A precise measurement of the direct CP violation parameter Re ( ε ′ / ε ) The NA 48 Collaboration

The direct CP violation parameter Re(ε′/ε) has been measured from the decay rates of neutral kaons into two pions using the NA48 detector at the CERN SPS. With 3.3 million KL → ππ events collected during the 1998 and 1999 running periods, a result of Re(ε′/ε) = (15.0 ± 2.7)×10−4 has been obtained. The result combined with the published 1997 sample is Re(ε′/ε) = (15.3 ± 2.6)×10−4.