With D. Nadler. Loop Spaces and Langlands Parameters.
We apply the technique of S^1-equivariant localization to sheaves on loop spaces in derived algebraic geometry, and obtain a fundametnal link between two families of categories at the heart of geometric representation theory. Namely, we categorify the well known relationship between free loop spaces, cyclic homology and de Rham cohomology to recover the category of D-modules on a smooth stack X as a localization of the category of S^1-equivariant coherent sheaves on its loop spaces LX. The main observation is that this procedure connects categories of equivariant D-modules on flag varieties with categories of equivariant coheernt sheaves on the Steinberg variety and its relatives. This provides a direct connetion between the geometry of finite and affine Hecke algebras and braid groups, and a uniform geometric construction of all of the categorical parameters for representations of real and complex reductive groups. This paper forms the first step in a project to apply the geometric Langlands program to the complex and real local Langlands programs, which we describe.