## Chapter 6 Heavy Quark Photon Production Radiation 

## 6.1 Introduction 

The confidence in the application of perturbative QCD methods to jet energy loss in heavy ion collisions gained from the early quantitative understanding of π and η suppression with null direct- γ control [238, 277] has recently come into serious doubt [438]. Evidence from measurements such as high- p$_{T}$ correlations [417, 439] and nonphotonic electrons [423, 424] demonstrates clear disagree- ment with perturbative models [152, 357, 374, 435, 440]. Several papers postu- late alternative nonperturbative energy loss mechanisms [250, 342, 343, 436], and a new measurement, the double ratio of charm to bottom nuclear mod- ification factors, has been suggested as a robust observable for testing some of these novel ideas [441]. Electromagnetic radiation from quark jets, as it is transparent to the partonic medium, holds enormous promise as a new tool for investigating jet energy loss mechanisms; naively one expects the spectrum of 

