Current algebra and soft pionic modes in asymmetric quark matter
  Pionic excitations are analyzed in isospin-asymmetric Fermi gas of constituent quarks. It is found that in addition to the usual pionic excitations, there exists a very low collective mode with quantum numbers of the charged pion (spin-isospin sound). In the chiral limit the excitation energy of one of the modes scales as the current quark mass, and the mode saturates current-algebraic sum rules. This is in agreement with predictions of current algebra for isospin-asymmetric medium. 
Wojciech Broniowski and Brigitte Hiller