Chern-Weil Constructions on $\mathrm{\Psi }$DO Bundles

We construct Chern-Weil classes on infinite dimensional vector bundles with structure group contained in the algebra $\text{cl}[\le 0](M,E)$ of non-positive order classical pseudo-differential operators acting on a finite rank vector bundle $E$ over a closed manifold $M$. Mimicking the finite dimensional Chern-Weil construction, we replace the ordinary trace on matrices by linear functionals on $\text{cl}[\le 0](M,E)$ built from the leading symbols of the operators. The corresponding Chern classes vanish for loop groups, but a weighted trace construction yields a non-zero class perviously constructed by Freed. For loop spaces, the structure group reduces to a gauge group of bundle automorphisms, and we produce non-vanishing universal Chern classes in all degrees, using a universal connection theorem for these bundles.