Decay Constants of Pseudoscalar Mesons in Bethe-Salpeter Framework with Generalized Structure of Hadron-Quark Vertex

We employ the framework of Bethe-Salpeter equation under Covariant Instantaneous Ansatz to study the leptonic decays of pseudoscalar mesons. The Dirac structure of hadron-quark vertex function $\Gamma$ is generalized to include various Dirac covariants besides $\gamma_5$ from their complete set. The covariants are incorporated in accordance with a power counting rule, order by order in powers of the inverse of the meson mass. The decay constants are calculated with the incorporation of leading order covariants. Most of the results are dramatically improved.