Penguin diagram
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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
They were first isolated and studied by Mikhail Shifman, Arkady Vainshtein, and Valentin Zakharov.<ref>Template:Cite journal
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The processes which they describe were first directly observed in 1991 and 1994 by the CLEO collaboration.Template:Citation needed
Origin of the nameEdit
John Ellis was the first to refer to a certain class of Feynman diagrams as "penguin diagrams" in a 1977 paper on b-quarks.<ref>Template:Cite journal</ref> The name came about in part due to their shape, and in part due to a legendary bar-room bet with Melissa Franklin. According to John Ellis:<ref name="ellis">Template:Cite arXiv</ref>
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