Semileptonic decays of the B-c 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 ru les of QCD and nonrelativistic QCD (NRQCD). Calculations of analytic expres sions for the spectral densities of QCD and NRQCD correlation functions wit h allowance for Coulomb-like alpha (s)/v terms are presented. At recoil mom enta 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 denot ing one of the leptons (e, mu, or tau). This allows one to express all NRQC D form factors in terms of a single universal quantity, an analog of the Is gur-Wise function at the maximal lepton-pair invariant mass. The gluon-cond ensate corrections to three-point functions are calculated both in full QCD in the Borel transform scheme and in NRQCD in the moment scheme. This enla rges the parametric-stability region of the sum-rule method, thereby render ing the results of the approach more reliable. Numerical estimates of the w idths for the transitions B-c J/psi(eta (c))lv(l) are presented. (C) 2000 M AIK "Nauka/Interperiodica".