Tau-lepton Physics at the FCC-ee circular e$^+$e$^-$ Collider

Abstract

The future FCC-ee collider is designed to deliver \boldsymbol{\mathrm{e^+e^-}}e+e− collisions to study with ultimate precision the Z, W, and Higgs bosons, and the top quark. In a high-statistics scan around the Z pole, \boldsymbol{1.3\times 10^{11}}1.3×1011 events \boldsymbol{\mathrm{Z}\to\tau\tau}Z→𝛕𝛕 will be produced, the largest sample of \boldsymbol{\tau\tau}𝛕𝛕 events foreseen at any lepton collider. With their large boost, \boldsymbol{\tau}𝛕 leptons from Z decays are particularly well suited for precision measurements. The focus of this report is on tests of lepton universality from precision measurement of \boldsymbol{\tau}𝛕 properties and on tests of charged lepton flavour violation in Z decays and in \boldsymbol{\tau}𝛕 decays. In both of these areas, FCC-ee promises sensitivities well beyond present experimental limits.