Beyond truth and beauty: a fourth family of particles.

Abstract

To understand why some physicist think a fourth family of quarks may exist, but that there are not many more than four, one must first understand what is currently explained and unexplained by the standard model of particle physics. The standard model is based on the assumption that ordinary matter is composed of two types of particles, quarks and leptons, and that the forces between them are transmitted by a third category of particles called bosons. Leptons include the familiar electron and neutrino; the less familiar quarks combine to make up such large particles as the proton and the neutron. Three families of quarks have been discovered experimentally, each consisting of two particles, making a total of six quarks. Each family of quarks is roughly 10 times as massive as the preceding family. Experimentally, it turns out that every family of quarks is associated with a family of leptons, each consisting of a charged lepton and a neutral one. The standard model has a number of series defects. It does not prescribe the number of families of quarks and leptons. It also fails to predict the mass of all the remaining particles. Another major mystery is the fact that whereasmore » different kinds of quarks are often observed to transform into one another, leptons are never observed to do so. The proliferation of mysteries has led some theorists to suspect the existence of a fourth family of quarks and leptons. The discovery of such a family might clear up some long-standing questions. One of them has to do with the phenomena known as charge-parity violation. 9 figs.« less