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The generalization of the Feynman-Hellmann theorem for resonance states inquantum field theory is derived. On the basis of this theorem, a criterion isproposed to study the possible exotic nature of certain hadronic statesemerging in QCD. It is shown that this proposal is supported by explicitcalculations in Chiral Perturbation Theory and by large-$N_c$ arguments.Analyzing recent lattice data on the quark mass dependence in the pseudoscalar,vector meson, baryon octet and baryon decuplet sectors, we conclude that, asexpected, these are predominately quark-model states, albeit the correctionsare non-negligible.

Feynman-Hellmann theorem for resonances and the quest for QCD exotica