Hadronic structure in the decay τ-→ντπ-π 0π0 and the sign of the tau neutrino helicity

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Based on a sample corresponding to 4.3×10 produced τ-pair events, we have studied hadronic dynamics in the decay τ →ν π π π in data recorded by the CLEO II detector operating at the CESR e e collider. The decay is dominated by the process τ →ν a (1260), with the a meson decaying to three pions predominantly via the lowest dimensional (mainly S-wave) ρ π 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 decay to σπ, f (1370)π, and f (1270)π, as well as higher dimensional a →ρπ and ρ′ π amplitudes. Notably, the squared σπ amplitude accounts for approximately 15% of the total τ →ν π π π 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 π π π Dalitz plot distributions. We have searched for additional contributions from τ →ν π′ -(1300). We place 90% confidence level upper limits on the branching fraction for this channel of between 1.0×10 and 1.9×10 , depending on the π′ decay mode considered. The π π π 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 →K*K amplitude. From a χ fit using this parametrization, we extract the pole mass and width of the a , as well as the magnitude of the K*K coupling. We have also investigated the impact of a possible contribution from the a ′(1700) meson on this spectrum. Finally, exploiting the parity-violating angular asymmetry in a →3π decay, we determine the signed value of the τ neutrino helicity to be h =-1.02±0.13 (stat)±0.03 (syst+model), confirming the left-handedness of the τ neutrino. ©1999 The American Physical Society. 6 - - 0 0 + - - - - - 0 - - 0 0 - - 0 0 - -4 -4 - 0 0 2 τ τ 1 1 1 0 2 1 τ τ τ 1 1 1 1 ντ

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Physical Review D - Particles, Fields, Gravitation and Cosmology

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