${\cal O}(\alpha_s^2)$ corrections to $b \to c$ decay at zero recoil

Abstract.

Analytic formulae are presented for the two-loop perturbative QCD corrections to $b \to c$ decay at the zero recoil point, which are required for the extraction of $|V_{bc}|$ from measurements of exclusive $B\to D^* l \nu$ decays. The results are in agreement with those in [8, 9]. Some comments on the numerical evaluation of the diagrams involved are made.