Abstract
The semileptonic decays of the B c meson into heavy quarkonia J/ψ(ηc) and a pair of leptons are investigated on the basis of three-point sum rules of QCD and nonrelativistic QCD (NRQCD). Calculations of analytic expressions for the spectral densities of QCD and NRQCD correlation functions with allowance for Coulomb-like αs/v terms are presented. At recoil momenta close to zero, generalized relations due to the spin symmetry of NRQCD are derived for the B c → J/ψ(ηc)/νl form factors, with l denoting one of the leptons (e, μ, or τ). This allows one to express all NRQCD form factors in terms of a single universal quantity, an analog of the Isgur-Wise function at the maximal lepton-pair invariant mass. 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 the sum-rule method, thereby rendering the results of the approach more reliable. Numerical estimates of the widths for the transitions B c → J/ψ(ηc)lνl are presented.
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