In this chapter we shall examine the role of the experiment of Christenson, Cronin, Fitch, and Turlay in refuting the concept of invariance of physical processes under CP (combined space inversion and particle–antiparticle interchange). We shall see that this experiment was a “convincing” experiment. It persuaded most of the physics community that CP was violated. There were, however, several alternative explanations offered to explain this result, and we shall also discuss the role that experiment played in refuting or eliminating these alternatives. We begin with the historical background of the experiment and then examine both the experiment itself and the reaction of the physics community to it.

HISTORICAL BACKGROUND

From the time of their discovery in 1947 by Rochester and Butler, the neutral K mesons, or kaons, have exhibited odd features. One puzzle, that certain K particles of equal masses and lifetimes (called θ and τ) seemed to decay into states of opposite parity, was resolved by the discovery of parity nonconservation (see Chapter 1). An equally puzzling observation concerning the kaons was put this way by C. N. Yang: “While the [se] strange particles are produced quite abundantly (say 5% of the pions) at BeV energies and up, their decays into pions and nucleons are rather slow (10−10 sec). Since the time scale of pion-nucleon interactions is of the order of 10−23 sec, it was very puzzling how to reconcile the abundance of these objects with their longevity (1013 units of time scale).”