The semi-infinite intersection cohomology sheaf-II: the Ran space version

Abstract: This paper is a sequel to [Ga1]. We study the semi-infinite category on the Ran version of the affine Grassmannian, and study a particular object in it that we call the semi-infinite intersection cohomology sheaf. Unlike the situation of [Ga1], this version of the semi-infinite intersection IC sheaf is defined as the middle of extension of the constant (more precisely, dualizing) sheaf on the basic stratum, in a certain t-structure. We give several explicit description and characterizations of our semi-infinite intersection IC sheaf: we describe its !- and *- stalks; we present it explicitly as a colimit; we relate it to the IC sheaf of Drinfeld's relative compactification; we describe it via the Drinfeld-Plucker formalism.