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A kaon is a strange meson. The ground-state flavor assignments are $K^{+}=u\bar{s}$ and $K^0=d\bar{s}$, with antikaons $K^{-}=s\bar{u}$ and ${\bar{K}}^0=s\bar{d}$. These ground states have spin-parity $0^{-}$. The anti-strange assignments have strangeness $+1$, with the opposite sign for their antiparticles.^[ref]

Charged and neutral kaons differ in more than electric charge. Through neutral kaon mixing, the neutral flavor states combine into propagation states with markedly different decay behavior. Their time-dependent decays provide sensitive tests of CP violation. Specifying whether a label denotes a flavor state or a propagation state is therefore essential when describing a neutral-kaon experiment.

The neutral vector mesons with isospin zero have the same conserved quantum numbers, so an off-diagonal mass matrix can mix the singlet $V_1=(u\bar{u}+d\bar{d}+s\bar{s})/\sqrt{3}$ and octet $V_8=(u\bar{u}+d\bar{d}-2s\bar{s})/\sqrt{6}$. Choose phases such thatAt the ideal mixing angle $\tan \theta =1/\sqrt{2}$, or $\theta \simeq 35.3^{\circ }$, these becomeThe overall sign of $\varphi$ is unobservable. The omega meson near $782\mathrm{MeV}$ decays predominantly to three pions, whereas the phi meson near $1020\mathrm{MeV}$ strongly favours kaon-antikaon channels when they are open. Suppression of its nonstrange channels agrees with the OZI rule and a predominantly strange state. The omega meson is below the two-kaon threshold, so its absence of that channel alone would not establish the mixing angle; masses and the broader decay pattern provide the evidence together.^[ref]