PERTURBATIVE QCD CALCULATIONS OF HEAVY MESON EXCLUSIVE DECAYS

We entertain the idea that decays of heavy mesons are perturbatively calculable. In the present study we confine our attention to decays of the B0 into two charged pseudoscalars (and the corresponding B+ decays), and to decays that are leading order in CKM factors, namely B0-->D-D(s)+ and D-pi+, plus the B0-->pi-pi+. We note that if the mass of the heavy quark is less than the mass of the heavy meson, then in general the decay amplitude can have an imaginary part. We also note that factorable graphs dominate non-factorable graphs more because of dynamical factors than color factors. Using a peaking approximation for the longitudinal momentum distribution of quarks in a heavy meson, and for plausible values of the parameters, we show that the perturbative calculation of the D-D+ and D-pi+ decays accords with experiment. For the pi-pi+ decay, the imaginary part of the amplitude is large enough so that the calculated rate is near to values obtained by ''non-perturbative'' models. Regarding the form factors f+/- (q2) of the B0-->D- current, we show that while f+ has a monopole form and f- a dipole form asymptotically, both may be best represented by dipole forms in the momentum transfer range relevant to B0 decay.