What can be learned from the Belle spectrum for the decay tau- -> nu_tau   K_S pi-
  A theoretical description of the differential decay spectrum for the decay tau- -> nu_tau K_S pi-, which is based on the contributing K pi vector and scalar form factors F_+^{K pi}(s) and F_0^{K pi}(s) being calculated in the framework of resonance chiral theory (R$\chi$T), additionally imposing constraints from dispersion relations as well as short distance QCD, provides a good representation of a recent measurement of the spectrum by the Belle collaboration. Our fit allows to deduce the total branching fraction B[tau- -> nu_tau K_S pi-] = 0.427 +- 0.024 % by integrating the spectrum, as well as the K^* resonance parameters M_{K^*} = 895.3 +- 0.2 MeV and Gamma_{K^*} = 47.5 +- 0.4 MeV, where the last two errors are statistical only. From our fits, we confirm that the scalar form factor F_0^{K pi}(s) is required to provide a good description, but we were unable to further constrain this contribution. Finally, from our results for the vector form factor F_+^{K pi}(s), we update the corresponding slope and curvature parameters lambda'_+ = (25.2 +- 0.3)*10^{-3} and lambda''_+ = (12.9 +- 0.3)*10^{-4}, respectively. 
Matthias Jamin, Antonio Pich, Jorge Portoles