First observation of τ → 3πηντ and τ → f1πντ 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 → γγ and η → 3π0 decay modes and 1-prong decays with two π0's using the η → γγ channel. The measured branching fractions are B(τ- → π- π- π+ ηντ) = (3.4-0.5+0.6 ± 0.6) × 10-4 and B(τ- → π- 2π0ηντ) = (1.4 ± 0.6 ± 0.3) × 10-4. We observe clear evidence for f1 → ηππ substructure and measure B(τ- → f1π- ντ) = (5.8;-1.3+1.4 ± 1.8) × 10-4. We have also searched for η′(958) production and obtain 90% C.L. upper limits B(τ- → π- η′ ντ) < 7.4 × 10-5 and B(τ- → π- π0 η′ντ < 8.0 × 10-5.