Nonrelativistic effective Lagrangians

Abstract

Chiral perturbation theory is extended to nonrelativistic systems with spontaneously broken symmetry. In the effective Lagrangian, order parameters associated with the generators of the group manifest themselves as effective coupling constants of a topological term, which is gauge invariant only up to a total derivative. In the case of the ferromagnet, a term connected with the Brouwer degree dominates the derivative expansion. The general analysis includes antiferromagnetic magnons and phonons, while the effective field theory of fluids or gases is beyond the scope of the method.