Generalizations of the Model

This chapter further develops the concepts introduced in the previous chapter at a level suitable for a reader without prior knowledge of spontaneous symmetry breaking. The material covered includes physical systems with multiple Nambu–Goldstone bosons and systems lacking Lorentz invariance. The focus is on understanding the connection between the pattern of symmetry breaking, the number of Nambu–Goldstone bosons, and their dispersion relations. It is shown how this connection can be conveniently addressed using a low-energy effective field theory. Altogether, this chapter makes the reader ready for the general, state-of-the-art discussion of spontaneous symmetry breaking, Nambu–Goldstone bosons and their effective field theory description, which follows later in the book.