Decay Constants of Heavy-Light Mesons from QCD

We summarize recently improved results for the pseudoscalar [1,2] and vector [3] meson decay constants and their ratios from QCD spectral sum rules where N2LO + estimate of the N3LO PT and power corrections up to d< 6 dimensions have been included in the SVZ expansion. The "optimal results" based on stability criteria with respect to the variations of the Laplace/Moments sum rule variables, QCD continuum threshold and subtraction constant \mu are compared with recent sum rules and lattice calculations. To understand the "apparent tension" between some recent results for f_B*/f_B, we present in Section 8 "a novel extraction" of this ratio from heavy quark effective theory (HQET) sum rules by including the normalization factor (M_b/M_B)^2 relating the pseudoscalar to the universal HQET correlators for finite b-quark and B-meson masses. We obtain f_B*/f_B=1.025(16) in good agreement with the one 1.016(16) from (pseudo)scalar sum rules in full QCD [3]. We complete the paper by including new improved estimates of the scalar, axial-vector and B^*_c meson decays constants (Sections 11-13). For further phenomenological uses, we attempt to extract a Global Average of different sum rules and lattice determinations of the decay constants which are summarized in Tables 2-6. We do not found any deviation of these SM results from the present data.