Heavy quark hadronic Lagrangian for s-wave quarkonium.

We use heavy quark effective theory ~HQET! techniques to parametrize certain nonperturbative effects related to quantum fluctuations that put both heavy quarks and antiquarks in quarkonium almost on shell. The large off-shell momentum contributions are calculated using Coulomb-type states. The almost on-shell momentum contributions are evaluated using an effective ‘‘chiral’’ Lagrangian which incorporates the relevant symmetries of the HQET for quarks and antiquarks. The cutoff dependence of both contributions matches perfectly. The decay constants and the matrix elements of bilinear currents at zero recoil are calculated. Their leading nonperturbative contributions are parametrized by a single constant and turn out to be O~a/LQCDan!, an being the Bohr radius and a the strong coupling constant, times the nonperturbative contribution coming from the multipole expansion ~gluon condensate!. We discuss the physical applications to Y, J/c, and Bc systems.