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

Abstract

In view of new precise measurements of the $\bar{B}\to D^{\ast }\ell \bar{\hat{I}}½$ 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 $\mathcal{F}(1)=\hat{I}{\cdot }_A\hat{\hat{I}}¾(1)=0.93\pm 0.03$ and ${\hat{\varrho }}^2=0.7\pm 0.2$. Combining this with the most recent experimental results, we obtain the model-independent value $|V_{cb}|=0.040\pm 0.003$.

11 pages LaTeX, 2 figures. Revision - minor typographic changes made