ar X iv : 0 90 3 . 16 54 v 1 [ he p - ph ] 1 0 M ar 2 00 9 Dispersive representation and shape of the K l 3 form factors : robustness

An accurate low-energy dispersive parametrization of the scalar Kπ form factor was constructed some time ago in terms of a single parameter guided by the Callan-Treiman low-energy theorem. A similar twice subtracted dispersive parametrization for the vector Kπ form factor will be investigated here. The robustness of the parametrization of these two form factors will be studied in great detail. In particular the cut-off dependence, the isospin breaking effects and the possible, though not highly probable, presence of zeros in the form factors will be discussed. Interesting constraints in the latter case will be obtained from the soft-kaon analog of the Callan-Treiman theorem and a comparison with the recent τ → Kπντ data. 1Email: [email protected] 2Email: [email protected] 3Email: [email protected] 4Jan Stern sadly passed away before the final version of the paper was written.