$D^*Dπ$ and $B^*Bπ$ couplings in QCD

We calculate the $D^*Dπ$ and $B^*Bπ$ couplings using QCD sum rules on the light-cone. 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 $g_{D^*Dπ}= 12.5\pm 1$ and $g_{B^*Bπ}= 29\pm 3$. From this we predict the partial width $ Γ(D^{*+} \ra D^0 π^+ )=32 \pm 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 $g_{B^*Bπ}$ and $g_{D^*Dπ}$ the pole dominance model for the $ B \ra π$ and $D\ra π$ semileptonic form factors is compared with the direct calculation of these form factors in the same framework of light-cone sum rules.

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