Energy dependent growth of the nucleon and hydrodynamic initial conditions

TL;DR

This paper investigates how gluon saturation causes the nucleon's size to grow with energy, smoothing initial conditions in heavy-ion collisions, and discusses implications for hydrodynamic modeling at RHIC and LHC energies.

Contribution

It introduces an energy-dependent nucleon size effect on initial conditions and analyzes its impact on hydrodynamic applicability in heavy-ion collisions.

Findings

Nucleon size increases with collision energy due to gluon saturation.

Initial energy density fluctuations become smoother at higher energies.

Hydrodynamics can be applied earlier at LHC energies than at RHIC.

Abstract

Due to gluon saturation, the growth of the inelastic nucleon-nucleon cross section with increasing collision energy sqrt(s) results in a broadening of the nucleon's density distribution in position space. This leads to a natural smoothing of the initial energy density distribution in the transverse plane of the matter created near midrapidity in heavy-ion collisions. We study this effect for fluctuating initial conditions generated with the Monte Carlo Kharzeev-Levin-Nardi (MC-KLN) model for Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). We argue that at the LHC viscous hydrodynamics is applicable at earlier times than at RHIC, not only because of the higher temperature but also since the length scale over which the initial pressure fluctuates increases with collision energy.