Abstract
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 α 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) f π , while in the static limit f B static = (2.07 ± 0.32) f π . The renormalization group invariant coupling in HQET is estimated to be f P ∞ ≅ (0.35 ± 0.07) GeV 3 2 . We find that the linear quark mass dependence of the decay constants is f P √ M P ≅ ( f P √ M P ) ∞ [1 - (0.88 ± 0.18)GeV/ M Q ]. We also show that the moments tend to the Laplace sum rule for M Q →∞.
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