Goldstone and pseudo-Goldstone bosons in nuclear, particle and condensed-matter physics

It is a common feature of many physical systems that their behaviour is relatively simple when examined only at low energies (or temperatures) compared to the systemʹs own characterisitic scales. It often happens that there are relatively few states which can participate in low-energy processes, and their interactions can sometimes become less and less important the lower the energies that are examined. Very general theoretical tools exist to exploit this simplicity, when it arises. Systems with spontaneously broken symmetries (i.e. for which a symmetry of the hamiltonian is not also a symmetry of its ground state) form a very important class of examples of this type, due to the Goldstone bosons which inevitably appear in their low-energy spectrum. This review develops the theory of Goldstone bosons, concentrating on their description in terms of an effective lagrangian formulation.