Measurement of the azimuthal anisotropy for charged particle production in √s[subscript NN]=2.76 TeV lead-lead collisions with the ATLAS detector

Abstract

Differential measurements of charged particle azimuthal anisotropy are presented for lead-lead collisions at √s[subscript NN]=2.76 TeV with the ATLAS detector at the LHC, based on an integrated luminosity of approximately 8 μb[superscript −1]. 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 v[subscript n] denoting the magnitude of the anisotropy. Significant v[subscript 2]–v[subscript 6] values are obtained as a function of transverse momentum (0.5<p[subscript T]<20 GeV), pseudorapidity (|η|<2.5), and centrality using an event plane method. The v[subscript n] values for n≥3 are found to vary weakly with both η and centrality, and their p[subscript T] dependencies are found to follow an approximate scaling relation, v[subscript n][superscript 1/n](p[subscript T])∝v[subscript 2][superscript 1/2](p[subscript T]), except in the top 5% most central collisions. A Fourier analysis of the charged particle pair distribution in relative azimuthal angle (Δφ=φ[subscript a]−φ[subscript b]) is performed to extract the coefficients v[subscript n,n]=〈cosnΔφ〉. For pairs of charged particles with a large pseudorapidity gap (|Δη=η[subscript a]−η[subscript b]|>2) and one particle with p[subscript T]<3 GeV, the v[subscript 2,2]–v[subscript 6,6] values are found to factorize as v[subscript n,n](p[subscript T][superscript a],p[subscript T][superscript b])≈v[subscript n](p[subscript T][superscript a])v[subscript n](p[subscript T][superscript b]) in central and midcentral events. Such factorization suggests that these values of v[subscript 2,2]–v[subscript 6,6] 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 v[subscript 1,1](p[subscript T][superscript a],p[subscript T][superscript b]) data are consistent with the combined contributions from a rapidity-even v[subscript 1] and global momentum conservation. A two-component fit is used to extract the v[subscript 1] contribution. The extracted v[subscript 1] is observed to cross zero at p[subscript T]≈1.0 GeV, reaches a maximum at 4–5 GeV with a value comparable to that for v[subscript 3], and decreases at higher p[subscript T].