Predicting the tau strange branching ratios and implications for V_{us}

Abstract

Hadronic tau decays provide several ways to extract the Cabbibo-Kobashi-Maskawa (CKM) matrix element V_{us}. The most precise determination involves using inclusive tau decays and requires as input the total branching ratio into strange final states. Recent results from B-factories have led to a discrepancy of about 3.4 sigma from the value of V_{us} implied by CKM unitarity and direct determination from Kaon semi-leptonic modes. In this paper we predict the three leading strange tau branching ratios, using dispersive parameterizations of the hadronic form factors and taking as experimental input the measured Kaon decay rates and the tau -> K pi nu_tau decay spectrum. We then use our results to reevaluate V_{us}, for which we find |V_{us}|=0.2207 \pm 0.0027, in better agreement with CKM unitarity.