%0 Journal Article
%T N-quantum approach to quantum field theory at finite T and $¦Ì$: the NJL model
%A Yasuo Umino
%A Vicente Vento
%J Physics
%D 1999
%I arXiv
%R 10.1016/S0370-2693(99)01370-2
%X We extend the N-quantum approach to quantum field theory to finite temperature ($T$) and chemical potential ($\mu$) and apply it to the NJL model. In this approach the Heisenberg fields are expressed using the Haag expansion while temperature and chemical potential are introduced simultaneously through a generalized Bogoliubov transformation. Known mean field results are recovered using only the first term in the Haag expansion. In addition, we find that at finite T and in the broken symmetry phase of the model the mean field approximation can not diagonalize the Hamiltonian. Inclusion of scalar and axial vector diquark channels in the SU(2)$_{rm f}$ $otimes$ SU(3)$_{\rm c}$ version of the model can lead to a lowering of the vacuum energy density. We discuss how to go beyond the mean field approximation by including higher order terms in the Haag expansion.
%U http://arxiv.org/abs/hep-ph/9907354v3