First Evidence for Direct CP Violation

Abstract

Since CP violation was first observed in the decay of the long-lived neutral kaon into two pions [1], it remains one of the enigmas in particle physics. Whilst CP violation is manifest in neutral kaon decays, the search for CP-violating effects has elsewhere been unsuccessful. In the phenomenology of CP violation in the neutral kaon system [2], the short- and long-lived mass eigenstates are usually defined in terms of the CP eigenstates K₁ (CP = + 1) and K₂ (CP = - 1) as K_s ≈ K₁ + ?K₂ and K_L ≈ K₂ + ?K₁. The parameter ? describes CP violation induced by kaon state-mixing. Direct CP violation may also occur in the decay of K₂ into two pions with a relative amplitude ?′, which is non-zero in the case of a phase difference between the amplitudes A₀ and A₂ for the decay into isospin O and 2 states of two pions. Before the present measurement, all experimental results were compatible with ? = 2.27 × 10^-3 exp (i 43.7⁰) and with the Superweak Model [3], in which state-mixing is the only source of CP violation and ?′ = 0. In the Standard Model with six weakly interacting quarks [4], direct CP violation as well as state-mixing is introduced by transitions via heavy-quark intermediate states. Based on this, a small, but non-zero, value of ?′ is predicted [5]. To a good approximation, ?′ /? is related to the double ratio R of the relative decay rates of the long- and short-lived neutral kaons into two neutral and two charged pions as Re (?′/?) = 1/6 × (1 - R).