CP violation
and similarly for . Thus occasionally the decays as a K1 with CP = +1, and likewise the can decay with CP = −1. This is known as indirect CP violation, CP violation due to mixing of and its antiparticle. There is also a direct CP violation effect, in which the CP violation occurs during the decay itself. Both are present, because both mixing and decay arise from the same interaction with the W boson and thus have CP violation predicted by the CKM matrix. Direct CP violation was discovered in the kaon decays in the early 2000s by the NA48 and KTeV experiments at CERN and Fermilab.