Threshold effect

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In particle physics, the term threshold effect usually refers to small corrections to rough calculations based on the renormalization group that arise from the detailed behavior near the scale where new physics takes place. In the context of renormalization group, we often "integrate out" modes of quantum fields with frequencies exceeding a certain energy scale (cutoff). If the cutoff is very close to the energy scale that we want to study, the threshold effects become important and contribute small terms to formulae such as those for the beta functions.