Determination of V_cb from exclusive decays in a relativistic quark model

In the framework of a relativistic covariant Bethe-Salpeter model for the quark-antiquark system we present a renewed determination of the Cabbibo-Kobayashi-Maskawa matrix element $\mathrm{\mid }{V}_{cb}\mathrm{\mid }$. Complementing an earlier analysis applied to the whole decay spectrum for $B\to D^{\ast }e\nu$ we now also employ the ``zero-recoil method’’ that uses the end point of the decay spectrum ($\omega =1$) and is suited for heavy-to-heavy transitions. The averaged experimental value extracted from the data at zero recoil, $|V_{cb}|{\cal F}(\omega=1)=0.0343\pm0.0015$,thenleadsto$|V_{cb}|=0.0360\pm 0.0016$. This value is somewhat larger than the one that uses the whole decay spectrum for the model analysis. We also contrast this result to a nonrelativistic model and to recent experiments on the $B\to De\nu$ semileptonic decay.