The $K\pi$ form factors from Analyticity and Unitarity
  Analyticity and unitarity techniques are employed to obtain bounds on the shape parameters of the scalar and vector form factors of semileptonic $K_{l3}$ decays. For this purpose we use vector and scalar correlators evaluated in pQCD, a low energy theorem for scalar form factor, lattice results for the ratio of kaon and pion decay constants, chiral perturbation theory calculations for the scalar form factor at the Callan-Treiman point and experimental information on the phase and modulus of $K\pi $ form factors up to an energy $\tin=1 {\rm GeV}^2$. We further derive regions on the real axis and in the complex-energy plane where the form factors cannot have zeros. 
Gauhar Abbas, B.Ananthanarayan, I.Caprini, I.Sentitemsu Imsong
hep-ph