Phenomenology of the little bang

TL;DR

This paper reviews recent advances in modeling ultrarelativistic heavy-ion collisions, emphasizing viscous hydrodynamics, initial condition uncertainties, and the role of fluctuations in observed correlation structures.

Contribution

It highlights the progress in understanding the expansion dynamics and introduces the concept of triangular flow as a mechanism for observed correlations.

Findings

Viscous hydrodynamics effectively models the expansion phase.

Initial condition modeling remains inadequate.

Triangular flow explains long-range correlation structures.

Abstract

I review recent selected developments in the theory and modeling of ultrarelativistic heavy-ion collisions. I explain why relativistic viscous hydrodynamics is now used to model the expansion of the matter formed in these collisions. I give examples of first quantitative predictions, and I discuss remaining open questions associated with the description of the freeze-out process. I argue that while the expansion process is now well understood, our knowledge of initial conditions is still poor. Recent analyses of two-particle correlations have revealed fine structures known as ridge and shoulder, which extend over a long range in rapidity. These correlations are thought to originate from initial state fluctuations, whose modeling is still crude. I discuss triangular flow, a simple mechanism recently put forward, through which fluctuations generate the observed correlation pattern.