or Majorana particles.The mixing between heavy and SM leptonsis severely constrained by LEP1 data and low-energy experiments [5]. Therefore, only thesearches for pair produced heavy leptons are con-sidered. Charged (L±) and neutral (N) leptonscould be pair produced in e+e− collisions via thediagrams shown in Figure 1. Unstable leptons2subsequently decay via charged (CC) or neutral(NC) current interactions (only CC decays are al-lowed for sequential leptons since flavour chang-ing neutral current (FCNC) is forbidden withinthe SM). Charged current decays generally dom-inate for the mass range and different type of un-stable leptons considered. Thus only final statesconsistent with CC decays are searched for.2.1. Heavy Unstable Charged LeptonsUnstable charged leptons decay via CC to a Wboson and a neutrino. The W boson in turn candecay either leptonically or hadronically.e+e− −→ L+ L−b→ ν` W−b→qq¯ or `−ν¯`→ ν¯` W+b→qq¯ or `+ν`The two experimental topologies considered areevents with four jets and missing momentum (dueto the presence of neutrinos from the heavy lep-ton CC decay) or events with two jets, one lep-ton and missing momentum. Final states whereboth W bosons decay leptonically are not consid-ered due to the rather small W leptonic branchingfraction. The efficiency of the different selectionsis of the order of 20% and the main backgroundcomes from hadronic and semi-leptonic W+W−events.2.2. Heavy Unstable Neutral LeptonsThe following decays of heavy unstable neutralleptons are considered.e+e− −→ N N¯b→ `± W∓b→qq¯ or `∓ (−)ν`→ `∓ W±b→qq¯ or `± (−)ν`Candidate events contain either four jets and twoisolated leptons or two jets, three leptons andmissing momentum. Furthermore, because Ma-jorana N can decay either to W+ or W−, eventsOPAL0510152025303540455010 -1 1 10 10 2p (GeV)dE/dx (keV/cm)(a)e±pi± K± p± d+051015202520 30 40 50 60 70 80 90100 200p (GeV)dE/dx (keV/cm)(b)mX=70 GeV, Q/e=±1mX=45 GeV, Q/e=±1Ecm=189 GeVEcm=183 GeVEcm=172 GeVEcm=161 GeVFigure 2. (a) Ionization energy loss (dE/dx) as afunction of momentum (p) for different SM par-ticles. The search regions are hatched. (b) Ex-panded view of the search regions. Theoreticalpredictions for different heavy long-lived particleare shown.can contain leptons of the same charge. There-fore, to remain sensitive to Majorana N, no re-quirement on the sign of the isolated leptons isimposed. The efficiency of the different selectionsis about 35% and the main background for thissearch comes from multihadronic qq¯ events andfour-fermion processes ``qq and qqqq.2.3. Heavy Stable/Long-Lived LeptonsHeavy stable or long-lived leptons produce twoback-to-back tracks. Both tracks are required tohave an ionization energy loss (dE/dx) outsidethe range of SM particle. Figure 2 shows thedE/dx distribution of SM particles as a functionof momentum (p). The hatched area is the searchregion [6]. In the dE/dx region overlapping withSM particles, heavy leptons are identified using3Table 195% CL lower mass limits for the searches ofheavy sequential leptons.Channel OPAL L3N → eW Dirac 97.8 97.1Majorana 88.8 85.7N → µW Dirac 97.1 97.0Majorana 87.7 88.0N → τW Dirac 78.9 89.0Majorana 67.5 79.6L± → ν`W± 91.3† 98.8L± stable/long-lived 98.0 93.5†† data recorded up to √s = 189 GeVkinematic properties of the event.2.4. Mass LimitsIn all searches for heavy leptons, the number ofobserved events agrees with the SM prediction [7–10]. Lower mass limits for heavy sequential lep-tons are calculated by different LEP experimentsand presented in Table 1. Mass limits for Ma-jorana neutral leptons are lower than for Diracleptons because the Majorana production cross-section decreases more rapidly with mass. Masslimits for both charged and neutral vector lep-tons are approximately 100 GeV [7]. Vector lep-tons have a larger production cross-section thansequential and mirror leptons which explains thehigher mass limits