Penguin diagram
Encyclopedia
In quantum field theory
Quantum field theory
Quantum field theory provides a theoretical framework for constructing quantum mechanical models of systems classically parametrized by an infinite number of dynamical degrees of freedom, that is, fields and many-body systems. It is the natural and quantitative language of particle physics and...

, penguin diagrams are a class of Feynman diagrams which are important for understanding CP violating
CP violation
In particle physics, CP violation is a violation of the postulated CP-symmetry: the combination of C-symmetry and P-symmetry . CP-symmetry states that the laws of physics should be the same if a particle were interchanged with its antiparticle , and left and right were swapped...

 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.

They were first isolated and studied by Mikhail Shifman
Mikhail Shifman
-External links:*****...

, Arkady Vainshtein, and Valentin Zakharov.
The processes which they describe were first directly observed in 1991 and 1994 by the CLEO
CLEO (particle detector)
CLEO was a general purpose particle detector at the Cornell Electron Storage Ring , and the name of the collaboration of physicists who operated the detector. The name CLEO is not an acronym; it is short for Cleopatra and was chosen to go with CESR . CESR was a particle accelerator designed to...

 collaboration.

Origin of the name

John Ellis
John Ellis (physicist)
Jonathan Richard Ellis FRS is a British theoretical physicist who is currently Clerk Maxwell Professor of Theoretical Physics at King's College London. After completing his secondary education at Highgate School, he attended Cambridge University, earning his Ph.D. in theoretical particle physics...

 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
Melissa Franklin
Melissa Franklin is an experimental particle physicist and the Department Chair and Mallinckrodt Professor of Physics at Harvard University. While working at the Fermi National Acceleration Laboratory in Chicago, her team found some of the first evidence for the existence of the top quark....

. According to John Ellis:
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