Modeling Heavy Ion Collisions in AdS/CFT

TL;DR

This paper models high energy heavy ion collisions using AdS/CFT, revealing rapid nuclear stopping at strong coupling and proposing a mixed approach to account for weak coupling effects, with implications for early-time energy density evolution.

Contribution

It introduces a shock wave collision model in AdS_5 that captures nuclear stopping and extends it to include small-coupling effects via unphysical shock waves, providing insights into early energy density dynamics.

Findings

Shock waves fully stop immediately after collision in AdS_5.

At strong coupling, the energy density remains constant initially.

A perturbative expansion in graviton exchanges reproduces early-time energy density behavior.

Abstract

We construct a model of high energy heavy ion collisions as two ultrarelativistic shock waves colliding in AdS_5. We point out that shock waves corresponding to physical energy-momentum tensors of the nuclei completely stop almost immediately after the collision in AdS_5, which, on the field theory side, corresponds to complete nuclear stopping due to strong coupling effects, likely leading to Landau hydrodynamics. Since in real-life heavy ion collisions the large Bjorken x part of nuclear wave functions continues to move along the light cone trajectories of the incoming nuclei leaving the small-x partons behind, we conclude that a pure large coupling approach is not likely to adequately model nuclear collisions. We show that to account for small-coupling effects one can model the colliding nuclei by two (unphysical) ultrarelativistic shock waves with zero net energy each (but with non-zero energy density). We use this model to study the energy density of the strongly-coupled matter created immediately after the collision. We argue that expansion of the energy density in the powers of proper time squared corresponds on the gravity side to a perturbative expansion of the metric in graviton exchanges. Using such expansion we reproduce our earlier result that the energy density of produced matter at mid-rapidity starts out as a constant (of time) in heavy ion collisions at large coupling.