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In perturbative quantum field theory, a loop-induced effect refers to any mechanism which is induced exclusively by loop diagrams. If no tree-level coupling exists in the Lagrangian between a particular set of fields (for example, the Higgs boson, which carries no colour charge, and gluons), and their lowest order correlation functions in perturbation theory are instead mediated through a closed loop of virtual particles (such as the top quark), this coupling is said to be loop-induced. In cases where symmetries of the classical theory are broken for the first time by loop diagrams, these effects are said to generate anomalies of the quantum theory.