Loop Groups and Twisted K-Theory II
  This is the second in a series of papers investigating the relationship between the twisted equivariant K-theory of a compact Lie group G and the "Verlinde ring" of its loop group. We introduce the Dirac family of Fredholm operators associated to a positive energy representation of a loop group. It determines a map from isomorphism classes of representations to twisted K-theory, which we prove is an isomorphism if $G$ is connected with torsion-free fundamental group. We also introduce a Dirac family for finite dimensional representations of compact Lie groups; it is closely related to both the Kirillov correspondence and the equivariant Thom isomorphism.   In Part III (math.AT/0312155) we extend the proof of our main theorem to arbitrary compact Lie groups G and provide supplements in various directions. In Part I (arXiv:0711.1906) we develop twisted equivariant K-theory and carry out some of the computations needed here. We refer to the announcements math.AT/0312155 and math.AT/0206237 for further expository material and motivation. 
Daniel S. Freed, Michael J. Hopkins, Constantin Teleman