Evidence for the decay

The inclusive production of η mesons in tau lepton decay has been studied using the High Resolution Spectrometer at the PEP e + e - facility. The data sample corresponds to an integrated luminosity of 300 pb −1 and the storage ring was operated at GeV. The η production appeaars to be only compatible with the decay τ + →π + η ν , which violates isospin and G -parity conservation. The branching ratio of 5.1±1.5% explains much of the current discrepancy between the one-prong topological branching ratio and the sum of the individual one-prong modes.