title: Massive Nambu-Goldstone bosons.
abstract: Nicolis and Piazza have recently pointed out the existence of Nambu-Goldstone-like excitations in relativistic systems at finite density, whose gap is exactly determined by the chemical potential and the symmetry algebra. We show that the phenomenon is much more general than anticipated and demonstrate the presence of such modes in a number of systems from (anti)ferromagnets in a magnetic field to superfluid phases of quantum chromodynamics. Furthermore, we prove a counting rule for these massive Nambu-Goldstone bosons and construct a low-energy effective Lagrangian that captures their dynamics.
authors.affiliation: Department of Physics, University of California, Berkeley, California 94720, USA. hwatanabe@berkeley.edu
authors.forename: Haruki
authors.initials: H
authors.lastname: Watanabe
authors.forename: Tomáš
authors.initials: T
authors.lastname: Brauner
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Murayama