Heavy quark symmetry in B→D(∗)ℓν̄

We calculate heavy quark symmetry breaking in the slopes and curvatures of the B→D(∗)ℓν̄ spectra at zero recoil, including the order αs2β0 corrections. We point out that the theoretical uncertainties in the differences between B→D and B→D∗ slopes and curvatures are smaller than in the deviations of the slopes and curvatures themselves from their infinite mass limits. We find that the central values of the current experimental results for the difference of the slopes differ from our calculations when QCD sum rules are used to estimate subleading Isgur–Wise functions. A better understanding of the shapes of the B→D(∗)ℓν̄ spectra may also help to reduce the error of |Vcb| extracted from the zero recoil limit of B→D∗ℓν̄. We argue that heavy quark symmetry requires that the same fitting procedure be used in the experimental determinations of the shape parameters and |Vcb| from the B→Dℓν̄ and B→D∗ℓν̄ spectra.