obtained.3. Searches for Excited LeptonsThe existence of excited leptons is severelyconstrained by g-2 precision measurements [11].Generally, composite models [12,13] assume thatboth left and right-handed components of excitedleptons form weak isodoublets. The couplingof excited leptons to a gauge boson (`∗`∗V) isthen purely vector. The single production cross-section and branching fractions of excited leptonsare determined by the strength of the ``∗V cou-pling. This interaction can be described by thefollowing SU(2)×U(1) gauge invariant effectiveLagrangian [12]L``∗V = 12Λ¯`∗σµν[gfτ2Wµν + g′f ′Y2Bµν]`L(a) (b)e–e+γ,Z l*,ν*l*,ν_*e–e+γ,Z l*,ν*l,ν_f/Λe–γ,Z,We+ e,ν_ee*,νe*f/ΛFigure 3. Dominant diagrams representing (a)pair and (b) single production of excited leptons.The vertices with a blob are suppressed by a fac-tor of f/Λ.where σµν is the covariant bilinear tensor, τ de-notes the Pauli matrices, Y is the weak hyper-charge, Wµν and Bµν represent the SM gaugefield tensors and the couplings g, g′ are the SU(2)and U(1) coupling constants of the SM. The pa-rameter Λ which has units of energy sets the com-positeness scale. Finally, the strength of the ``∗Vcoupling is governed by the constants f and f ′.These constants can be interpreted as weight fac-tors associated to the different gauge groups. Theassignment f = f ′ or f = −f ′ is usually chosensuch that the only free parameter (apart from themass of the excited lepton) is f/Λ. Excited lep-tons can be singly or pair produced via the di-agrams shown in Figure 3. They decay to SMleptons via the emission of γ, Z0 or W±.3.1. Pair ProductionPair produced excited leptons decaying pho-tonically are identified by events containingtwo leptons and two photons (``γγ, ννγγ) inthe final state. Excited leptons decaying viacharged current produce the same topologies(ννWW, ``WW ) as heavy leptons and thereforethe same analysis is used. In addition to thesefinal states, the L3 Collaboration [14] search for4Table 295% CL lower mass limits of excited leptons.f = f ′ f = −f ′ Coupl. Ind.L3 OPAL DELPHI ALEPH L3 OPAL L3e∗ 100.1 100.1 94.1† 94.3† 96.2 91.3† 96.0µ∗ 100.3 100.1 94.1† 94.3† 96.2 91.3† 96.2τ∗ 99.9 100.0 93.3† 94.3† 96.2 91.3† 94.9ν∗e99.3 99.5 93.6† 94.2† 99.5 94.3† 98.5ν∗µ99.4 99.5 93.7† 94.2† 99.5 94.3† 98.5ν∗τ93.9 91.9 89.5† 94.2† 99.4 94.3† 92.7† limits calculated from data recorded up to √s = 189 GeVmixed decay events where one excited lepton de-cays photonically and the other via CC (e+e− →`∗`∗ → `γνW). In all the different final statesconsidered, no excess is found in the data. The95% CL lower mass limits [9,10,14,16] of excitedleptons are shown in Table 2.3.2. Single ProductionFor single production searches, all possible ex-cited lepton decays are consider which resultsin various possible final states. Typical eventtopologies include ``γ, ννγ, `νW , ``Z and ννZ.If excited leptons exist, a peak should appear inthe invariant mass distribution of the excited lep-ton decay products. Figure 4 shows the `γ invari-ant mass for electron, muon and tau candidates.Typical mass resolutions are of the order of 0.3to 1 GeV for eγ and µγ, and around 1.5 to 2GeV for τγ events. No mass peak is observed inthe data. Limits on the coupling over compos-iteness scale (f/Λ) as a function of the excitedlepton mass are calculated assuming f = f ′ andf = −f ′ [14,15,17]. Example of these limits areshown in Figure 5 and 6.4. SummaryWith the increase in centre-of-mass energy atLEP, the search for heavy and excited leptonscontinues. The four LEP experiments have sofar not observed any excess in the data consistentwith the presence of new leptons. Using data col-lected as a centre-of-mass energy up to 202 GeV,95% lower mass limits for heavy and excited lep-01020300 25 50 75 100 125 150 175 20002468100 25 50 75 100 125 150 175 200OPAL PreliminaryMeγ (GeV)Entries/2.0 GeVMµγ (GeV)Entries/2.0 GeVMτγ (GeV)Entries/2.0 GeV024680 25 