You discovered CP-violation in the Kaon system.
In the Standard Model of Particle Physics the CP-symmetry states that the laws of physics should be the same if a particle is interchanged with its antiparticle (C symmetry), and the its spatial coordinates are inverted (P symmetry). The violation of the CP-symmetry is the consequence of the appearing of a complex phase in the Cabibbo-Kobayashi-Maskawa (CKM) matrix describing the quark mixing, or the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix describing the neutrino mixing.
The CP-violation was discovered in 1964 by James Cronin and Val Fitch at the Alternating Gradient Synchrotron at the Brookhaven laboratory, during the investigation of the deacy of kaon. For this discovery they received the Nobel Prize in Physics in 1980.