Gluonic Hot Spot Initial Conditions in Heavy-Ion Collisions

TL;DR

This paper evaluates the MAGMA gluonic hot spot model for initial conditions in heavy-ion collisions, finding it fails to match experimental flow data when integrated with hydrodynamic simulations, and discusses alternative models.

Contribution

It critically assesses the MAGMA initial condition model against experimental data and explores alternative approaches within the Color Glass Condensate framework.

Findings

MAGMA does not reproduce flow coefficients in hydrodynamic simulations.

Hot spots from one nucleus do not resolve hot spots from the other in the model.

Alternative initial condition models are discussed for better data agreement.

Abstract

The initial conditions in heavy-ion collisions are calculated in many different frameworks. The importance of nucleon position fluctuations within the nucleus and sub-nucleon structure has been established when modeling initial conditions for input to hydrodynamic calculations. However, there remain outstanding puzzles regarding these initial conditions, including the measurement of the near equivalence of the elliptical $v_{2}$ and triangular $v_{3}$ flow coefficients in ultra-central 0-1% Pb+Pb collisions at the LHC. Recently a calculation termed MAGMA incorporating gluonic hot spots via two-point correlators in the Color Glass Condensate framework, and no nucleons, provided a simultaneous match to these flow coefficients measured by the ATLAS experiment, including in ultra-central 0-1% collisions. Our calculations reveal that the MAGMA initial conditions do not describe the experimental data when run through full hydrodynamic SONIC simulations or when the hot spots from one nucleus resolve hot spots from the other nucleus, as predicted in the Color Glass Condensate framework. We also explore alternative initial condition calculations and discuss their implications.