Perverse Sheaves on Loop Grassmannians and Langlands Duality

Abstract

We outline a proof of a geometric version of the Satake isomorphism. Given a connected, complex algebraic reductive group G we show that the tensor category of representations of the dual group $\check G$ is naturally equivalent to a certain category of perverse sheaves on the loop Grassmannian of G. The above result has been announced by Ginsburg in \cite{G} and some of the arguments are borrowed from \cite{G}. However, we use a more "natural" commutativity constraint for the convolution product, due to Drinfeld. Secondly, we give a direct geometric proof that the global cohomology functor is exact and decompose this cohomology functor into a direct sum of weights. We avoid the use of the decomposition theorem of \cite{BBD} which makes our techniques applicable to perverse sheaves with coefficients over arbitrary commutative rings. This note includes sketches of (some of the) proofs. The details, as well as the generalization to representations over arbitrary fields and commutative rings will appear elsewhere.