= Charged weak box contribution to kaon mixing
{title2=$\sum_{i,j}\lambda_i\lambda_jF(m_i,m_j)$}
A <Standard Model> charged-current <box diagram> converts $d\bar s$ into $s\bar d$. Internal <quarks> run over $u,c,t$, and the flavor sum contains $\lambda_i\lambda_j$ with $\lambda_i=V_{is}^*V_{id}$. The loop function depends on internal masses; <CKM matrix> unitarity gives $\sum_i\lambda_i=0$, producing the <GIM mechanism> cancellation of flavor-independent terms. The amplitude is of order $g^4$ and second order in the <weak interaction>.
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