Measurement of the azimuthal anisotropy for charged particle production in \(\sqrt{s_{NN}}\) = 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 \(\sqrt{s_{NN}}\) = 2.76 TeV with the ATLAS detector at the LHC, based on an integrated luminosity of approximately 8 \(\mu b^{-1}\). This anisotropy is characterized via a Fourier expansion of the distribution of charged particles in azimuthal angle \(\phi\) , 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<pT<20 GeV), pseudorapidity (|\).eta\).<2.5) and centrality using an event plane method. The \(v_n\) values for \(n\ge3\) are found to vary weakly with both \(\eta\) and centrality, and their pT dependencies are found to follow an approximate scaling relation, \(v_n^{1/n}(pT) \propto v_2^{1/2}(pT)\). A Fourier analysis of the charged particle pair distribution in relative azimuthal angle \((\Delta\phi=\phi_a-\phi_b)\) is performed to extract the coefficients \(v_{n,n}=<cos (n \Delta\phi)>\). For pairs of charged particles with a large pseudorapidity gap \((|\Delta\eta=\eta_a-\eta_b|>2)\) and one particle with pT<3 GeV, the \(v_{2,2}-v_{6,6}\) values are found to factorize as \(v_{n,n}(pT^a,pT^b) \sim v_n(pT^a)v_n(pT^b)\) in central and mid-central events. Such factorization suggests that these values of \(v_{2,2}-v_{6,6}\) are primarily due to the response of the created matter to the fluctuations in the geometry of the initial state. A detailed study shows that the \(v_{1,1}(pT^a,pT^b)\) 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 \(v_1\) is observed to cross zero at \(pT\sim\) 1.0 GeV, reaches a maximum at 4-5 GeV with a value comparable to that for \(v_3\). and decreases at higher pT.