We apply the infinite mass effective theory, when a heavy quark mass tends to infinity, and Chiral perturbation theory at the quark level, based on the extended Nambu -- Jona -- Lasinio model with linear realization of chiral $U(3)\times U(3)$ symmetry, to calculate the form factors of the $D^{ \,+}_{ \,s}\to\phi~\mu^+~\nu_{ \,\mu}$ decay up to the first order in current $s$ -- quark mass. The theoretical results are compared with experimental data and found to be in good agreement.
Categories: hep-ph