The $$\uptau $$ challenges at FCC-ee

Abstract At FCC-ee, about $$1.7 \times 10^{11}\,\hbox {Z} \rightarrow \uptau ^+\uptau ^-$$ 1.7 × 10 11 Z → τ + - events will be produced. This high statistics in the clean $$\hbox {e}^+\hbox {e}^-$$ e environment opens possibility of much improved determinations $$\uptau $$ -lepton properties and, via measurement polarisation, neutral-current couplings electrons and $$\tau s. Improved measurements properties—lifetime, leptonic branching fractions, mass—allow important tests lepton universality. The experimental challenge is to match as far possible statistical uncertainties generally at $$10^{-5}$$ 5 level. applies particular lifetime measurement, which derived from 2.2-mm average flight distance, for fraction polarisation measurements, where cross-channel contamination concern. These issues raise strict detector requirements, particular, on accuracy construction alignment vertex precise calorimetric separation photons $$\uppi ^0$$ π 0 s collimated decay topologies.