Measurement of the τ−→f1(1285)π−ντ branching fraction and a search for second-class currents in τ→η′(958)π−ντ

The τ−→ηπ−π+π−ντ decay with the η→γγ 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)×10−4. It is found that τ−→f1(1285)π−ντ→ηπ−π+π−ντ is the dominant decay mode with a branching fraction of (1.11±0.06±0.05)×10−4. The first error is statistical and the second systematic. In addition, a 90% confidence level upper limit on the branching fraction of the τ−→η′(958)π−ντ decay is measured to be 7.2×10−6. This last decay proceeds through a second-class current and is expected to be forbidden in the limit of isospin symmetry.