Theoretical Uncertainties in the Extraction of $|V_{cb}|$ from $\bar{B}\to D^{\ast }\ell \bar{\nu }$ Decays near Zero Recoil

Abstract

I discuss the theoretical uncertainties in the extraction of $|V_{cb}|$ from a measurement of the $\bar{B}\to D^{\ast }\ell \bar{\nu }$ decay rate close to zero recoil. In particular, I combine previous estimates of the $1/m_Q^2$ 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 uncertainty. I also give a prediction for the slope of the form factor $\hat{\xi }(w)$ at zero recoil: ${\hat{\varrho }}^2=0.7\pm 0.2$. Using the most recent experimental results, I obtain the model-independent value $|V_{cb}|=0.0395\pm 0.0030$.

Comments: invited talk presented at the 27th International Conference on High Energy Physics, Glasgow, Scotland, July 1994, 9 pages LaTeX, 1 figure appended after \end{document}, CERN-TH.7396/94