Up to now, the AdS/CFT duality has been used to model thermal transport processes in a strongly coupled gauge plasma (see the Review in this VJ). It was unclear what theories with an AdS/CFT dual predict for hard processes, for which QCD permits rigorous calculations. In several recent articles,
- Deep inelastic scattering at strong coupling from gauge/string duality: the saturation line
- Deep inelastic scattering off a N=4 SYM plasma at strong coupling
- Jet evolution in the N=4 SYM plasma at strong coupling
Hatta et al. have shown how such processes can be explored in strongly coupled super-Yang Mills theories with a gravity dual. The authors couple a localized color-singlet current to the Minkowski space boundary of the thermal AdS5 space-time and study how it propagates into the bulk. The spread into the bulk represents the quantum mechanical spread of the orininally localized probe. The explicit solution permits a complete study of the space-time response of the thermal medium as a function of the energy and virtuality of the probe, including all quantum effects at strong coupling. For example, the authors are able to map out in detail how a jet fragments and ultimately disappears into the thermal medium.
This work opens the door to a complete study of hard processes, akin to deep-inelastic scattering and jet formation and quenching, in the gauge theories with a gravity dual. It will be interesting to see how the results map onto the formalism of perturbative QCD in the presence of a medium. It will also be interesting to find out which of the observed features are characteristic of strong coupling, in general, and which ones are peculiar to a theory without asymptotic freedom.