Bπ Couplings in Qcd

We calculate the D∗Dπ and B∗Bπ couplings using QCD sum rules on the lightcone. In this approach, the large-distance dynamics is incorporated in a set of pion wave functions. We take into account two-particle and three-particle wave functions of twist 2, 3 and 4. The resulting values of the coupling constants are gD∗Dπ = 12.5 ± 1 and gB∗Bπ = 29± 3. From this we predict the partial width Γ(D∗+ → D0π+) = 32± 5 keV . We also discuss the soft-pion limit of the sum rules which is equivalent to the external axial field approach employed in earlier calculations. Furthermore, using gB∗Bπ and gD∗Dπ the pole dominance model for the B → π and D → π semileptonic form factors is compared with the direct calculation of these form factors in the same framework of light-cone sum rules. a on leave from Institute of Theoretical and Experimental Physics, 117259 Moscow, Russia b on leave from St.Petersburg Nuclear Physics Institute, 188350 Gatchina, Russia; now at DESY, D–22603 Hamburg, Germany c on leave from Yerevan Physics Institute, 375036 Yerevan, Armenia Alexander von Humboldt Fellow + Work supported by the German Federal Ministry for Research and Technology under contract No. 05 6MU93P