Measurement of the [formula omitted] branching fraction and a search for second-class currents in [formula omitted

Abstract

The τ − → η π − π + π − ν τ decay with the η → γ γ 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 − → f 1 ( 1285 ) π − ν τ → η π − π + π − ν τ 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 − ν τ 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.