Centrality dependence of the charged-particle multiplicity density at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV (1012.1657v3)

Abstract: The centrality dependence of the charged-particle multiplicity density at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV is presented. The charged-particle density normalized per participating nucleon pair increases by about a factor 2 from peripheral (70-80%) to central (0-5%) collisions. The centrality dependence is found to be similar to that observed at lower collision energies. The data are compared with models based on different mechanisms for particle production in nuclear collisions.

• The paper reveals a centrality-dependent increase in charged-particle density, rising from about 4.4 in peripheral to 8.4 in central collisions.
• It employs precise tracklet reconstruction with ALICE detectors and uses Glauber modeling to determine collision geometry.
• The findings validate and challenge theoretical models by highlighting the crucial role of gluon dynamics in particle production.

Analysis of Centrality Dependence of Charged-Particle Multiplicity Density in Pb-Pb Collisions at 2.76 TeV

This paper presents a detailed investigation into the centrality dependence of charged-particle multiplicity density at mid-rapidity in lead-lead (Pb-Pb) collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}} = 2.76$ TeV, as observed by the ALICE Collaboration at CERN. The paper expands upon previous research that focused solely on the most central collisions by examining a broader range of centrality classes, specifically covering 0-80% of the hadronic cross-section.

Methodological Insights

The measurement leverages the capabilities of the ALICE detector, utilizing its Inner Tracking System (ITS) and Silicon Pixel Detector (SPD) layers. The experimental setup allows for high-precision detection of charged particles, crucial for estimating multiplicity densities across different collision geometries. Centrality classes are determined using the Glauber model, which calculates the average number of participating nucleons ($\langle N_{\text{part}} \rangle$) and binary collisions.

Tracklet reconstruction, defined by hit combinations across detector layers, forms the basis for evaluating the charged-particle pseudo-rapidity density $dN_{\text{ch}}/d\eta$ in the mid-rapidity region $|\eta| < 0.5$. This data undergoes extensive corrections for acceptance, efficiency, contamination, and background subtraction, ensuring robustness across different collision centralities.

Key Findings

The investigation reveals a significant dependence of charged-particle density on collision centrality. Specifically, the multiplicity density normalized per participating nucleon pair shows a marked increase from around 4.4 in the most peripheral (70-80%) to approximately 8.4 in the most central (0-5%) collisions. This variation is consistent with observed trends at lower collision energies, indicating a similar pattern in particle production mechanisms across these energy regimes.

Theoretical Comparisons

The findings are compared against diverse theoretical models, including two-component models and saturation models. While dual-parton model implementations exhibit a steeper centrality rise than observed, the two-component HIJING 2.0 model, modified with a centrality-dependent gluon shadowing mechanism, aligns closely with the experimental data. This alignment suggests a moderation of particle production with increased centrality, a feature attributed to the substantial role of gluon dynamics in high-energy nuclear interactions. Conversely, saturation models show a reduced centrality dependence, highlighting the complexity of modeling particle production in heavy-ion collisions.

Implications and Future Directions

The implications of this research are multifaceted. The consistency of centrality dependence across energy scales reinforces existing theoretical constructs about strong interactions under extreme conditions, particularly in the context of Quark-Gluon Plasma (QGP) formation. Practically, these insights aid in refining models essential for interpreting results from ongoing high-energy physics experiments.

Future research directions could focus on integrating more sophisticated modeling techniques to account for the non-linear phenomena observed in these collisions. Additionally, extending similar analyses to different colliding systems or higher energies could provide more insights into the universality of these mechanisms and the intricate role of initial conditions in QGP dynamics.

In conclusion, this paper offers comprehensive empirical data, facilitating an enhanced understanding of charged-particle production across a broad spectrum of collision centralities and laying groundwork for further explorations into the behavior of strongly interacting matter at unprecedented energy scales.