CP violation in neutral kaons

In 1964 a small experiment found the first crack in the mirror between matter and antimatter, and it came from a peculiar particle called the neutral kaon. A kaon is a meson — a quark bound to an antiquark — and the neutral kaon has the strange property that it and its own antiparticle can turn into each other, blurring the line between the two. By watching how these particles decay, James Cronin and Val Fitch caught nature, for the first time, treating matter and antimatter slightly differently.

Here is the mechanism in words. The neutral kaon comes in a combination of two states distinguished by their CP behavior: one combination should decay only into two pions, the other only into three pions, if CP were a perfect symmetry. The two-pion decay is fast; the three-pion decay is slow. So if you let a beam of kaons travel a long way, the fast-decaying part dies off, and far downstream you should have a pure beam of the slow, three-pion-only kind. Cronin and Fitch found that a tiny fraction — about two in a thousand — of those supposedly three-pion-only kaons decayed into two pions anyway. That forbidden decay was the smoking gun: the long-lived kaon was not a pure CP state after all, and CP symmetry was broken.

This discovery, which won the 1980 Nobel Prize, was a turning point in physics. It proved that the matter-antimatter symmetry is not exact and forced theorists to build CP violation into the Standard Model. Even more strikingly, combined with the belief that the combined CPT symmetry is exact, CP violation implies that the laws of physics also distinguish a direction of time at the microscopic level — a deep and surprising link. The honest caveat is that kaon CP violation, while historic, is small and somewhat indirect; the cleaner, larger effects later found in B mesons gave physicists a much sharper tool for understanding the same underlying mechanism.