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First Observation of $τ\to 3πην_τ$ and $τ\to f_{1}πν_τ$ Decays

We have observed new channels for $τ$ decays with an $η$ in the final state. We study 3-prong tau decays, using the $η\toγγ$ and $η\to 3\piz$ decay modes and 1-prong decays with two $\piz$'s using the $η\toγγ$ channel. The measured branching fractions are $\B(τ^{-}\to π^{-}π^{-}π^{+}ην_τ) =(3.4^{+0.6}_{-0.5}\pm0.6)\times10^{-4}$ and $\B(τ^{-}\to π^{-}2\pizην_τ =(1.4\pm0.6\pm0.3)\times10^{-4}$. We observe clear evidence for $f_1\toηππ$ substructure and measure $\B(τ^{-}\to f_1π^{-}ν_τ)=(5.8^{+1.4}_{-1.3}\pm1.8)\times10^{-4}$. We have also searched for $η'(958)$ production and obtain 90% CL upper limits $\B(τ^{-}\to π^{-}η'ν_τ)<7.4\times10^{-5}$ and $\B(τ^{-}\to π^{-}\pizη'ν_τ)<8.0\times10^{-5}$.