This section reports averages and elaborations of τ branching fractions, and combinations of upper limits on τ branching fractions to lepton-flavour-violating τ decay modes.
Branching fractions averages are obtained with a fit of τ branching fractions measurements aimed at optimally exploiting the available experimental information and described in Section 2. The fit results are used in Section 3 to test the lepton-flavour universality of the charged-current weak interaction. The “universality-improved” [1] branching fraction Be= B(τ → e ν ν) and the ratio between the hadronic branching fraction and Be, are obtained in Section 4. The value of the Cabibbo-Kobayashi-Maskawa (CKM) matrix element |Vus| from τ decays is given in Section 5. Combinations of upper limits on lepton-flavour-violating τ branching fractions are computed in Section 6. All results are obtained from inputs available through the end of 2018.