Pion condensation and mean field theory

Developments in the last decade in both physics and astrophysics have stimulated interest in the theory of matter above nuclear densities (-- 10/sup 14/ gm/cc). At high densities new states of matter involving additional particles or field configurations become possible. In recent years several such states have been discussed - the pion condensate, the density isomer and the quark matter. In these lectures the authors discuss pion condensation. Nuclear matter may exist in a state where pions or more accurately pion-like excitations spontaneously appear. Such a state has been called the pion condensate state since pions (like helium atoms) would form a condensate. Thus nuclear matter ground state may at a certain density make a transition to another state in which there are pions with a macroscopic occupation number