Hadronic structure in the decayτ−→ντπ−π0π0and the sign of the tau neutrino helicity
D. M. Asner, A. Eppich, J. Gronberg, T. S. Hill, D. J. Lange, R. J. Morrison, H. N. Nelson, T. K. Nelson, D. Roberts, B. H. Behrens, W. T. Ford, A. Gritsan
Physical Review D, Particles and fields
Based on a sample corresponding to 4.3 million produced tau-pair events, we have studied hadronic dynamics in the decay tau- --> nu_tau pi- pi0 pi0 in data recorded by the CLEO II detector operating at the CESR e+e- collider. The decay is dominated by the process tau --> nu_tau a_1(1260), with the a_1 meson decaying to three pions predominantly via the lowest dimensional (mainly S-wave) a_1 --> rho pi Born amplitude. From fits to the Dalitz plot and angular observables, we find significant
... ional contributions from amplitudes for a_1 decay to sigma pi, f_0(1370) pi and f_2(1270) pi, as well as higher dimensional a_1 --> rho pi and rho' pi amplitudes. The squared sigma pi amplitude accounts for 15 the models considered. We have searched for additional contributions from tau --> nu_tau pi'(1300). We place 90 branching fraction for this channel of between 1.0*10^-4 and 1.9*10^-4, depending on the pi' decay mode. The pi- pi0 pi0 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 a coupling to an a_1 --> K* K amplitude. From a chi^2 fit, 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 --> 3pi decay, we determine the signed value of the tau neutrino helicity to be h_ν_τ = -1.02 +- 0.13(stat.) +- 0.03(syst.+model), confirming the left-handedness of the tau neutrino.