= Penguin diagram
A <Penguin diagram>[penguin] diagram is a <Loop diagram>[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 quark>[down-type quark] <Loop diagram>[loop]. Individual attachments need not be gauge invariant; the physical <Probability amplitude>[amplitude] includes the required companion graphs and <Flavor eigenstate>[flavour] sums.
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