Semileptonic decays of the Bc meson

The semileptonic decays of the B-c meson into heavy quarkonia J/psi(eta (c)) 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 alpha (s)/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/psi(eta (c))lv(l) form factors, with I denoting one of the leptons (e, mu, or tau). 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/psi(eta (c))lv(l) are presented. (C) 2000 MAIK "Nauka/Interperiodica".