Heavy quark mass expansion and intrinsic charm in light hadrons

We review the technique of heavy quark mass expansion of various operators made of heavy quark fields using a semiclassical approximation. It corresponds to an operator product expansion in the form of a series in the inverse heavy quark mass. This technique applied recently to the axial vector current is used to estimate the charm content of the $\eta ,$ ${\eta }^{\prime }$ mesons and the intrinsic charm contribution to the proton spin. The derivation of heavy quark mass expansion for $〈\overline{Q}{\gamma }_{5}Q〉$ is given here in detail and the expansions of the scalar, vector and tensor current and of $〈\overline{Q}{\nabla }_{\mu }{\gamma }_{\nu }Q〉$ (a contribution to the energy-momentum tensor) are presented as well. The obtained results are used to estimate the intrinsic charm contribution to various observables.