We present averages of a selection of τ lepton quantities with the goal to provide the best tests of the universality of the charged-current weak interaction (Section 3) and of the Cabibbo-Kobayashi-Maskawa (CKM) matrix coefficient |Vus| from τ decays (Section 5). We focus on the averages that benefit most from the adoption of the HFLAV methodology [1], namely a global fit of the τ branching fractions that best exploits the available experimental information. Since the 2016 edition, the HFLAV-Tau group has collaborated to the determination of the τ-lepton branching fractions based on a global fit and to the related mini-review that are included in the “Review of particle physics” [2]. The differences between the PDG 2016 fit and the fit presented here are detailed in Section 2.4.
All relevant published statistical correlations are used, and a selection of measurements, particularly the most precise and the most recent ones, was studied to take into account the significant systematic dependencies from external parameters and common sources of systematic uncertainty.
Finally, we report in Section 6 the latest limits on the lepton-flavour-violating τ branching fractions and in Section 8 we determine the combined upper limits for the branching fractions that have multiple experimental results.
The τ lepton results are obtained from inputs available through summer 2016 and have been published on the web in 2016 with the label “Summer 2016”. However, there have been minor revisions since then, and we have updated tables and plots in this report with the label “Spring 2017”.