Extensions of the Standard Model

Abstract

Abstract The Standard Model described in Chapter 3, which has given a magnificently accurate account of a huge range of data from accelerator experiments since its inception, does, however, have limitations. As examples, it assumes neutrinos are massless, in conflict with recent experimental results, and has theoretical difficulties with topics as diverse as the so-called hierarchy problem, or of accounting for the baryon asymmetry of the universe. As will be indicated in later chapters, it is indeed on the scale of the universe that it fails to take into account completely new forms of matter and energy, which have only really become apparent since the Standard Model was first introduced in the mid-1970s. Finally of course, it does not include gravity. Including gravity with the other fundamental interactions is still an unsolved problem. Of course, we do not know what better theory will eventually replace the Standard Model, although whatever that is, the Standard Model will surely be a part of it. In this chapter, we just outline some new directions in physics going beyond it. As was pointed out in Chapter 1, the question as to the nature of neutrinos— whether they are Dirac or Majorana particles—is still open. In the Standard Model, neutrinos are assumed to be massless—an assumption in accord with all the data available when the Standard Model first appeared. But, as described below, evidence from neutrino flavour oscillations, appearing after 1990 showed the neutrino masses to be finite, although very small. The fact that the masses of the light neutrinos, of order 0.1 eV/c, are some ten orders of magnitude less than the typical masses of the known Dirac particles, such as quarks and charged leptons, suggests that they might indeed be Majorana particles, the smallness of the mass arising from the so-called see-saw mechanism described below.