Theoretical Update on the Model-Independent Determination of |V_{cb}| Using Heavy Quark Symmetry

Abstract

In view of new precise measurements of the $\bar B\to D^*\ell \bar\nu$ decay rate near zero recoil, we reconsider the theoretical uncertainties in the extraction of $| V_{cb}|$ using heavy quark symmetry. In particular, we combine our previous estimate of $1/m_Q^2$ corrections to the normalization of the hadronic form factor at zero recoil with sum rules derived by Shifman {\it et al}. to obtain a new prediction with less theoretical uncertainty. We also summarize the status of the calculation of short-distance corrections, and of the slope of the form factor at zero recoil. We find ${\cal F}(1)=\eta_A \hat\xi(1)=0.93\pm 0.03$ and $\hat\varrho^2=0.7\pm 0.2$. Combining this with the most recent experimental results, we obtain the model-independent value $| V_{cb}|=0.040\pm 0.003$.