Measurement of the $\tau^- \to \eta \pi^-\pi^+ \nu_\tau$ Branching Fraction and a Search for a Second-Class Current in the $\tau^- \to \eta'(958) \pi ^-\nu_\tau$ Decay

The "\tau\to 3\pi\eta\nu" decay with the "\eta \to \gamma\gamma" mode is studied using 384 fb^-1 of data collected by the BaBar detector. The branching fraction is measured to be (1.60+/-0.05+/-0.11)x10^-4. It is found that "\tau \to f1 \pi \nu to 3\pi \eta \nu" is the dominant decay mode with a branching fraction of (1.11+/-0.06+/-0.05)x10^-4. The first error on the branching fractions is statistical and the second systematic. In addition, a 90% confidence level upper limit on the branching fraction of the "\tau \to \eta'(958) \pi \nu" decay is measured to be 7.2x10^-6. This last decay proceeds through a second-class current and is expected to be forbidden in the limit of isospin symmetry.