First Observation of τ→ 3πηντ and τ→ f₁πντ Decays

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We have observed new channels for τ decays with an η in the final state. We study 3-prong tau decays, using the η→γγ and η→ 3 decay modes and 1-prong decays with two 's using the η→γγ channel. The measured branching fractions are (τ⁻→ π⁻π⁻π⁺ηντ) =(3.4⁺⁰.⁶₋₀.₅±0.6)×10⁻⁴ and (τ⁻→ π⁻2ηντ =(1.4±0.6±0.3)×10⁻⁴. We observe clear evidence for f₁→ηππ substructure and measure (τ⁻→ f₁π⁻ντ)=(5.8⁺¹.⁴₋₁.₃±1.8)×10⁻⁴. We have also searched for η'(958) production and obtain 90% CL upper limits (τ⁻→ π⁻η'ντ)<7.4×10⁻⁵ and (τ⁻→ π⁻η'ντ)<8.0×10⁻⁵.