1 Introduction

This section reports the averages of the τ mass (Section 2) and lifetime (Section 3), and a global fit of the available measurements of the τ branching fractions (Section 4). When the averaged measurements have asymmetric uncertainties, they are symmetrized using the quadratic average, i.e., the square root of the mean of the squares of the positive and negative uncertainties. The fit results are used in Section 5 to test the lepton-flavour universality of the charged-current weak interaction. A “universality-improved” [1] branching fraction B_e= B(τ → e ν ν) and the ratio between the hadronic branching fraction and B_e are obtained in Section 6. The value of the Cabibbo-Kobayashi-Maskawa (CKM) matrix element |V_us| obtained from τ decays is given in Section 7. Combinations of upper limits on lepton-flavour-violating τ branching fractions are computed in Section 8. We use measurements reported through September 2023 and the following ones that were made available at a later time:

• ( B(τ⁻ → µ⁻ ν_µν_τ))/( B(τ⁻ → e⁻ ν_e ν_τ)) by the Belle II experiment [2],
• B(τ⁻ → µ⁻ µ⁺ µ⁻) by the CMS experiment [3],
• B(τ⁻ → µ⁻ µ⁺ µ⁻) by the Belle II experiment [4],
• B(τ⁻ → π⁻ Λ) by the Belle II experiment [5], and
• B(τ⁻ → π⁻ Λ) by the Belle II experiment [5].