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 series in the inverse heavy quark mass. This technique applied recently to the axial current is used to estimate the charm content of the \eta, \eta' mesons and the intrinsic charm contribution to the proton spin. The derivation of heavy quark mass expansion for Qbar \gamma_5 Q is given here in detail and the expansions of the scalar, vector and tensor current and of 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.