Loop-induced

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.