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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 tree 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.〔http://www.quantumdiaries.org/2012/03/19/dissecting-the-penguin/〕
They were first isolated and studied by Mikhail Shifman, Arkady Vainshtein, and Valentin Zakharov.〔''JETP Letters'' 22, 55 (1975); Nucl. Phys. B 120, 316 (1977)〕
The processes which they describe were first directly observed in 1991 and 1994 by the CLEO collaboration.
== Origin of the name ==
John Ellis was the first to refer to a certain class of Feynman diagrams as penguin diagrams, due in part to their shape, and in part to a legendary bar-room bet with Melissa Franklin. According to John Ellis:
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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