Semileptonic decays of Bc meson

The semileptonic Bc-meson decays into the heavy quarkonia J/Ψ(ηc) and a pair of leptons are investigated in the framework of three-point sum rules of QCD and NRQCD. Calculations of analytical expressions for the spectral densities of QCD and NRQCD correlators with account for the Coulomb-like αs/v terms are presented. The generalized relations due to the spin symmetry of NRQCD for the form factors of Bc → J/Ψ(ηc)/vl transitions with l denoting one of the leptons e, μ, or τ, are derived at the recoil momentum close to zero. This allows one to express all NRQCD form factors through a single universal quantity, an analog of Isgur-Wise function at the maximum invariant mass of lepton pair. The gluon condensate corrections to three-point functions are calculated both in full QCD in the Borel transform scheme and in NRQCD in the moment scheme. This enlarges the parametric stability region of sum rule method, that makes the results of the approach to be more reliable. Numerical estimates of widths for the transitions of Bc → J/Ψ(ηc)/vl are presented