Heavy-quark symmetry and chiral dynamics

The flavor and spin symmetry of the heavy quarks and the spontaneously broken approximate ${\mathrm{SU}\mathrm{}\left(3\right)\mathrm{}}_L\times {\mathrm{SU}\mathrm{}\left(3\right)\mathrm{}}_R$ chiral symmetry of the light quarks are exploited to formulate a theory describing the low-energy interactions of the heavy mesons ($Q\overline{q}$ bound states) and heavy baryons (${\mathrm{Qq}}_1{q}_2$ bound states) with the Goldstone bosons $\pi$, $K$, and $\eta$. The theory contains only three parameters independent of the number of heavy-quark species involved. They can be determined by the decays $D^{*}\to D+\pi$, ${\Sigma }_c\to {\Lambda }_c+\pi$, and ${\Sigma }_c^{*}\to {\Sigma }_c+\pi$. Theoretically, these coupling constants are related, through partial conservation of axial-vector current, to the axial charges of the heavy mesons and the heavy baryons. They are all calculable in the nonrelativistic quark model by using the spin wave functions of these particles alone. The theory is applied to strong decays and semileptonic weak decays of the heavy mesons and baryons. The implications are also discussed.