Variational density matrices in quantum field theory at finite temperature and chemical potential.

Abstract

I evaluate the Helmholtz free energy of finite temperature {lambda}{var_phi}{sup 4} theory, both real and complex, using a variational quadratic {ital ansatz} for the density matrix. Minimizing with respect to the variational parameters produces results identical to those obtained by summing the daisy and superdaisy diagrams. In the nonrelativistic limit this is equivalent to a Hartree-Fock mean field with an effective mass. Quartic terms are then included by means of a relativistic generalization of the hypernetted-chain approximation without exchange terms, called the {open_quote}{open_quote}direct approximation.{close_quote}{close_quote} In this way infinite groups of rings and ladders are summed, giving nonperturbative expressions for the internal energy and four-point function in terms of a small number of Dyson-like integral equations. An expression is obtained for the internal energy of a zero-temperature system in terms of only two variational parameters. Because the hypernetted-chain approximation preserves the Euler-Lagrange variational principle, minimizing the internal energy with respect to these parameters should provide a semiquantitative upper bound on the ground state energy of an interacting relativistic system at zero temperature. For the full finite temperature theory in the direct approximation, there are now three variational parameters and it is necessary to obtain the entropy in a approximation comparable tomore » that for the internal energy. This is done in an analogous manner to the separability approximation of nonrelativistic hypernetted-chain theory. Finally, an improvement on the direct approximation is attained by including exchange terms of all types. This proceeds along the lines of parquet summations, resulting in a set of integral equations that, when solved self-consistently, includes all series and parallel connections of direct and exchange diagrams. {copyright} {ital 1996 The American Physical Society.}« less