Abstract
Differential í measurements of charged particle azimuthal anisotropy are presented for lead-lead collisions at ísNN = 2.76 TeV with the ATLAS detector at the LHC, based on an integrated luminosity of approximately 8 _b-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 _n denoting the magnitude of the anisotropy. Significant _2-_6 values are obtained as a function of transverse momentum (0.5 < pT < 20 GeV), pseudorapidity (|_| < 2.5), and centrality using an event plane method. The _n values for n _ 3 are found to vary weakly with both _ and centrality, and their pT dependencies are found to follow an approximate scaling relation, _ n1/n (pT) _ _ 21/2 (pT), except in the top 5% most central collisions. A Fourier analysis of the charged particle pair distribution in relative azimuthal angle (__ = _a - _b) is performed to extract the coefficients _n,n = _cos n__ _. For pairs of charged particles with a large pseudorapidity gap (|__ = _a - _b| > 2) and one particle with pT < 3 GeV, the _2,2-_6,6 values are found to factorize as _n,n(pTa, pTb) _n(pTa)_n(pTb) in central and midcentral events. Such factorization suggests that these values of _2,2-_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 _1,1(pTa, pTb) data are consistent with the combined contributions from a rapidity-even _1 and global momentum conservation.A two-component fit is used to extract the _1 contribution. The extracted _1 isobserved to cross zero at pT 1.0 GeV, reaches a maximum at 4-5 GeV with a value comparable to that for _3, and decreases at higher pT.