A penguin diagram is a loop topology in which a neutral gauge boson is emitted during a flavour-changing transition. A representative electroweak top-quark diagram contains a charged vector and a down-type quark loop. Individual attachments need not be gauge invariant; the physical amplitude includes the required companion graphs and flavour sums.
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A penguin diagram is a type of visual representation used in particle physics to illustrate processes involving the interactions of particles, particularly in quantum field theory. The name derives from the graphical resemblance of the arrangement of particles and lines to a penguin. These diagrams help physicists visualize and analyze interactions such as scattering processes or decay events, typically involving fundamental particles like quarks and leptons.