**$|V_{us}|$ from $\tau$ decays in theory**

*K. Maltman, R. Hudspith, T. Izubuchi, R. Lewis, H. Ohki, J. Zanotti*

in 9th International Workshop on the CKM Unitarity Triangle

Contribution: pdf

**Abstract**

The puzzle of the $>3\sigma$ low (c.f. three-family-unitarity expections) determination of $V_{us}$ from the conventional implementation of flavor-breaking (FB) finite-energy sum rules (FESRs) employing inclusive hadronic $\tau$ decay data is revisited, problems with this implementation

identified, and an alternative implementation which cures them described. Applying this new implementation using preliminary BaBar results for the exclusive $\tau\rightarrow K^-\pi^0\nu_\tau$ branching fraction, we find $\vert V_{us}\vert=0.2229(22)_{exp}(4)_{th}$, in good agreement with results from other sources. Limitations on near-term possibilities for reducing experimental errors are discussed, and a new approach to improving this situation via dispersive analyses of strange hadronic $\tau$ data using lattice input described.