in quantum field theory, penguin diagrams are a class of feynman diagrams which are important for understanding cp violating processes in the standard model. they refer to one-loop processes in which a quark temporarily changes flavor (via a w or z loop), and the flavor-changed quark engages in some tree interaction, typically a strong one. for the interactions where some quark flavors (e.g. very heavy ones) have much higher interaction amplitudes than others, such as cp-violating or higgs interactions, these penguin processes may have amplitudes comparable to or even greater than those of the direct tree processes. a similar diagram can be drawn for leptonic decays.