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The PHOBOS Glauber Monte Carlo

B. Alver, M. Baker, C. Loizides, P. Steinberg

TL;DR

This paper presents a detailed implementation of the Glauber Monte Carlo model used to estimate the initial geometric configuration in heavy-ion collisions. It describes event-by-event nucleon placement using nuclear density parameterizations and an eikonal collision criterion that yields $N_{part}$, $N_{coll}$, $b$, and eccentricities such as $\epsilon_{RP}$ and $\epsilon_{part}$. The authors provide a ROOT-based user guide for the PHOBOS code, including class definitions, example routines, and sample results across RHIC and LHC energies. The work delivers a practical tool to estimate initial-state geometry and its event-by-event fluctuations, enabling comparisons across systems and energies and informing analyses of eccentricity-driven observables like $\epsilon_{RP}$ and $\epsilon_{part}$.

Abstract

``Glauber'' models are used to calculate geometric quantities in the initial state of heavy ion collisions, such as impact parameter, number of participating nucleons and initial eccentricity. The four RHIC experiments have different methods for Glauber Model calculations, leading to similar results for various geometric observables. In this document, we describe an implementation of the Monte Carlo based Glauber Model calculation used by the PHOBOS experiment. The assumptions that go in the calculation are described. A user's guide is provided for running various calculations.

The PHOBOS Glauber Monte Carlo

TL;DR

This paper presents a detailed implementation of the Glauber Monte Carlo model used to estimate the initial geometric configuration in heavy-ion collisions. It describes event-by-event nucleon placement using nuclear density parameterizations and an eikonal collision criterion that yields , , , and eccentricities such as and . The authors provide a ROOT-based user guide for the PHOBOS code, including class definitions, example routines, and sample results across RHIC and LHC energies. The work delivers a practical tool to estimate initial-state geometry and its event-by-event fluctuations, enabling comparisons across systems and energies and informing analyses of eccentricity-driven observables like and .

Abstract

``Glauber'' models are used to calculate geometric quantities in the initial state of heavy ion collisions, such as impact parameter, number of participating nucleons and initial eccentricity. The four RHIC experiments have different methods for Glauber Model calculations, leading to similar results for various geometric observables. In this document, we describe an implementation of the Monte Carlo based Glauber Model calculation used by the PHOBOS experiment. The assumptions that go in the calculation are described. A user's guide is provided for running various calculations.

Paper Structure

This paper contains 9 sections, 3 equations, 3 figures, 1 table.

Table of Contents

  1. Introduction
  2. The Model
  3. Makeup of Nuclei
  4. Collision Process
  5. Users' Guide
  6. Running the Code
  7. Example functions
  8. Sample Results
  9. Conclusion

Figures (3)

  • Figure 1: Typical events for Cu+Cu (top panel), Au+Au (middle panel), and Pb+Pb (lower panel) collisions, the first two performed at RHIC energies and the latter at the LHC. Wounded nucleons (participants) are indicated as solid circles, while spectators are dotted circles.
  • Figure 2: Distributions of $N_{\rm part}$ and $N_{\rm coll}$ for 10k events for Cu+Cu and Au+Au at RHIC, and Pb+Pb at the LHC.
  • Figure 3: $\hbox{$\epsilon_{\rm RP}$}$ (open symbols) and $\hbox{$\epsilon_{\rm part}$}$ (closed symbols) as a function of $N_{\rm part}$ for Cu+Cu and Au+Au collisions at RHIC and Pb+Pb collisions at the LHC.