fB from finite size effects in lattice QCD

We discuss a novel method to calculate fB on the lattice based on the study of the dependence of finite size effects upon the heavy quark mass of flavoured mesons and on a non--perturbative recursive finite size technique. This method avoids the systematic errors related to extrapolations from the static limit or to the tuning of the coefficients of effective Lagrangian and the results admit an extrapolation to the continuum limit. We show the results of a first estimate at finite lattice spacing, but close to the continuum limit, giving fB = 170(11)(5)(22)MeV. We also obtain fBs = 192(9)(5)(24)MeV. The first error is statistical, the second is our estimate of the systematic error from the method and the third the systematic error from the specific approximations adopted in this first exploratory calculation. The method can be generalized to two--scale problems in lattice QCD.