Uniformly positive correlations in the dimer model and phase transition in lattice permutations on ${\mathbb{Z}}^d$, $d>2$, via reflection positivity

Our first main result is that correlations between monomers in the dimer model in ${\mathbb{Z}}^d$ do not decay to zero when $d>2$. This is the first rigorous result about correlations in the dimer model in dimensions greater than two and shows that the model behaves drastically differently than in two dimensions, in which case it is integrable and correlations are known to decay to zero polynomially. Such a result is implied by our more general, second main result, which states the occurrence of a phase transition in the model of lattice permutations, which is related to the quantum Bose gas. More precisely, we consider a self-avoiding walk interacting with lattice permutations and we prove that, in the regime of fully-packed loops, such a walk is long' and the distance between its end-points grows linearly with the diameter of the box. These results follow from the derivation of a version of the infrared bound from a new general probabilistic settings, with coloured loops and walks interacting at sites and walks entering into the system from some virtual’ vertices.