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

Abstract

I discuss the theoretical uncertainties in the extraction of $|\,V_{cb}|$ from a measurement of the $\bar B\to D^*\ell\,\bar\nu$ 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 $\widehat\xi(w)$ at zero recoil: $\widehat\varrho^2=0.7\pm 0.2$. Using the most recent experimental results, I obtain the model-independent value $|\,V_{cb}|=0.0395\pm 0.0030$.