\b[Forbidden at tree level.] A photon, gluon, $Z$ boson, or neutral Higgs interaction is flavor diagonal in the quark mass basis, while a $W$ boson connects an up-type quark to a down-type quark. The process $u\bar c\to c\bar u$ would therefore require a <flavor-changing neutral current>, which first appears through loops in the <Standard Model>.

Solution

Past exam of the mathematics course of the University of Cambridge / 2019 / iii / Paper 305 / 3 / a / i / Solution

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A flavor-changing neutral current changes fermion flavor without changing electric charge. Such currents are absent at tree level in the Standard Model and are strongly suppressed at loop level by the <Glashow-Iliopoulos-Maiani mechanism>.

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 Incoming links: Flavor-changing neutral current