= Solution
At the <Z boson> pole, the accessible primary <fermion> pairs are $e^+e^-$, $\mu^+\mu^-$, $\tau^+\tau^-$, the three $\nu_i\bar\nu_i$ channels, and $u\bar u,d\bar d,s\bar s,c\bar c,b\bar b$. The <top quark> is too heavy. The charged channels also have <photon> exchange, and the electronic channel has a substantial nonresonant <Bhabha scattering> contribution. On-shell <W boson> pairs are above threshold; they are not primary $Z$ decays.
A <tau lepton> pair gives two approximately back-to-back narrow low-multiplicity cones, usually one or three charged tracks per cone, with overall charge zero and missing <energy> and <momentum> from <neutrinos>. Opposite hemispheres can show electron, muon or hadronic tags. A <decay vertex> or track impact-parameter information helps establish the short <tau lepton> flight. Reject <Bhabha scattering> and dimuon events by their nearly full visible beam <energy>, particle identification and little missing <momentum>; reject ordinary <quark> multijets by their broader high-multiplicity activity and larger visible <invariant masses>. Control two-<photon> backgrounds with visible transverse <momentum>, forward activity and event geometry. Selection efficiencies depend on decay mode and must be calibrated.
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