Abstract
A search for charged excited leptons decaying into a lepton and a photon has been performed using approximately 680 pb−1 of e+e− collision data collected by the OPAL detector at LEP at centre-of-mass energies between 183 and 209 GeV. No evidence for their existence was found. Upper limits on the product of the cross-section and the branching fraction are inferred. Using results from the search for singly produced excited leptons, upper limits on the ratio of the excited lepton coupling constant to the compositeness scale are calculated. From pair production searches, 95% confidence level lower limits on the masses of excited electrons, muons and taus are determined to be 103.2 GeV.
Highlights
Models in which fermions have substructure attempt to explain, among other things, the wellordered pattern of fermion generations observed in nature
This paper presents results from a search for excited electrons (e∗), muons (μ∗) and tau leptons (τ ∗) decaying electromagnetically, using data collected by the OPAL experiment at LEP
No excess of data indicating the existence of excited leptons is found in either the single or pair production search
Summary
Models in which fermions have substructure attempt to explain, among other things, the wellordered pattern of fermion generations observed in nature. The results presented in this paper are interpreted in the context of the framework described in [5, 6] In this phenomenological model, the interaction between excited leptons and a gauge boson (l∗l∗V ), which largely determines the cross-section for pair production of excited leptons, is vector-like. The single production cross-section and branching fractions of excited leptons are determined by the strength of the ll∗V coupling This interaction can be described by the following SU(2)×U(1) gauge invariant effective Lagrangian [5, 6]. For easy comparison with previously published results, limits calculated in this paper correspond to the coupling choice f = f ′ This assignment is a natural choice which forbids excited neutrinos from decaying electromagnetically. For this particular coupling choice, the electromagnetic branching fraction of charged excited leptons drops smoothly from 100% for masses below the Z0 and W± mass thresholds to about 30% for masses in excess of 200 GeV
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