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][3] 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:[4]


  1. ^
  2. ^ Vainshtein, A. I.; Zakharov, V. I.; Shifman, M. A. (1975). Pisma Zh. Eksp. Teor. Fiz. 22: 123.  Missing or empty |title= (help)
    Translated in Vainshtein, A. I.; Zakharov, V. I.; Shifman, M. A. (1975). JETP Letters. 22: 55.  Missing or empty |title= (help)
  3. ^ Shifman, M. A.; Vainshtein, A. I.; Zakharov, V. I. (1977). "Asymptotic freedom, light quarks and the origin of the ΔT = {1}/{2} rule in the non-leptonic decays of strange particles". Nuclear Physics B. 120 (2): 316. Bibcode:1977NuPhB.120..316S. doi:10.1016/0550-3213(77)90046-3. 
  4. ^ Mikhail Shifman (1995). "ITEP Lectures in Particle Physics". arXiv:hep-ph/9510397Freely accessible [hep-ph].