THE VALUE OF FB FROM QSSR

We extract f(B) from new moments sum rule in the heavy quark effective theory (HQET). We check and improve previous results from Laplace sum rules in HQET by considering all sources of errors including the ones due to the estimated alpha(s)2-term. Averaging these with the ones from the ''standard'' relativistic sum rules, we deduce the unique QCD spectral sum rules (QSSR) value f(B) = (1.60 +/- 0.32 )fpi, while in the static limit f(B)static = (2.07 +/- 0.32)fpi. The renormalization group invariant coupling in HQET is estimated to be f(p)infinity congruent-ti (0.35 +/- 0.07) GeV 3/2. We find that the linear quark mass dependence of the decay constants is f(p) square-root M(p) congruent-to (f(p) square-root M(p))infinity [ 1 - (0.88 +/- 0.18)GeV/M(Q) ]. We also show that the moments tend to the Laplace sum rule for M(Q) --> infinity.