Anomalous Z decays: excited leptons?

Abstract

We study the possibility that the recently observed ℓ +ℓ − γ decays of the Z are due to the existence of an excited electron (muon) with approximately 75 GeV (60 GeV) mass. We point out that in a composite picture of leptons such states should fall in vectorlike SU(2) ⊗ U(1) multiplets, with weak isospin I ⩽ 3/2, and be coupled with a magnetic moment type transition to the light leptons. We study the relative rates of the Z→ ℓ +ℓ − γ or νν¯γ and W + → ℓ +νγ, ℓ +νγ, ℓ +ℓ −(ff¯) +, νν¯(ff¯) + channels, as means of determining the weak isospin assignment of the excited leptons. All present information is seen to be compatible with the existence of the required excited leptons, including the absence of observed anomalies in the e +e − → γγ cross sections and in the muon ( g − 2). For spin 1/2, the distribution of the invariant mass for the direct lepton photon pair in ℓ +ℓ − γ events is expected to be flat, leaving unexplained why, in the observed three events, this mass is found to be less than about 10 GeV.