Results on Direct CP Violation from NA48

A long-standing question in high energy physics has been the origin of the phenomenon of CP violation. CP violation was first observed in the decay KL → π+π− [1]. Effects have subsequently been found in KL → π0π0 [2], the charge asymmetry of KL → e±π∓ν (Ke3) [3] and KL → μ±π∓ν (Kμ3) [4], KL → π+π−γ [5] and most recently in KL → ππee [6]. All of these effects can be explained by applying a single CP violating effect in the mixing between K0 and K0 which proceeds through the box Feynman diagram and are characterized by the parameter ε. A second form of CP violation with different characteristics can also be investigated in the decay of the neutral K-mesons. The CP = −1 kaon state (the K2) can decay directly into a 2π final state without first mixing into a kaon with CP = +1. This effect, referred to as direct CP violation, may proceed by the penguin Feynman diagram, and is characterized by the parameter ε′. The measured quantity is the double ratio of the decay widths, which, in the NA48 experiment is equivalent to the double ratio of four event-counts