Theorem on the Form Factors inKl3Decay

We derive a formula for the matrix element $〈K\left|{\partial }_{\mu }\mathcal{F}_a^{}{}_{}{}^{\mu }\right|\pi 〉$ correct up to second order in both the momentum transfer $t$ and the breaking of SU(3) ⊗ SU(3) symmetry. This leads to a prediction that $\xi$ is very small compared with unity. Our result is independent of any assumption concerning the structure of the symmetry-breaking Hamiltonian.