= Solution
\b[Allowed through second-order weak interactions, not a tree-level neutral-current vertex.] This is the flavour-changing transition responsible for <neutral kaon mixing>, with change of strangeness by two. Box <Feynman diagrams> contain two <W bosons> and internal up-type quarks. Their vertices come from the <weak charged current> and contain <CKM matrix> factors. At low energy they generate an operator $(\bar s_L\gamma_\mu d_L)(\bar s_L\gamma^\mu d_L)$ plus its conjugate. The <GIM mechanism> cancels the flavour-independent part of the internal-quark sum; unequal quark masses leave a nonzero loop amplitude.
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