Penguin diagram

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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.[1]

They were first isolated and studied by Mikhail Shifman, Arkady Vainshtein, and Valentin Zakharov.[2] The processes which they describe were first directly observed in 1991 and 1994 by the CLEO collaboration.

Origin of the name[edit]

Example of a penguin diagram

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:[3]


  1. ^
  2. ^ JETP Letters 22, 55 (1975); Nucl. Phys. B 120, 316 (1977)
  3. ^ Mikhail Shifman (1995). "ITEP Lectures in Particle Physics". arXiv:hep-ph/9510397Freely accessible [hep-ph].