Measurement of Direct Cp Violation by the Na48 Experiment at Cern

CP violation occurs in the Standard Model through the imaginary phase in the CKM mixing matrix. An alternative mechanism, proposed shortly after the discovery of the effect, is a superweak interaction. The latter can be ruled out by experimental observation of direct CP violation, parametrized by ε. Theoretical calculations of ε within the Standard Model are hard, but most predictions give ε/ε ∼ O(10−6) . The previous generation of experiments to measure direct CP violation (NA31 at CERN and E731 at Fermilab) gave inconclusive results. New experiments were setup to clarify the situation. In 1999 both new experiments presented results based on the first set of their statistics: Re(ε/ε)=(28.0 ± 4.1)×10−4 (KTeV, FNAL) and Re(ε′/ε)=(18.5±7.3)×10−4 (NA48, CERN). The existence of direct CP violation is thus confirmed. The final results of these two experiments, with substantially smaller uncertainties, are expected to conclude on the size of the direct CP violation effect. The experimental determination of Re(ε/ε) is based on the fact that the two CP violating neutral kaon decay amplitudes into two pions