(particle physics) any of four unstable subatomic particles, mesons, they are a combination of a strange quark or antiquark and either an up or down quark or antiquark
K⁰: strange quark + down antiquark
K (antiparticle of K⁰): down quark + strange antiquark
K⁺: strange quark + up antiquark
K⁻ (antiparticle of K⁺): up quark + strange antiquark