kaons

• lightest mesons containing a strange quark (strange quark or antiquark, with an up or down one)
• decay only via the weak interaction → long-lived (charged kaons ≈ 12 ns)
• neutral kaons oscillate between kaon and antikaon → a short-lived state (≈ 0.09 ns) and a long-lived one (≈ 51 ns)
• CP violation first seen here: long-lived neutral kaon → two pions (Cronin and Fitch, 1964)