Chapter 6 Improving the IR of Holographic Descriptions of QCD As we saw chapter 5, a surprisingly good description of QCD can be obtained from a simple five dimensional gauge theory on a truncated AdS space [2]. However, there are many short-comings of the model: • The use of an AdS geometry implicitly means that the background gauge configuration is conformal • Hence the UV does not become asymptotically free • The existence of a mass gap is imposed by hand by the introduction of an infrared cutoff. It is not the result of a running coupling • The fields that holographically describe the quark bilinears are in- cluded phenomenologically • The solution for the field holographic to the quark mass and conden- sate is put in by hand - it is not dynamically determined by either the 85