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MC-EKRT: Monte Carlo event generator with saturated minijet production for initializing 3+1 D fluid dynamics in high energy nuclear collisions

Harri Niemi, Jussi Auvinen, Kari J. Eskola, Henry Hirvonen, Yuuka Kanakubo, Mikko Kuha

TL;DR

The paper addresses the challenge of generating realistic, fluctuating, three-dimensional initial conditions for $(3+1)D$ relativistic hydrodynamics in high-energy nuclear collisions. It introduces MC-EKRT, which combines LO collinear pQCD with the EKRT saturation conjecture and implements spatially dependent, event-by-event fluctuating nPDFs to produce fully specified partonic final states that are converted to initial energy-density profiles via Gaussian smearing. Key contributions include dynamical fluctuations beyond geometry, rigorous energy-momentum and valence-quark conservation, and a fast pathway to 3D initial states suitable for hydrodynamic evolution. Preliminary comparisons to ALICE and PHOBOS data show good overall agreement across centrality and pseudorapidity, supporting the robustness of the saturation mechanism across rapidities and highlighting the method's potential for improving predictive power in heavy-ion collisions.

Abstract

We present a novel saturation and leading order collinear factorization based Monte-Carlo implementation of the EKRT model for computing QCD matter initial states in high-energy nuclear collisions. As new features the MC implementation gives a full 3-dimensional initial state event-by-event, introduces new event-by-event fluctuating spatially dependent nuclear parton distribution functions, includes dynamical fluctuations in minijet production and saturation, and accounts for the energy-momentum and valence-quark number conservation.

MC-EKRT: Monte Carlo event generator with saturated minijet production for initializing 3+1 D fluid dynamics in high energy nuclear collisions

TL;DR

The paper addresses the challenge of generating realistic, fluctuating, three-dimensional initial conditions for relativistic hydrodynamics in high-energy nuclear collisions. It introduces MC-EKRT, which combines LO collinear pQCD with the EKRT saturation conjecture and implements spatially dependent, event-by-event fluctuating nPDFs to produce fully specified partonic final states that are converted to initial energy-density profiles via Gaussian smearing. Key contributions include dynamical fluctuations beyond geometry, rigorous energy-momentum and valence-quark conservation, and a fast pathway to 3D initial states suitable for hydrodynamic evolution. Preliminary comparisons to ALICE and PHOBOS data show good overall agreement across centrality and pseudorapidity, supporting the robustness of the saturation mechanism across rapidities and highlighting the method's potential for improving predictive power in heavy-ion collisions.

Abstract

We present a novel saturation and leading order collinear factorization based Monte-Carlo implementation of the EKRT model for computing QCD matter initial states in high-energy nuclear collisions. As new features the MC implementation gives a full 3-dimensional initial state event-by-event, introduces new event-by-event fluctuating spatially dependent nuclear parton distribution functions, includes dynamical fluctuations in minijet production and saturation, and accounts for the energy-momentum and valence-quark number conservation.

Paper Structure

This paper contains 3 sections, 3 equations, 2 figures.

Table of Contents

  1. Introduction
  2. MC-EKRT
  3. Results

Figures (2)

  • Figure 1: The impact of the EKRT saturation, energy-conservation and valence-quark number conservation filters on the rapidity dependence of minijet transverse energy in $\sqrt{s_{NN}}=5.02\,$TeV central (left panel) and peripheral Pb+Pb collisions (right panel). Figure is from Ref. Kuha:2024kmq.
  • Figure 2: Charged particle multiplicity $\mathrm{d}N_{\mathrm{ch}}/\mathrm{d}\eta$ as a function of pseudorapidity in Pb+Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV, compared with ALICE data ALICE:2016fbtALICE:2015juo (top panels), in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV, compared with PHOBOS data Back:2002wb (middle panels). The bottom panels show the charged particle $v_2\{4\}$ as a function of pseudorapidity in $\sqrt{s_{NN}}=2.76$ Pb+Pb collisions, compared with ALICE data ALICE:2016tlx (left), and in $\sqrt{s_{NN}}=200$ GeV Au+Au collisions compared with PHOBOS event plane $v_2$ data PHOBOS:2004vcu. Figures are from Ref. Kuha:2024kmq.