Pseudodifferential Calculus, Twisted gerbes and twisted index theory for Lie groupoids

Abstract: The goal of this paper is to construct a calculus whose higher indices are naturally elements in the twisted K-theory groups for Lie groupoids. Given a Lie groupoid $G$ and a $PU(H)$-valued groupoid cocycle, we construct an algebra of projective pseudodifferential operators. The subalgebra of regularizing operators identifies with the naturally associated smooth convolution algebra of the associated twisted gerbe. We develop the associated symbolic calculus, symbol short exact sequences and existence of parametrices. In particular the algebra of projective operators appears as a quantization of the twisted symbol algebra. As the (untwisted) Lie groupoid case that it encompasses, the negative order operators extend to the twisted $C^*$-algebra and the zero order operators act as bounded multipliers on it. We obtain an analytic index morphism in twisted K-theory associated in a classic way by the corresponding pseudodifferential extension. We prove that this index morphism only depends on the isomorphism class of the cocycle, {\it i.e.,} on the twisting as the associated class in $H^1(G;PU(H))$. We also show that this twisted analytic index morphism is compatible with the index we constructed in a previous work, in collaboration with Bai-Ling Wang, by means of the Connes tangent groupoid, obtaining as a consequence the analytic interpretation, in terms of projective pseudodifferential operators and ellipticity, of the twisted longitudinal Connes-Skandalis index theorem. Our construction encompasses and unifies several previous cases treated in the literature, we discuss in the final section some examples of classes of operators unified by our setting.