Evidence for the decay τ- → π-η ντ

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 √s = 29 GeV. The η production appears 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