How big are the decay constants fp of heavy-light mesons?

The quark loop diagram for fP for a heavy-light (Qanti q) meson is employed within a Lorentz-invariant framework for addressing a two-fold issue: (I) Finite corrections to the asymptotic scaling law (fP ∝ mQ-1/2) of Shifman, Voloshin, Politzer and Wise; (II) 1/MQ corrections to the static approximation for the heavy-quark propagator in lattice calculations. The loop diagram yields a general two-component structure fP = fO + δf in which the second term is an unequal-mass cut-off shell contribution. The ratio δf/fO provides a rough index of the nature and extent of both these corrections for heavy-light mesons. Concrete estimates are made in terms of a Bethe-Salpeter model for the vertex function Γ(q) whose parameters are firmly rooted in meson and baryon spectroscopy. It is found that δf/fO increases up to the D-B region (where its effect is 15-20%), before tapering off for higher mQ values. The total fP values are on the low side (fD < 150 MeV, fB < 100 MeV). A brief comparison is given with lattice measurements. (orig.)