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

TL;DR

This ATLAS study delivers a comprehensive measurement of charged-particle azimuthal anisotropy in Pb–Pb collisions at 2.76 TeV, extracting v_n (n=1–6) with both event-plane and two-particle-correlation methods. It demonstrates that v_2–v_6 at low to intermediate p_T largely reflect collective flow from initial geometry fluctuations, with v_{n,n} factorizing into single-particle v_n under a large pseudorapidity gap, while v_1 contains both a rapidity-even dipole component and a momentum-conservation contribution. A two-component fit isolates v_1, revealing a sign change and a peak near 4–5 GeV, highlighting the interplay between dipole flow and jet quenching. The results constrain the initial-state fluctuations and the medium's viscosity, and show how nonflow effects become relevant at higher p_T, providing a bridge between hydrodynamic and jet-quenching descriptions.

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.

Figures (34)

• Figure 1: (Color online) The $\chi_n$ (top) and EP resolution factor (bottom) vs. centrality (smaller value refers to more central events) for $n=2$--6, together with the systematic uncertainty as shaded bands. The EP is measured by both sides of the FCal detector (denote by "full FCal"). Note that the Res$\{n\Psi_n\}$ value can not be greater than one (see Eq. \ref{['eq:mep2']}), thus its systematic uncertainty shrinks as it approaches one.
• Figure 2:
• Figure 3: (Color online) $v_n$ vs. $\eta$ for $2<\pT<3$ GeV from the FCal$_{\mathrm{P(N)}}$ method (i.e the EP is measured by either FCal$_{\mathrm N}$ or FCal$_{\mathrm P}$) with each panel representing one centrality interval. The shaded bands indicate systematic uncertainties from Tables \ref{['tab:v2']}--\ref{['tab:v6']}.
• Figure 4: (Color online) $v_n$ vs. $\pT$ for several centrality intervals. The shaded bands indicate the systematic uncertainties from Tables \ref{['tab:v2']}--\ref{['tab:v6']}.
• Figure 5: (Color online) $v_n$ vs. centrality for six $\pT$ ranges from the full FCal event plane method. The shaded bands indicate systematic uncertainties from Tables \ref{['tab:v2']}--\ref{['tab:v6']}.