Upsilon \bar BB$ Couplings, Slope of the Isgur-Wise Function and Improved Estimate of \boldmath{$V_{cb}$}}

Abstract

We estimate the sum of the $\Upsilon \bar BB $ couplings using QCD Spectral Sum Rules (QSSR). Our result implies the phenomenological bound $\xi'(vv'=1) \geq -1.04$ for the slope of the Isgur-Wise function. An analytic estimate of the (physical) slope to two loops within QSSR leads to the accurate value $\xi'(vv'=1) \simeq -(1.00 \pm 0.02)$ due to the (almost) complete cancellations between the perturbative and non-perturbative corrections at the stability points. Then, we deduce, from the present data, the improved estimate $\vert V_{cb} \vert \simeq \ga 1.48 \mbox{ps}/\tau_B \dr ^{1/2}(37.3 \pm 1.2 \pm 1.4)\times 10^{-3}$ where the first error comes from the data analysis and the second one from the different model parametrizations of the Isgur-Wise function.