Lattice QCD calculation of (B)over-bar→Dl(ν)over-bar decay form factors at zero recoil -: art. no. 014502

A lattice QCD calculation of the (B) over bar --> Dl (v) over bar decay form factors is presented. We obtain the value of the form factor h(+)(w) at the zero-recoil limit w=1 with high precision by considering a ratio of correlation functions in which the bulk of the uncertainties cancels. The other form factor h(-)(w) is calculated, for small recoil momenta, from a similar ratio. In both cases, the heavy quark mass dependence is observed through direct calculations with several combinations of initial and final heavy quark masses. Our results are h(+)(1) =1.007(6)(2)(3) and h(-)(1)=-0.107(28)(04)(10/30). For both the first error is statistical; the second sterns from the uncertainty in adjusting the heavy quark masses and the last from omitted radiative corrections. Combining these results, we obtain a precise determination of the physical combination FB --> D(1) =1.058(20/17), where the mentioned systematic errors are added in quadrature. The dependence on lattice spacing and the effect of quenching are not yet included, but with our method they should be a fraction of FB --> D-1.