Symmetry Violations and Quark Flavour Physics

Abstract

One of the surprising facts in our present understanding of the development of the Universe is the complete absence of "primordial" antimatter from the Big Bang about 13.7 billion years ago. The detection of charged cosmic-ray particles by magnetic spectrometers borne by balloons, satellites, and the space shuttle has shown no evidence for such primordial (high-energy) antibaryons; nor has the search for gamma rays from antimatter-matter annihilation yielded any such observation. In the early phases of the expanding Universe, a hot (10 32 K) and dense plasma of quarks, antiquarks, leptons, antileptons and photons coexisted in equilibrium. This plasma expanded and cooled down, and matter and antimatter could recombine and annihilate into photons. If all interactions were symmetric with respect to matter and antimatter, and if baryon and lepton numbers were conserved, then all particles would finally convert to photons, and the expansion of the Universe would shift the wavelength of these photons to the far infrared region. This cosmic microwave background radiation was indeed observed by Penzias and Wilson in 1965 [1], and its wavelength distribution corresponds exactly to Planck black-body radiation at a temperature of 2.73 K (see Fig. 9.1). The density of this radiation is about 5 × 10 2 photons/cm 3 .