Measurement of the azimuthal anisotropy for charged particle production in sqrt(s_NN) = 2.76 TeV lead-lead collisions with the ATLAS detector (1203.3087v3)

Abstract: Differential measurements of charged particle azimuthal anisotropy are presented for lead-lead collisions at sqrt(s_NN) = 2.76 TeV with the ATLAS detector at the LHC, based on an integrated luminosity of approximately 8 mb^-1. This anisotropy is characterized via a Fourier expansion of the distribution of charged particles in azimuthal angle (phi), with the coefficients v_n denoting the magnitude of the anisotropy. Significant v_2-v_6 values are obtained as a function of transverse momentum (0.5<pT\<20 GeV), pseudorapidity (|eta|\<2.5) and centrality using an event plane method. The v_n values for n>=3 are found to vary weakly with both eta and centrality, and their pT dependencies are found to follow an approximate scaling relation, v_n^{1/n}(pT) \propto v_2^{1/2}(pT). A Fourier analysis of the charged particle pair distribution in relative azimuthal angle (Dphi=phi_a-phi_b) is performed to extract the coefficients v_{n,n}=<cos (n Dphi)>. For pairs of charged particles with a large pseudorapidity gap (|Deta=eta_a-eta_b|>2) and one particle with pT<3 GeV, the v_{2,2}-v_{6,6} values are found to factorize as v_{n,n}(pT^a,pT^b) ~ v_n(pT^a)v_n(pT^b) in central and mid-central events. Such factorization suggests that these values of v_{2,2}-v_{6,6} are primarily due to the response of the created matter to the fluctuations in the geometry of the initial state. A detailed study shows that the v_{1,1}(pT^a,pT^b) data are consistent with the combined contributions from a rapidity-even v_1 and global momentum conservation. A two-component fit is used to extract the v_1 contribution. The extracted v_1 is observed to cross zero at pT\sim1.0 GeV, reaches a maximum at 4-5 GeV with a value comparable to that for v_3, and decreases at higher pT.

• The paper presents detailed measurements of Fourier coefficients (v2–v6) that characterize the azimuthal anisotropy in high-energy Pb-Pb collisions.
• It employs an event-plane method to extract anisotropy signals as functions of transverse momentum, pseudorapidity, and centrality.
• The analysis reveals scaling behavior and factorization of harmonics, providing new insights into the fluid-dynamic evolution of the Quark-Gluon Plasma.

Measurement of Azimuthal Anisotropy in Lead-Lead Collisions

The paper "Measurement of the azimuthal anisotropy for charged particle production in $\sqrt{s_{\mathrm {NN}=2.76}$ TeV lead-lead collisions with the ATLAS detector" presents an important analysis of lead-lead collisions at the Large Hadron Collider (LHC), specifically at the center-of-mass energy per nucleon pair, $\sqrt{s_{\mathrm {NN}} = 2.76$ TeV. The research, conducted by the ATLAS Collaboration, aims to provide a comprehensive understanding of the azimuthal anisotropy in these high-energy heavy-ion collisions, an essential observable in the paper of Quark-Gluon Plasma (QGP).

Azimuthal Anisotropy and its Measurement

In high-energy nuclear collisions, azimuthal anisotropy refers to the variation in the distribution of emitted particles with respect to the reaction plane. This anisotropy is quantified using a Fourier expansion, with coefficients $v_n$ describing the magnitude of each harmonic component. The first few coefficients, such as $v_2$ or elliptic flow, are predominantly used to characterize the initial geometric configuration and dynamic evolution of the collision.

The paper utilizes the ATLAS detector at the LHC to perform differential measurements of these coefficients for charged particle production. The analysis employs an event-plane method to capture $v_2$--$v_6$ as functions of transverse momentum ($0.5 < \pT < 20$ GeV), pseudorapidity ($|\eta| < 2.5$), and collision centrality.

Key Results

The ATLAS Collaboration reports significant $v_2$--$v_6$ values, observing a weak dependency of these coefficients on pseudorapidity and centrality for $n \geq 3$. Furthermore, the transverse momentum dependencies adhere to a scaling behaviour approximately expressed as $v_n^{1/n}(\pT) \propto v_2^{1/2}(\pT)$, applicable generally except in the most central 5% of collisions. These results suggest an intricate relationship between the higher-order harmonics and the primary elliptic flow component.

The analysis further explores the factorization properties of the azimuthal anisotropies via charged particle pair distributions. By evaluating the distribution coefficients $v_{n,n}$, the authors demonstrate that for large pseudorapidity gap pairs and low transverse momentum conditions, the coefficients approximately factorize into products of single-particle harmonics, supporting a collectivity-based interpretation of the observed anisotropies.

Implications and Future Directions

The paper's results carry implications for understanding the initial geometric perturbations and their translation into final-state momentum space via fluid-dynamic expansion. The precise measurement of higher harmonics beyond $v_2$ offers more stringent constraints on theoretical models, particularly those addressing the shear viscosity to entropy density ratio, a critical parameter describing the perfect fluid nature of QGP.

Moving forward, the insights gained from these measurements are poised to enhance the development of hydrodynamic models and improve the description of initial state fluctuations within heavy-ion collision physics. Moreover, as model comparisons and integration of these results into global analyses continue, there is potential for refined understanding of QGP properties and the initial conditions driving heavy-ion collision dynamics.