Semileptonic B-c-meson decays in sum rules of QCD and NRQCD

The semileptonic B-c-meson decays into the heavy quarkonia J/psi(eta(c)) an d a pair of leptons are investigated in the framework of three-point sum ru les of QCD and NRQCD. Calculations of analytical expressions for the spectr al densities of QCD and NRQCD correlators with account for the Coulomb-like alpha(s)/upsilon terms are presented. The generalized relations due to the spin symmetry of NRQCD for the form factors of B-c --> J/psi(eta(c))l nu(l ) transitions with I denoting one of the leptons e,mu or tau, are derived a t the recoil momentum close to zero. This allows one to express all NRQCD f orm factors through a single universal quantity, an analogue of Isgur-Wise function at the maximal invariant mass of lepton pair. The gluon condensate corrections to three-point functions are calculated both in full QCD in th e Borel transform scheme and in NRQCD in the moment scheme. This enlarges t he parameteric stability region of sum rule method that makes the results o f the approach to be more reliable. Numerical estimates of widths for the t ransitions of B-c --> J/psi(eta(c))l nu(l) are presented. (C) 2000 Elsevier Science B.V. All rights reserved.