Review of NA48 CP violation measurements with neutral and charged kaons

The main goal of the NA48 experiments at the CERN SPS has been the search for direct CP violation (CPV) in kaon decays. In this paper results from NA48 and NA48/2 are presented. The direct CPV parameter Re(ε′/ε) has been measured by NA48 from the decay rates of neutral kaons into two pions: combining the results from data collected in 1997, 98, 99 and 2001, an overall value of Re(ε′/ε) = (14.7 ± 2.2) × 10 is obtained. Using a sample of 47000 KL→ and five million Ke3 decays collected during a dedicated run in 1999 with a pure KL beam, NA48 measured the ratio of the decay rates Γ(KL→)/Γ(KL→eν), from which the branching ratio of the CPV decay KL→, BR(KL→) = (1.941 ± 0.019)×10 and the CPV parameter |η+−| = (2.223 ± 0.012) × 10 were derived. In the charged kaon sector, NA48/2 has measured the asymmetry Ag = (g − g)/(g + g) of the linear slope parameter g in the Dalitz plot of K → 3 decays from data collected in 2003 and 2004. Any non-zero value of Ag would be evidence for direct CPV. A new technique of asymmetry measurement, involving simultaneous K and K beams and a large data sample allowed a result of unprecedented precision. The charge asymmetries were measured to be Ag = (-1.5 ± 2.2) × 10 with 3.11×10 K →   decays and Ag = (1.8 ± 1.8) × 10 with 9.13×10 K →   decays.