## Abstract

Differential measurements of charged particle azimuthal anisotropy are presented for lead-lead collisions at √s_{NN} = 2.76 TeV with the ATLAS detector at the LHC, based on an integrated luminosity of approximately 8 μb^{− 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_{n} denoting the magnitude of the anisotropy. Significant v_{2}–v_{6} values are obtained as a function of transverse momentum (0.5 < p_{T} < 20 GeV), pseudorapidity (|η| < 2.5), and centrality using an event plane method. The v_{n} values for n ≥ 3 are found to vary weakly with both η and centrality, and their p_{T} dependencies are found to follow an approximate scaling relation, v^{1/n} _{n} (p_{T}) ∝ v^{1/2} _{2} (p_{T}), except in the top 5% most central collisions. A Fourier analysis of the charged particle pair distribution in relative azimuthal angle (Δϕ = ϕ_{a} − ϕ_{b}) is performed to extract the coefficients v_{n, n} = (cos nΔϕ). For pairs of charged particles with a large pseudorapidity gap (|Δη = η_{a} − η_{b}| > 2) and one particle with p_{T} < 3 GeV, the v_{2,2}–v_{6,6} values are found to factorize as v_{n, n}(p^{a} _{T}, p^{b} _{T}) ≈ v_{n}(p^{a} _{T})v_{n}(p^{b} _{T}) in central and midcentral events. Such factorization suggests that these values of v_{2,2}–v_{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_{1,1}(p^{a} _{T}, p^{b} _{T}) 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} isobserved to cross zero at p_{T} ≈ 1.0 GeV, reaches a maximum at 4–5 GeV with a value comparable to that for v_{3}, and decreases at higher p_{T}.

Original language | English (US) |
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Article number | 014907 |

Pages (from-to) | 014907-1-014907-41 |

Journal | Physical Review C - Nuclear Physics |

Volume | 86 |

Issue number | 1 |

DOIs | |

State | Published - 2012 |

## ASJC Scopus subject areas

- Nuclear and High Energy Physics