We discuss theoretical approaches to form factors in heavy-meson decays, e.g. weak decay constants, transition form factors and effective heavy-to-light meson couplings, which are hadronic expressions of nonperturbative QCD. After motivating their origin in QCD factorization and heavy quark effective theories, we retrace their evolution from earlier quark-model calculations to nonperturbative QCD techniques with an emphasis on the formulation of bound states within the framework of the quark’s gap equation and the meson’s Bethe-Salpeter equation, both of which are nonperturbative continuum approaches to QCD. The resulting Bethe-Salpeter amplitudes can be projected onto the light front to obtain the meson’s light-cone distribution amplitudes (LCDA). The latter are an important ingredient in QCD factorization approaches to weak decays of heavy mesons.

Pertubative and nonperturbative QCD