This phenomenon was explained theoretically by the extension of the Cabibbo mixing angle formalism to three quark generations by Kobayashi and Maskawa.
One experiment conducted to measure the degree of this phenomenon relied on  measuring the mass-difference between two states of the neutral B-meson; that was the BaBar experiment at SLAC. This phenomenon was confirmed experimentally with the discovery that a long-lived particle had a small two pion decay mode, rather than only a three-pion decay mode. This phenomenon was explained theoretically by the extension of the Cabibbo mixing angle formalism to three quark generations by Kobayashi and Maskawa. This phenomenon may be responsible for the matter-antimatter asymmetry in the universe, according to a hypothesis of Sakharov.  Discovered in the neutral kaon system, thereby helping to resolve the tau-theta puzzle, for 10 points, identify this phenomenon experimentally verified by Cronin and Fitch, in which the combination of two symmetries is broken.
One experiment conducted to measure the degree of this phenomenon relied on  measuring the mass-difference between two states of the neutral B-meson; that was the BaBar experiment at SLAC.
CP violation {or charge parity symmetry violation]