Quenched lattice calculation of the vector channel B→D⋆ℓν decay rate

We calculate, in the continuum limit of quenched lattice QCD, the form factor that enters the decay rate of the semileptonic decay B→D⋆ℓν. By using the step scaling method (SSM), previously introduced to handle two scale problems in lattice QCD, and by adopting flavor twisted boundary conditions we extract FB→D⋆(w) at finite momentum transfer (w⩾1) and at the physical values of the heavy quark masses. Our results can be used in order to extract the CKM matrix element Vcb by the experimental decay rate without model dependent extrapolations. The value of Vcb agrees with the one obtained from the B→Dℓν channel and makes us confident that the quenched approximation well applies to these transitions.