Higher harmonic anisotropic flow measurements of charged particles in Pb-Pb collisions at 2.76 TeV

Abstract: We report on the first measurement of the triangular $v_3$, quadrangular $v_4$, and pentagonal $v_5$ charged particle flow in Pb-Pb collisions at 2.76 TeV measured with the ALICE detector at the CERN Large Hadron Collider. We show that the triangular flow can be described in terms of the initial spatial anisotropy and its fluctuations, which provides strong constraints on its origin. In the most central events, where the elliptic flow $v_2$ and $v_3$ have similar magnitude, a double peaked structure in the two-particle azimuthal correlations is observed, which is often interpreted as a Mach cone response to fast partons. We show that this structure can be naturally explained from the measured anisotropic flow Fourier coefficients.

• The paper presents the first measurements of triangular (v3), quadrangular (v4), and pentagonal (v5) flow coefficients to quantify azimuthal anisotropy in Pb-Pb collisions.
• The paper demonstrates that higher harmonic flows are highly sensitive to shear viscosity, providing evidence for a near-perfect fluid behavior of the quark-gluon plasma.
• The paper links two-particle correlations and centrality dependence to initial geometry fluctuations, thereby validating hydrodynamic models of QGP evolution.

Analysis of Higher Harmonic Anisotropic Flow in Pb-Pb Collisions

The study conducted by the ALICE Collaboration provides an insightful analysis of higher harmonic anisotropic flow in lead-lead (Pb-Pb) collisions at $\sqrt{s_{NN}} = 2.76$ TeV using the ALICE detector at CERN's Large Hadron Collider. The research represents a significant stride in our understanding of the collective behavior of the quark-gluon plasma (QGP), a state predicted by Quantum Chromodynamics (QCD) under extreme conditions.

Key Findings and Analysis

1. Measurement of Higher Harmonic Flow Coefficients:
   • The study focuses on the first measurements of the triangular $v_3$, quadrangular $v_4$, and pentagonal $v_5$ flow coefficients. These coefficients represent the azimuthal anisotropy of charged particles produced in high-energy nuclear collisions.
   • The data shows that triangular flow ($v_3$) can be correlated with initial spatial anisotropy and its fluctuations. This provides solid constraints on its origin, which are essential for accurately modeling the QGP.
2. Correlation with Initial Conditions:
   • The conversion of initial spatial asymmetry into anisotropic momentum distributions is a critical phenomenon in relativistic heavy-ion collisions. The study quantitatively analyzes this by establishing relationships between $v_n$ and the corresponding spatial eccentricities $\varepsilon_n$.
   • Calculations using both Glauber and MC-KLN CGC models to estimate initial spatial eccentricities suggest different sensitivities of $v_n$ to these initial conditions, particularly in the context of shear viscosity to entropy density ratio ($\eta/s$).
3. Effect of Viscosity:
   • Perhaps the most intriguing result is the apparent sensitivity of higher harmonic flows ($v_3$, $v_4$, $v_5$) to the shear viscosity of the QGP. The damping effect of shear viscosity on Fourier coefficients is more pronounced for higher-order harmonics, making these measurements particularly sensitive to $\eta/s$.
   • The comparison of experimental data to hydrodynamic models suggests a low $\eta/s$, indicative of a near-perfect fluid, which aligns with previous findings at both RHIC and LHC energies.
4. Centrality Dependence:
   • The investigation explores the centrality dependence of anisotropic flow coefficients. For the most central collisions, $v_2$ increases with proximity to head-on collisions, while $v_3$ remains relatively constant, reflecting its derivation from geometric fluctuations rather than the mean geometry of interactions.
5. Interpretation of Two-Particle Correlations:
   • A noteworthy aspect of the research is the explanation of the double-peaked structure in two-particle azimuthal correlation distributions, previously hypothesized as a Mach cone effect. The study shows that this structure can be well accounted for by the measured harmonics without requiring jet modifications, underlining the robustness of the anisotropic flow description.

Practical and Theoretical Implications

The results presented in this study provide crucial benchmarks for theoretical models of QGP behavior and open new pathways to refine these models. The ability to extract precise information on initial conditions and transport properties such as $\eta/s$ highlights the power of harmonic flow measurements as diagnostic tools. In future work, it is expected that these methods will continue to evolve, offering deeper insights into the nature of the QGP and its transition from hadronic matter.

Future Directions

The research outcomes warrant further exploration of higher harmonic flow coefficients in different collision systems and at varying energies. Such studies could provide a more comprehensive understanding of QGP properties across different experimental settings. In addition, multi-particle cumulant techniques offer potential for enhancing resolution and accuracy in identifying the contributions of flow fluctuations.

In conclusion, this research exemplifies the utility of anisotropic flow measurements in exploring the fundamental properties of QGP, providing detailed insight into its initial state geometry and dynamical evolution.