Trinity

Chapter 3 explains evidence for three generations of quarks and leptons, as needed to provide natural means for standard model CP violation. It describes the cross-generational mixing of quarks and of neutrinos of different flavor, and the matrices that characterize the mixing. CP violation from quark mixing is well measured but insufficient to explain the universe’s matter–antimatter imbalance, while CP violation in neutrino mixing is the subject of ongoing searches. Discoveries revealing and quantifying flavor oscillations among neutrinos from the sun and the atmosphere are reviewed. In describing attempts to understand the lightness and nature of neutrinos—whether they are Majorana neutrinos that are their own antiparticles—the chapter introduces the concept of chirality and searches for neutrinoless double beta decay. It also notes the strong CP problem, wherein nature maintains CP symmetry in interactions among the three quark colors even though quantum chromodynamics allows for sizable violations.