Kπ vector form factor, dispersive constraints and τ→ντKπ decays

Recent experimental data for the differential decay distribution of the decay τ-→ντKSπ- by the Belle collaboration are described by a theoretical model which is composed of the contributing vector and scalar form factors F+Kπ(s) and F0Kπ(s). Both form factors are constructed such that they fulfil constraints posed by analyticity and unitarity. A good description of the experimental measurement is achieved by incorporating two vector resonances and working with a three-times-subtracted dispersion relation in order to suppress higher-energy contributions. The resonance parameters of the charged K*(892) meson, defined as the pole of F+Kπ(s) in the complex s-plane, can be extracted, with the result MK*=892.0±0.9 MeV and ΓK*=46.2±0.4 MeV. Finally, employing a three-times-subtracted dispersion relation allows one to determine the slope and curvature parameters λ'+=(24.7±0.8) x 10-3 and λ''+=(12.0±0.2) x 10-4 of the vector form factor F+Kπ(s) directly from the data. (orig.)