This section reports a global fit of the available measurements of τ branching fractions and some elaborations of the fit results. In this edition, we do not include the combinations of upper limits on τ lepton-flavour-violating branching fractions, which were published in previous editions, since it was not possible to update this contribution in due time.
Branching fractions averages are obtained with a fit of branching fractions measurements so as to optimally exploit the available experimental information. The fit is described in Section 2. The fit results are used in Section 3 to test the lepton-flavour universality of the charged-current weak interaction. A “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| obtained from τ decays is given in Section 5.