Measurement of the 2π spectral function in the decay τ−→π−π0ντ and the contribution to the muon anomalous magnetic moment

We report a high-statistics measurement of the branching fraction for τ−→π−π0ντ and the invariant mass spectrum of the produced π−π0 system using 72.2 fb−1 of data recorded with the Belle detector at the KEKB asymmetric-energy e+e− collider. The branching fraction obtained is (25.24±0.01±0.39)%, where the first error is statistical and the second is systematic. The unfolded π−π0 mass spectrum is used to determine resonance parameters for the ρ(770), ρ′(1450), and ρ″(1700) mesons. We also use this spectrum to estimate the hadronic (2π) contribution to the anomalous magnetic moment of the muon (). Our result for integrated over the mass range is , where the first error is due to the experimental uncertainties, the second is due to the uncertainties in the branching fractions and the third is due to the uncertainties in the isospin-violating corrections. A full description of these results is available in Phys. Rev. D 78, 072006 (2008) [M. Fujikawa, H. Hayashii, S. Eidelman et al., (Belle collaboration), Phys. Rev. D 78, 072006 (2008)].