$|V_{cb}|$ from OPE Sum Rules for Heavy Flavor Transitions

Abstract

We derive a model-independent upper bound for the axial-vector form factor of the $B\ra D^*$ transition at zero recoil, $F_{B\ra D^*}$. The form factor turns out to be noticeably less than unity. The deviation of $F_{B\ra D^*}$ from unity is larger than previously anticipated. Using our estimate we extract $|V_{cb}|$ from the measured exclusive rate of $B\ra D^* l\nu$ extrapolated to the point of zero recoil. The central `` exclusive" value of $|V_{cb}|$ is in agreement with the value obtained from the inclusive semileptonic width $\Gamma (B\ra X_c l\nu)$. We argue that the theoretical uncertainty in determining $|V_{cb}|$ 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.