= Solution
With the <valence quark> assignments $K^0=d\bar s$ and $\bar K^0=s\bar d$, <neutral kaon mixing> changes <strangeness> by two units. A representative leading <Standard Model> contribution is a <box Feynman diagram> with two <W bosons> and two internal up-type <quarks>:
$$
d\longrightarrow u_i\longrightarrow s,\qquad
\bar s\longrightarrow\bar u_j\longrightarrow\bar d,
\qquad u_i,u_j\in\{u,c,t\}.
$$
The two lines exchange the two <W bosons>. There are four <weak charged current> <interaction vertices>, so the <scattering amplitude> is fourth order in the weak <gauge coupling>, or second order in the <Fermi constant>. The full result sums the internal <quark flavors> with their <Cabibbo-Kobayashi-Maskawa matrix> factors; one box alone illustrates the mechanism.
\Image[/past-exam-of-the-nuclear-and-particle-physics-course-of-the-university-of-oxford/2022-kaon-diagrams.png]
{title=Representative charged-current box and two- and three-pion kaon decay diagrams}
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