\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

 Solution (source code)