Abstract
Precision measurements of τ lepton properties, its mass, lifetime, and leptonic branching fractions, provide interesting tests of lepton universality.
At Z-pole energies, where the separation of a clean and relatively unbiased sample of τ+τ− events has been proven possible, accurate measurements of the τ leptonic branching fractions rely primarily on the ability to separate precisely the two leptonic decay modes, on the one hand, from the single-prong hadronic modes, on the other. The separation relies critically on a fine-grained calorimeter system combined with a dedicated muon system. The electron/pion separation can potentially be improved by the addition of a powerful dE/dx measurement providing a welcome experimental redundancy. A lesson from LEP is that the most precise leptonic branching fraction measurement (provided by ALEPH) was obtained via an global procedure where all τ decays were classified concurrently into the the various modes.