Constraining the properties of K_{l3} form factors with analyticity and   unitarity
  Bounds on the shape parameters of K_{l3} form factors are investigated in a general model independent framework. Using the method of unitarity bounds for correlators evaluated in pQCD, chiral symmetry and experimental information on the phase and modulus of \pi K form factors up to an energy t_{in}=1 GeV^2, we constrain the shape parameters of the scalar and vector form factors, the value of the scalar form factor at t=M_\pi^2-M_K^2, and exclude regions where zeros can exist on the real energy line and in the complex energy plane. 
Gauhar Abbas, B. Ananthanarayan, Irinel Caprini and I. Sentitemsu   Imsong
hep-ph hep-ex hep-lat