$\mathrm{\mid }{V}_{cb}\mathrm{\mid }$ from OPE Sum Rules for Heavy Flavor Transitions

We derive a model-independent upper bound for the axial-vector form factor of the $B\text{ra}{D}^{\ast }$ transition at zero recoil, $F_{B\text{ra}{D}^{\ast }}$. The form factor turns out to be noticeably less than unity. The deviation of $F_{B\text{ra}{D}^{\ast }}$ from unity is larger than previously anticipated. Using our estimate we extract $\mathrm{\mid }{V}_{cb}\mathrm{\mid }$ from the measured exclusive rate of $B\text{ra}{D}^{\ast }l\nu$ extrapolated to the point of zero recoil. The central `` exclusive" value of $\mathrm{\mid }{V}_{cb}\mathrm{\mid }$ is in agreement with the value obtained from the inclusive semileptonic width $\Gamma (B\ra X_c l\nu)$\mathrm{.}Wearguethatthetheoreticaluncertaintyindetermining$|V_{cb}|$ from the total inclusive width is significantly reduced if a constraint on the quark mass difference $m_b-m_c$ stemming from the heavy quark expansion is taken into account.