50 75 100 125 150 175 200Figure 4. Reconstructed `γ invariant mass forsingle production electron, muon and tau candi-date events. There are two entries per event dueto pairing ambiguity. The light grey histogramsrepresent excited lepton events with a mass of180 GeV and normalised to a coupling over com-positeness scale (f/Λ) of 0.5 TeV−1 for electronand 5 TeV−1 for muon and tau.510-410-310-210-11120 130 140 150 160 170 180 190 20010-310-210-11120 130 140 150 160 170 180 190 200DELPHIDELPHIPreliminaryPreliminaryml* (GeV/c2)λ/ml* (GeV-1 )ml* (GeV/c2)λ/ml* (GeV-1 )τ*ντ*µ*νµ*e*νe*(a)(b)Figure 5. 95% confidence level upper limits onthe ratio of the coupling to compositeness scaleas a function of the excited lepton mass, assum-ing f = f ′. The parameterisation used by theDELPHI Collaboration corresponds to λ/m`∗ =f/√2Λ. The regions above the curves are ex-cluded.ml* (GeV)| f | /Λ   (GeV-1 )e*µ*τ*f = f 'a) L3EXCLUDED10-410-310-2100 120 140 160 180 200mν* (GeV)| f | /Λ   (GeV-1 )νe*νµ*ντ*f = f 'b) L3EXCLUDED10-310-210-1100 120 140 160 180 200Figure 6. 95% confidence level upper limits onthe ratio of the coupling to compositeness scale,f/Λ, as a function of the excited lepton mass,assuming f = f ′. The regions above the curvesare excluded.6tons are calculated to be about 90 to 100 GeV.Furthermore, the search for singly produced ex-cited leptons yields limits on f/Λ as a functionof mass. The ongoing increase in energy and in-tegrated luminosity at LEP continues to open anew window for discovery.REFERENCES1. Super-Kamiokande Collaboration, D. Kiel-czewska for the collaboration, Nucl. Phys.Proc. Suppl. 81 (2000) 133-142;Super-Kamiokande Collaboration, Y. Fukudaet al., Phys. Rev. Lett. 81 (1998) 1562-1567.2. MACRO Collaboration, M. Spurio for the col-laboration, Nucl. Phys. A663 (2000) 779-782.3. M. Gell-Mann, P. Ramond and R. Slansky,Rev. Mod. Phys. 50 (1978) 721;T. Yanagida, Phys. Rev. D20 (1979) 2986.4. A. Djouadi, Z. Phys. C63 (1994) 317;F. Boudjema, A. Djouadi and J.L. Kneur, Z.Phys. C57 (1993) 425.5. D. London, “Exotic Fermions”, PrecisionTests of the Standard Model, 1993, ed.P.Langacher, World Scientific.6. OPAL Collaboration, K. Ackerstaff et al.,Phys. Lett. B433 (1998) 195-208.7. L3 Collaboration, “Search for Exotic Un-stable Heavy Leptons in e+e− Collisions at√s = 192-202 GeV”, L3 Note 2550, ICHEP2000 Contributed Paper 501, June 2000.8. L3 Collaboration, M. Acciarri et al., Phys.Lett. B462 (1999) 354-364.9. OPAL Collaboration, “New Particle Searchesin e+e− Collisions at√s = 189 GeV”, OPALPhysics Note PN370, November 1998.10. OPAL Collaboration, “New Particle Searchesin e+e− Collisions at√s = 192-202 GeV”,OPAL Physics Note PN418, ICHEP 2000Contributed Paper 185, November 1999.11. S.J. Brodsky and S.D. Drell, Phys. Rev. D22(1980) 2236;P. Mery, S.E. Moubarik, M. Perrottet andF.M. Renard, Z. Phys. C46 (1990) 229;F.M. Renard, Phys. Lett. B116 (1982) 264;J.I. Aranda, R. Martinez and O.A. Sampayo,Phys. Rev. D62 (2000) 013010.12. F. Boudjema, A. Djouadi and J.L. Kneur, Z.Phys. C57 (1993) 425.13. K. Hagiwara, D. Zeppenfeld and S. Ko-mamiya, Z. Phys. C29 (1985) 115.14. L3 Collaboration, “Search for Excited Lep-tons at√s = 192-202 GeV”, L3 Note 2572,ICHEP 2000 Contributed Paper 498, June2000.15. OPAL Collaboration, “Search for ChargedExcited Leptons with Photonic Decays ine+e− Collisions at√s = 189-202 GeV atLEP”, OPAL Physics Note PN434, ICHEP2000 Contributed Paper 236, July 2000.16. ALEPH Collaboration, “Search for Ex-cited Leptons in e+e− Collisions at√s = 188.6 GeV”, ALEPH Note 99-068CONF 99-042, EPS-HEP 99 ContributedPaper 7-419, June 1999;DELPHI Collaboration, “Update on theSearch for Excited Fermions at LEP”, DEL-PHI Note 99-71 CONF 258, EPS-HEP 99Contributed Paper 7-115, March 2000.17. 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Search for heavy and excited leptons at LEP

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