Quantum statistics with internal symmetry

We formulate a group-theoretical projection technique for the quantum-statistical description of systems with exactly conserved charges corresponding to local non-Abelian gauge symmetries. The formalism is specified for SU(N) internal symmetry and a partition function related to a mixed canonical–grand-canonical ensemble is defined. Its perturbation expansion is derived, and we point out potential applications. We also study single-particle Green’s functions for the calculation of mixed ensemble averages with the help of a generalized Wick’s theorem and find that a connected-graphs expansion is impossible.