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.
Highlights
The future 100-km circular collider FCC at CERN is planned to operate, in its first mode, as an electron-positron machine, FCC-ee [1, 2]
The FCC-ee is designed to deliver e+e− collisions to study with the highest possible statistics the Z, W, and Higgs bosons, and the top quark
The largest systematic uncertainty (1.0 fs) came from the 7.5 μm accuracy of the vertex detector alignment. This was estimated from samples of hadronic Z decays with three tracks in one hemisphere, and its value resulted from the statistical power of the test samples
Summary
The future 100-km circular collider FCC at CERN is planned to operate, in its first mode, as an electron-positron machine, FCC-ee [1, 2]. In its first phase of operation, FCC-ee is planned to produce 3 × 1012 visible Z decays in a scan around the Z pole, more than five orders of magnitude more than at LEP This will allow an extreme precision on the determination of the Z-boson parameters that are important inputs to precision tests of the Standard Model (SM). This report focuses on two main topics: i) test of lepton universality via precision measurements of τ-lepton properties, and ii) tests of cLFV in the decay of Z bosons and in the decay of τ leptons Within both areas, FCC-ee promises sensitivities far beyond current experimental bounds. The O(103) times larger statistics at the b-factories has allowed an improvement in the lifetime measurement by a relatively modest factor of three, whereas most LEP branching fraction measurements, in particular those for the leptonic final states, still stand unchallenged. For the tests of cLFV in Z decays, LEP measurements still stand largely unchallenged
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