[Paper Review] Effect of initial-state nucleon-nucleon correlations on collective flow in ultra-central heavy-ion collisions

Gabriel S. Denicol, Charles Gale+3 more|arXiv (Cornell University)|Jun 30, 2014

High-Energy Particle Collisions Research8 citations

TL;DR

This study investigates how nucleon-nucleon correlations in the initial state of ultra-central Pb-Pb collisions at LHC energies affect collective flow. Using MC-Glauber and IP-Glasma models, it demonstrates that including these correlations significantly reduces initial-state eccentricities (ε₂, ε₃), which persist through fluid-dynamical evolution and lead to a marked reduction in final-state elliptic and triangular flow harmonics (v₂{2}, v₃{2}), improving agreement with CMS data—particularly in the 0–1% centrality class.

ABSTRACT

We investigate the effect of nucleon-nucleon correlations on the initial condition of ultra-central heavy ion collisions at LHC energies. We calculate the eccentricities of the MC-Glauber and IP-Glasma models in the 0--1% centrality class and show that they are considerably affected by the inclusion of such type of correlations. For an IP-Glasma initial condition, we further demonstrate that this effect survives the fluid-dynamical evolution of the system and can be observed in its final state azimuthal momentum anisotropy.

Motivation & Objective

• To assess the impact of nucleon-nucleon correlations on initial-state geometry in ultra-central heavy-ion collisions at LHC energies.
• To determine whether such correlations affect the final-state momentum anisotropies (collective flow) in fluid-dynamical simulations.
• To evaluate whether including correlations improves theoretical predictions for flow harmonics compared to experimental data.
• To explore the potential of ultra-central collisions as a probe of initial-state wave function correlations and nuclear structure effects.

Proposed method

• Employed a Monte Carlo generator based on Alvioli et al. (2009) to sample nucleon configurations that include nucleon-nucleon correlations, preserving the Woods-Saxon single-particle density.
• Calculated initial-state eccentricities (ε₂, ε₃) in the 0–1% centrality class using both MC-Glauber and IP-Glasma models with and without correlations.
• Applied a 3+1D relativistic viscous hydrodynamic model to evolve the initial state to the final state, computing flow harmonics vₙ{2} for n=2,3.
• Used a transverse momentum cut-off of 0.3 GeV in momentum integrals to match experimental analysis conditions.
• Compared simulated flow harmonics with CMS data in both 0–1% and 2.5–5% centrality classes.
• Quantified the effect of correlations by comparing results with and without correlated nucleon configurations in the IP-Glasma framework.

Experimental results

Research questions

• RQ1 How do nucleon-nucleon correlations affect the initial-state eccentricities (ε₂, ε₃) in ultra-central Pb-Pb collisions?
• RQ2 To what extent do these initial-state modifications persist through fluid-dynamical evolution and influence final-state momentum anisotropies?
• RQ3 Does including nucleon-nucleon correlations improve the agreement between theoretical flow predictions and experimental data from CMS?
• RQ4 How does the effect of correlations differ between ultra-central (0–1%) and central (2.5–5%) collision classes?
• RQ5 What are the implications of these findings for probing initial-state wave function structure in high-multiplicity heavy-ion collisions?

Key findings

• Nucleon-nucleon correlations significantly reduce initial-state eccentricities ε₂ and ε₃ in both MC-Glauber and IP-Glasma models for 0–1% centrality collisions.
• The reduction in initial eccentricities due to correlations persists through fluid-dynamical evolution, leading to a substantial decrease in final-state elliptic and triangular flow harmonics (v₂{2}, v₃{2}).
• In the 0–1% centrality class, the inclusion of correlations reduces v₂{2} to values closer to experimental CMS data, improving theoretical agreement despite still being slightly above the data.
• The effect on flow harmonics is considerably smaller in the 2.5–5% centrality class, indicating a stronger sensitivity in ultra-central collisions.
• Theoretical predictions for v₃{2} remain slightly below the CMS data even after including correlations, suggesting additional effects may be missing.
• The study identifies that short-range attractive interactions and 3-nucleon correlations—currently unaccounted for—may further modify the results, pointing to future research directions.