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

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 v₂–v₆ 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 v₁ and global momentum conservation.A two-component fit is used to extract the v₁ contribution. The extracted v₁ isobserved to cross zero at p_T ≈ 1.0 GeV, reaches a maximum at 4–5 GeV with a value comparable to that for v₃, and decreases at higher p_T.