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Measurement of the azimuthal anisotropy for charged particle production in sNN=2.76 TeV lead-lead collisions with the ATLAS detector

G. Aad et al. (ATLAS Collaboration)
Phys. Rev. C 86, 014907 – Published 24 July 2012

Abstract

Differential measurements of charged particle azimuthal anisotropy are presented for lead-lead collisions at sNN=2.76 TeV with the ATLAS detector at the LHC, based on an integrated luminosity of approximately 8 μb1. This anisotropy is characterized via a Fourier expansion of the distribution of charged particles in azimuthal angle relative to the reaction plane, with the coefficients vn denoting the magnitude of the anisotropy. Significant v2v6 values are obtained as a function of transverse momentum (0.5<pT<20 GeV), pseudorapidity (|η|<2.5), and centrality using an event plane method. The vn values for n3 are found to vary weakly with both η and centrality, and their pT dependencies are found to follow an approximate scaling relation, vn1/n(pT)v21/2(pT), except in the top 5% most central collisions. A Fourier analysis of the charged particle pair distribution in relative azimuthal angle (Δφ=φaφb) is performed to extract the coefficients vn,n=cosnΔφ. For pairs of charged particles with a large pseudorapidity gap (|Δη=ηaηb|>2) and one particle with pT<3 GeV, the v2,2v6,6 values are found to factorize as vn,n(pTa,pTb)vn(pTa)vn(pTb) in central and midcentral events. Such factorization suggests that these values of v2,2v6,6 are primarily attributable to the response of the created matter to the fluctuations in the geometry of the initial state. A detailed study shows that the v1,1(pTa,pTb) data are consistent with the combined contributions from a rapidity-even v1 and global momentum conservation. A two-component fit is used to extract the v1 contribution. The extracted v1 is observed to cross zero at pT1.0 GeV, reaches a maximum at 4–5 GeV with a value comparable to that for v3, and decreases at higher pT.

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  • Received 14 March 2012

DOI:https://doi.org/10.1103/PhysRevC.86.014907

This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

©2012 CERN, for the ATLAS Collaboration

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Vol. 86, Iss. 1 — July 2012

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