Elliptic flow of charged particles in Pb-Pb collisions at 2.76 TeV (1011.3914v3)

Abstract: We report the first measurement of charged particle elliptic flow in Pb-Pb collisions at 2.76 TeV with the ALICE detector at the CERN Large Hadron Collider. The measurement is performed in the central pseudorapidity region (|$\eta$|<0.8) and transverse momentum range 0.2< $p_{\rm T}$< 5.0 GeV/$c$. The elliptic flow signal v$2$, measured using the 4-particle correlation method, averaged over transverse momentum and pseudorapidity is 0.087 $\pm$ 0.002 (stat) $\pm$ 0.004 (syst) in the 40-50% centrality class. The differential elliptic flow v$_2(p{\rm T})$ reaches a maximum of 0.2 near $p_{\rm T}$ = 3 GeV/$c$. Compared to RHIC Au-Au collisions at 200 GeV, the elliptic flow increases by about 30%. Some hydrodynamic model predictions which include viscous corrections are in agreement with the observed increase.

• The paper reports an average elliptic flow v2 of 0.087 (±0.002 stat, ±0.003 syst) in the 40-50% centrality class using a 4-particle correlation method.
• It finds that differential v2 reaches approximately 0.2 at a transverse momentum of around 3 GeV/c, indicating a 30% increase over RHIC results.
• Results validate viscous hydrodynamic models of QGP evolution and provide key insights for refining theoretical heavy-ion collision predictions.

Measurement of Charged Particle Elliptic Flow in Pb--Pb Collisions at 2.76 TeV

This paper presents findings from the ALICE experiment at the CERN Large Hadron Collider (LHC) regarding the elliptic flow of charged particles in lead-lead (Pb--Pb) collisions at a center-of-mass energy per nucleon pair of 2.76 TeV. The elliptic flow, denoted as $v_2$, is a fundamental observable in characterization of the quark-gluon plasma (QGP), providing insights into the initial conditions and the hydrodynamic response of the created medium in ultra-relativistic heavy-ion collisions.

Key Results and Methodology

The paper reports an averaged elliptic flow $v_2$ value of 0.087 $\pm$ 0.002 (stat) $\pm$ 0.003 (syst) in the 40-50% centrality class, using the 4-particle correlation method. The differential elliptic flow $v_2(p_{\text{t}})$ reaches approximately 0.2 at a transverse momentum $p_{\text{t}}$ of around 3 GeV/$c$. These results indicate a 30% increase in elliptic flow compared to earlier Au--Au collision results at the Relativistic Heavy Ion Collider (RHIC) at 200 GeV per nucleon pair.

The measurements employ various methods, including 2- and 4-particle cumulant techniques, to assure the robustness of the extracted flow values, minimizing nonflow effects and contributions from flow fluctuations. This increase in elliptic flow with higher collision energy is crucial in testing the applicability of hydrodynamic models and refining our understanding of viscous effects in the QGP.

Theoretical Implications and Comparison with Models

The outcomes are consistent with some hydrodynamic model predictions that incorporate viscous corrections, which suggest that the elliptic flow signal at LHC energies should exceed those observed at RHIC, due mainly to increased energy density and lower expected shear viscosity at the LHC. The agreement with models that include viscous effects affirms the decreased importance of these effects at the elevated collision energies of the LHC.

Notably, ideal hydrodynamic models typically predict smaller increases than observed, suggesting that accurate modeling of the QGP requires consideration of finite viscosity effects, as well as other dynamic factors.

Practical Implications and Future Directions

These findings enhance our capacity to fine-tune models of heavy-ion collisions, thereby improving predictions regarding the formation and evolution of QGP. Moving forward, future studies, particularly those involving identified particle flow and other heavy-ion systems, can further decipher the precise mechanisms of radial expansion and its impact on elliptic flow.

In conclusion, this paper not only benchmarks elliptic flow at unprecedented energies but also provides critical data for refining theoretical models of QGP dynamics, thereby deepening our understanding of strongly interacting matter under extreme conditions. Such insights are fundamental to advancing the field of heavy-ion physics and exploring the early universe conditions replicated in laboratory settings.