Analysis of the decayτ- →π-π-π+v τ and determination of thea1(1260) resonance parameters

Using the ARGUS detector at the DORIS IIe+e− storage ring we have studied the three-pion hadronic final state in the decay τ- → π-π-π+vτ. From about 7500 events we conclude that the three-pion system is dominated by theJP=1+ resonancea1 (1260) decaying preferentially via ρ0π-. Our data restrict a possible ρπ contribution of three-pion states without intermediate resonances to less than 6% at 95% CL. Fitting a model by Isgur et al. to the data yields\(m_{a_1 } = (1.211 \pm 0.007 \pm _{0.0}^{0.050} ) GeV/c^2 \) and\(\Gamma _{a_1 } = (0.446 \pm 0.0021 \pm _{0.0}^{0.140} ) GeV/c^2 \). The second errors reflect the model dependence of the result. This is the currently most significant determination of thea1 resonance parameters. The ratio ofS- andD-wave amplitudes for the ρ0π- intermediate state of thea 1 − decay at the nominala 1 − mass was found to beD/S=−0.11±0.02. Using thisD/S ratio we update our former measurement of the parity violating asymmetry parameter\(\gamma _{AV} = 2g_A g_V /(g_A^2 + g_V^2 )\) to 1.25±0.23± 0.08 0.15 . The branching ratio of the decay τ- → π-π-π+vτ is determined to be Br(τ- → π-π-π+vτ)=(6.8±0.1±0.5)%.