### Abstract

Based on a sample corresponding to 4.3×10^{6} produced τ-pair events, we have studied hadronic dynamics in the decay τ^{-}→ν_{τ}π^{-}π ^{0}π^{0} in data recorded by the CLEO II detector operating at the CESR e^{+}e^{-} collider. The decay is dominated by the process τ^{-}→ν_{τ}a_{1}^{-}(1260), with the a_{1}^{-} meson decaying to three pions predominantly via the lowest dimensional (mainly S-wave) ρ^{-}π^{0} Born amplitude. From model-dependent fits to the Dalitz plot and angular observables in bins of 3 π mass, we find significant additional contributions from amplitudes for a_{1} decay to σπ, f_{0}(1370)π, and f_{2}(1270)π, as well as higher dimensional a_{1}→ρπ and ρ′ π amplitudes. Notably, the squared σπ amplitude accounts for approximately 15% of the total τ^{-} →ν_{τ}π^{-}π^{0}π^{0} rate in the models considered. The data are well described using couplings to these amplitudes that are independent of the 3π mass. These amplitudes also provide a good description for the τ^{-} →ν_{τ}π^{-}π^{0}π^{0} Dalitz plot distributions. We have searched for additional contributions from τ^{-}→ν_{τ}π′ -(1300). We place 90% confidence level upper limits on the branching fraction for this channel of between 1.0×10^{-4} and 1.9×10^{-4}, depending on the π′ decay mode considered. The π^{-}π^{0}π^{0} mass spectrum is parametrized by a Breit-Wigner form with a mass-dependent width which is specified according to the results of the Dalitz plot fits plus an unknown coupling to an a_{1}→K*K amplitude. From a χ^{2} fit using this parametrization, we extract the pole mass and width of the a_{1}, as well as the magnitude of the K*K coupling. We have also investigated the impact of a possible contribution from the a_{1}′(1700) meson on this spectrum. Finally, exploiting the parity-violating angular asymmetry in a_{1}→3π decay, we determine the signed value of the τ neutrino helicity to be h_{ντ}=-1.02±0.13 (stat)±0.03 (syst+model), confirming the left-handedness of the τ neutrino.

Original language | English |
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Article number | 012002 |

Pages (from-to) | 1-22 |

Number of pages | 22 |

Journal | Physical Review D - Particles, Fields, Gravitation and Cosmology |

Volume | 61 |

Issue number | 1 |

Publication status | Published - Jan 1 2000 |

### ASJC Scopus subject areas

- Physics and Astronomy (miscellaneous)

## Fingerprint Dive into the research topics of 'Hadronic structure in the decay τ<sup>-</sup>→ν<sub>τ</sub>π<sup>-</sup>π <sup>0</sup>π<sup>0</sup> and the sign of the tau neutrino helicity'. Together they form a unique fingerprint.

## Cite this

^{-}→ν

_{τ}π

^{-}π

^{0}π

^{0}and the sign of the tau neutrino helicity.

*Physical Review D - Particles, Fields, Gravitation and Cosmology*,

*61*(1), 1-22. [012002].