k-meson

They concluded that if there exist at least three doublets of quarks, it is possible to accommodate one non-erasable complex phase in the transition amplitudes between up-type quarks and down-type quarks in the quark doublets, and this complex phase induces the CP violation in K-meson decays.

Since the discovery of the CP violation in K-meson decay, many attempts were made to look for CP violation processes other than neutral K-meson decays.

They realised that "strange" particles, known as K-mesons, did not exactly copy the behaviour of their anti-matter counterparts.

The trouble is that K-mesons are unstable, and die within less than a millionth of a second, leaving a trail of other particles or antiparticles.