On representations of star product algebras over cotangent spaces on   Hermitian line bundles
  For every formal power series $B=B_0 + \lambda B_1 + O(\lambda^2)$ of closed two-forms on a manifold $Q$ and every value of an ordering parameter $\kappa\in [0,1]$ we construct a concrete star product $\star^B_\kappa$ on the cotangent bundle $\pi : T^*Q\to Q$. The star product $\star^B_\kappa$ is associated to the formal symplectic form on $T^*Q$ given by the sum of the canonical symplectic form $\omega$ and the pull-back of $B$ to $T^*Q$. Deligne's characteristic class of $\star^B_\kappa$ is calculated and shown to coincide with the formal de Rham cohomology class of $\pi^*B$ divided by $\im\lambda$. Therefore, every star product on $T^*Q$ corresponding to the Poisson bracket induced by the symplectic form $\omega + \pi^*B_0$ is equivalent to some $\star^B_kappa$. It turns out that every $\star^B_kappa$ is strongly closed. In this paper we also construct and classify explicitly formal representations of the deformed algebra as well as operator representations given by a certain global symbol calculus for pseudodifferential operators on $Q$. Moreover, we show that the latter operator representations induce the formal representations by a certain Taylor expansion. We thereby obtain a compact formula for the WKB expansion. 
Martin Bordemann, Nikolai Neumaier, Markus J. Pflaum, Stefan Waldmann