Understanding the fragmentation region of heavy-ion collisions

The fragmentation region of a heavy-ion collision is the area of phase-space in which the products and fragments of the collision have the same (or very similar) rapidity as that of the beam. This region is critically understudied, despite offering the possibility of studying high-density regions of the QCD phase-diagram in existing collider experiments. I will present a series of initial explorations of the fragmentation region, beginning with a simple model that provides evidence that densities of 2-3 times the nuclear saturation density may be reached in this region. I will then show the most recent rigorous calculations of bremsstrahlung in the fragmentation region, a calculation that incorporates both the non-perturbative physics of gluon saturation (as described by the color-glass condensate) and the correct perturbative physics of high-frequency bremsstrahlung. These results offer insight into how one might correctly model the hydrodynamics of the fragmentation region, but also has implications for bremsstrahlung calculations in other theories such as QED.