Status and progress of the HFLAV-Tau group activities

Abstract

We report the status and progress of the Heavy Flavour Averaging Group work on the global fit of τ tau lepton branching fractions, on the lepton universality tests and on the measurement of the Cabibbo-Kobayashi-Maskawa matrix element |V us | using τ lepton measurements. We also review the prospects for improving the precision of the |V us | measurement.

Measurement of ${\cal{B}}(\tau \to K\,n\pi^0 \nu)$, $n=0,1,2,3$, and ${\cal{B}}(\tau \to \pi\,n\pi^0 \nu)$, $n=3,4$ by $BABAR$

“…The resulting |V us | value [4] is more than 3σ lower than the value that is obtained from the the |V ud | and |V ub | measurements with the assumption that the CKM matrix is unitary [5,4]. The experimental uncertainty of this |V us | determination is dominated by the uncertainties on the τ branching fractions into states with an odd number of kaons, which are summed to obtain B(τ → X s ν) [6].…”

On the statistical treatment of the Cabibbo angle anomaly

“…In the foreseeable future, τ decays may provide a further precision determination of the Cabibbo angle via the ratio Γ(τ → Kν τ )/Γ(τ → πν τ ), see refs. [57,58], and the number of precision observables for the determination of the Cabibbo angle rises to four. Further input is also coming up from pion beta decays [59].…”

Lepton flavour violation and neutrino masses

“…Due to its comparatively large mass, tau leptons decay via numerous leptonic and semileptonic modes, several of them violating charged lepton flavour. Figure 1 summarises the current bounds on τ cLFV decay modes, as reported by the HFLAV Collaboration [22,23]. In the future, Belle II is expected to improve these bounds by 1-2 orders of magnitude [24].…”