|Fig. 1: Schematic diagram of the Fitch-Cronin Experiment. The beam of Kaons enter the decay region as pure K2. The decay products are measured in a series of detectors that determine the momentum, mass, and timing information, thus allowing the decay angle θ to be determined.|
There are three important symmetries in nature: charge (C), parity (P), and time (T). Each of these corresponds to a change in a particle’s state: C reverses the sign of the charge, P flips left-handed into right-handed, and T reverses the direction of time. Until the 1950s it was believed that interactions governed by all four of the basic forces (strong, weak, electromagnetic, gravity) were invariant under any of these transformations. In 1956 Yang and Lee reviewed previous work and found that while there was more than enough data to see that the strong and electromagnetic interactions obeyed P, it hadn’t been tested with the weak force. They proposed an experiment on 60Co atoms that was carried out by Wu and showed that beta decay (governed by the weak interaction) violated P. However, the experiment would still be symmetric under both C and P (CP), leading many to believe that the laws of nature were actually invariant under CP and T. But if CP violation occurred in nature, it would be found in weak interactions.
The first experimental test of CP violation came in 1964 with the Fitch-Cronin experiment. The experiment involved particles called neutral K-mesons, which fortuitously have the properties needed to test CP. First, as mesons, they're a combination of a quark and an anti-quark, in this case down and antistrange, or anti-down and strange. Second, the two different particles have different CP values and different decay modes: K1 has CP = 1 and decays into two pions; K2 has CP = 1 and decays into three. Because decays with larger changes in mass occur more readily, the K1 decay happens 100 times faster than the K2 decay. This means that a sufficiently long beam of neutral Kaons will become arbitrarily pure K2 after a sufficient amount of time.
The Fitch-Cronin experiment exploits this. If all the K1s are allowed to decay out of a beam of mixed Kaons, only K2 decays should be observed. If any K1 decays are found, it means that a K2 flipped to a K1, and the CP for the particles flipped from -1 to +1, and CP wasn’t conserved.
Fitch and Cronin measured this using the apparatus shown in Fig. 1. The Kaons are sent down a 57-foot collimator, into a chamber of Helium. The K2's decay in the chamber, and the decay products are measured in a series of detectors at the end of the chamber. The detectors each consist of a spark chamber to determine direction, a magnet/scintillator to determine mass, and a water Cerenkov for timing information. The goal was to find θ, the sum of the angles from the horizontal of two particles hitting the detectors at the same time. In a three-body decay, this angle will very rarely be 0, while in a two-body decay it will almost always be 0 (the y-components will be equal, and the x-components will be swamped by the relativistic motion in the x-direction).
The results of the experiment are shown in Fig. 2. The experiment resulted in an excess of 45±9 events around cos(θ) = 1 in the correct mass range for 2-pion decays. This means that for every decay of K2 into three pions, there are (2.0±0.4)×10-3 decays into two pions. Because of this, neutral K mesons violate CP.
It’s still unclear, 44 years later, why CP is violated is violated in weak interactions. There’s no indication of CP violation in any other force. However, this does have important implications. Before the Fitch-Cronin experiment there was no known interaction that treated anti-matter differently from matter, and so there was no explanation for Baryogenesis, the observed asymmetry between matter and antimatter. CP violation in the weak force provides the only possibility so far discovered, although exactly how the difference forms is still a mystery. CP violation remains an active research area, with experiments such as BaBar at SLAC (and other B-factories) exploring the extent and character of the violation more accurately than Fitch-Cronin, with the eventual goal of providing answers to these deep questions.
© 2008 Stuart Coleman. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 J. H. Christenson, J. W. Cronin, V. L. Fitch and R. Turlay, "Evidence for the 2π Decay of the K20 Meson," Phys. Rev. Lett. 13, 138 (1964).
 T. D. Lee, and C.-N. Yang, "Question of Parity Conservation in Weak Interactions," Phys. Rev. 104, 254 (1956).
 C. S. Wu et al., "Experimental Test of Parity Conservation in Beta Decay," Phys. Rev. 105, 1413 (1957).