  Recently a new bosonization method has been used to derive, at zero fermion
density, an effective action for relativistic field theories whose partition
function is dominated by fermionic composites, chiral mesons in the case of
QCD. This approach shares two important features with variational methods: the
restriction to the subspace of the composites, and the determination of their
structure functions by a variational calculation. But unlike standard
variational methods it treats excited states at the same time and on the same
footing as the ground state. I extend this method including states of
nonvanishing fermion (baryon) number and derive an effective action for QCD at
finite temperature and baryon density. I test the result on a four-fermion
interaction model.
