Linear and higher order power corrections in semileptonic B decays

In previous work we have developed a relativistic quark model of mesons which is consistent with all QCD constraints at zeroth and first order in the heavy quark expansion. Here we obtain first order model predictions for the differential decay spectrum, the forward-backward asymmetry $A_{FB}$ and the $D^{\ast}$ polarization parameter $\alpha$ in the decay $B\to D^{\ast}\ell\nu$. We compare these with the predictions of QCD sum rules at first order. The model suggests why the corrections allowed at first order are small, concurrently with substantial corrections at second order.