Introduction to Elementary Particle Physics

In this chapter we shall discuss two important aspects, different but correlated, of the weak interactions: the CP-violation phenomena and the oscillations between members of flavoured, electrically neutral meson–antimeson pairs, the K0s, the B0s, the B0ss and the D0s. In each case, the states of definite flavour differ from those of definite mass and lifetime.

We begin with an elementary discussion of the neutral K system that will elucidate which are the states of definite strangeness, those of definite CP and those with definite mass and lifetime. We shall describe the oscillation between the former states, giving the relevant mathematical expressions and discussing the experimental evidence, including the observation of the regeneration of the initial flavour.

We then define the different modes of CP violation: in the wave function (or in the mixing), in the interference between decay with and without mixing, and in the decays. All modes happen for neutral mesons, the last one happens also for charged ones. We will describe how CP violation was discovered in the neutral K system in 1964.

In Section 8.6 we describe oscillations and CP violation in the B0 system, which needs a somewhat more-advanced formalism. In the same section we shall mention the discoveries of B0s oscillation, D0 mixing. These beautiful experimental results have been obtained at high-luminosity electron-positron colliders, KEKB in Japan and PEP2 in California, built on purpose and called beauty factories.