Gaussian $β$-Markov Path Spaces without a Spectral Gap and the Araki--Woods Representations

Dereziński and Gérard constructed the Gaussian $β$-Markov path space of a free Bose field in a form equivalent to the Araki--Woods representation, which suits nonequilibrium statistical mechanics with a quantum-field environment, but only for one-particle Hamiltonians with a spectral gap. We extend this construction, together with the equivalence to the Araki--Woods representation, to one-particle Hamiltonians without a spectral gap. The extension is intended for constructive quantum field theory at positive temperature and equilibrium statistical mechanics, for instance in the problem of the absence of Bose--Einstein condensation of quasiparticles whose number is not conserved.