KMS conditions, standard real subspaces and reflection positivity on the circle group

Abstract: In the present paper we continue our investigations of the representation theoretic side of reflection positivity by studying positive definite functions \psi on the additive group (R,+) satisfying a suitably defined KMS condition. These functions take values in the space Bil(V) of bilinear forms on a real vector space V. As in quantum statistical mechanics, the KMS condition is defined in terms of an analytic continuation of \psi to the strip { z \in C: 0 \leq Im z \leq b} with a coupling condition \psi (ib + t) = \oline{\psi (t)} on the boundary. Our first main result consists of a characterization of these functions in terms of modular objects (\Delta, J) (J an antilinear involution and \Delta > 0 selfadjoint with J\Delta J = \Delta^{-1}) and an integral representation. Our second main result is the existence of a Bil(V)-valued positive definite function f on the group R_\tau = R \rtimes {\id_\R,\tau} with \tau(t) = -t satisfying f(t,\tau) = \psi(it) for t \in R. We thus obtain a 2b-periodic unitary one-parameter group on the GNS space H_f for which the one-parameter group on the GNS space H_\psi is obtained by Osterwalder--Schrader quantization. Finally, we show that the building blocks of these representations arise from bundle-valued Sobolev spaces corresponding to the kernels 1/(\lambda^2 - (d^2)/(dt^2}) on the circle R/bZ of length b.