Centrality determination of Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV with ALICE

Abstract: This publication describes the methods used to measure the centrality of inelastic Pb-Pb collisions at a center-of-mass energy of 2.76 TeV per colliding nucleon pair with ALICE. The centrality is a key parameter in the study of the properties of QCD matter at extreme temperature and energy density, because it is directly related to the initial overlap region of the colliding nuclei. Geometrical properties of the collision, such as the number of participating nucleons and number of binary nucleon-nucleon collisions, are deduced from a Glauber model with a sharp impact parameter selection, and shown to be consistent with those extracted from the data. The centrality determination provides a tool to compare ALICE measurements with those of other experiments and with theoretical calculations.

• The paper introduces a refined Glauber Monte Carlo simulation to link charged-particle multiplicity and spectator energy with collision centrality.
• The paper employs ALICE's integrated detectors (SPD, VZERO, TPC, ZDC) to accurately classify events and estimate participant numbers.
• The paper systematically evaluates nuclear geometry parameters, providing actionable insights for improved QCD analyses in heavy-ion collisions.

Centrality Determination of Pb–Pb Collisions at √sNN = 2.76 TeV with ALICE

The work presented in the CERN report outlines the methodology adopted by the ALICE collaboration for determining the centrality of Pb–Pb collisions at a center-of-mass energy per nucleon pair of 2.76 TeV, a critical parameter for probing Quantum Chromodynamics (QCD) matter under extreme conditions. The centrality reflects the overlap region of colliding nuclei and correlates with the number of participating nucleons and binary collisions, typically described through a Glauber model.

Methodology

The centrality measurements leverage two main experimental observables that relate to collision geometry: the average charged-particle multiplicity $N_{ch}$ and the zero-degree energy $E_{ZDC}$, the latter denoting the energy of spectator nucleons captured by Zero-Degree Calorimeters. The Glauber model, which treats nucleus-nucleus interactions as numerous binary nucleon-nucleon collisions, was pivotal in interpreting these observables. The ALICE team implemented a refined Glauber Monte Carlo simulation to estimate the mean number of participants $N_{part}$ and binary collisions $N_{coll}$ based on varying impact parameters.

Experimental Approach

ALICE's complex detector composition, incorporating the Silicon Pixel Detector (SPD), VZERO arrays, Time Projection Chamber (TPC), and Zero Degree Calorimeters (ZDC), facilitated the comprehensive data acquisition needed for centrality determination. The VZERO amplitude and energy deposition in the ZDC provided the variables for centrality calibration. Events were categorized into centrality classes founded on the VZERO and ZDC readings compared against Monte Carlo simulations.

Key Findings and Results

The Glauber Monte Carlo simulations determined that events fall within specific centrality classes when demarcated by their VZERO amplitude. The systematic evaluations included varying parameters like the Woods-Saxon distribution nucleon spacing and the inelastic nucleon-nucleon cross-section. Table 1 in the report exhibits the resulting geometric properties (participants and collisions) for different percentiles, with uncertainties calculated from the variance in Glauber model assumptions.

Operational challenges included managing large electromagnetic cross-sections that contribute peripheral event contamination, mitigated by strategic simulations and multiplicity distributions fitting. Two contrasting methodologies were employed: ground-up simulation coupled with empirically adjusted multiplicity distribution modeling, providing improved event categorization precision.

Implications and Future Directions

ALICE's centrality determination approach is a robust framework for analyzing heavy-ion collisions at varying energies, offering a feasible blueprint for similar studies across different experiments and theoretical validations in QCD. The insights gained enhance comparative analysis potential against other LHC experiments' findings and allow for precise theoretical calculation benchmarking.

Future work involves refining detector sensitivities and Glauber model parameters to better capture nuances in nucleon interactions, bolstering the accuracy of particle event categorization. As the understanding of high-energy nucleus collisions deepens, the adaptability of these methodologies will be vital for future energy scales and nuclear species studied at the LHC and beyond.