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Harmonic decomposition of two-particle angular correlations in Pb--Pb collisions at $\mathbf{\sqrt{s_{\rm NN}} = 2.76}$ TeV

K. Aamodt Sang Un Ahn 1 Laurent Aphecetche 2 N. Arbor 3 Y.W. Baek 4 A. Baldisseri 5 A. Baldit 4 V. Barret 4 N. Bastid 4 Guillaume Batigne 2 I. Belikov 6 H. Borel 5 B. Boyer 7 M. Bregant 2 J. Castillo Castellanos 5 J.L. Charvet 5 B. Cheynis 8 J.-P. Coffin 6 G. Conesa Balbastre 3 P. Crochet 4 H. Delagrange 2 O. Driga 2 L. Ducroux 8 P. Dupieux 4 B. Espagnon 7 Magali Estienne 2 J. Faivre 3 C. Furget 3 Marie Germain 2 C. Geuna 5 J.-Y. Grossiord 9 R. Guernane 3 M. Guilbaud 8 C. Hadjidakis 7 B. Hippolyte 6 I. Hrivnacova 7 R. Ichou 2 S. Jangal 6 S. Kox 3 C. Kuhn 6 A. Lardeux 2 Y. Le Bornec 7 F. Lefevre 2 M. Lenhardt 2 X. Lopez 4 L. Luquin 2 A. Maire 6 F. Manso 4 Y. Mao 3 M. Marchisone 4 Gines Martinez Garcia 2 A. Mas 2 L. Massacrier 8 A. Michalon 6 S. Pal 5 Philippe Pillot 2 S. Porteboeuf 4 A. Rakotozafindrabe 5 J.S. Real 3 P. Rosnet 4 C. Roy 6 Diego Stocco 2 C. Suire 7 R. Tieulent 8 A. Uras 8 L. Valencia Palomo 7 R. Vernet 10 B. Vulpescu 4 R. Wan 6 H. Yang 5 X. Zhang 4 Y. Zoccarato 8
Abstract : Angular correlations between unidentified charged trigger ($t$) and associated ($a$) particles are measured by the ALICE experiment in \PbPb\ collisions at $\snn=2.76$ TeV for transverse momenta $0.25 < p_{T}^{t,\, a} < 15$ GeV/$c$, where $p_{T}^t > p_{T}^a$. The shapes of the pair correlation distributions are studied in a variety of collision centrality classes between 0 and 50% of the total hadronic cross section for particles in the pseudorapidity interval $|\eta| < 1.0$. Distributions in relative azimuth $\Delta\phi \equiv \phi^t - \phi^a$ are analyzed for $|\Delta\eta| \equiv |\eta^t - \eta^a| > 0.8$, and are referred to as "long-range correlations". Fourier components $V_{n\Delta} \equiv <\cos(n\Delta\phi)>$ are extracted from the long-range azimuthal correlation functions. If the particle pair correlation arises dominantly from production mechanisms that distribute according to a common plane of symmetry, then the pair $\vnd$ coefficients are expected to factorize as the product of single-particle anisotropies $v_n (\pt)$, i.e. $V_{n\Delta}(p_{T}^t, p_{T}^a) = v_n(p_{T}^t) \, v_n(p_{T}^a)$. This expectation is tested for $1 \leq n \leq 5$ by applying a global fit of all $\vnd (p_{T}^t, p_{T}^a)$ to obtain the best values $\vngf (\pt)$. It is found that for $2 \leq n \leq 5$, the factorization holds for associated particle momenta up to $\pta \sim 3$-4 GeV/$c$, with a trend of increasing deviation between the data and the factorization hypothesis as $p_{T}^t$ and $p_{T}^a$ are increased or as collisions become more peripheral. $V_{1\Delta}$ does not factorize precisely at any $\pt$ or centrality, as indicated by the lack of a good global fit over the full $\ptt, \pta$ range. The $\vngf$ values for $2 \leq n \leq 5$ from the global fit are in close agreement with previous measurements.
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Submitted on : Tuesday, September 13, 2011 - 8:32:37 AM
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K. Aamodt, Sang Un Ahn, Laurent Aphecetche, N. Arbor, Y.W. Baek, et al.. Harmonic decomposition of two-particle angular correlations in Pb--Pb collisions at $\mathbf{\sqrt{s_{\rm NN}} = 2.76}$ TeV. Physics Letters B, Elsevier, 2012, 708, pp.249-264. ⟨10.1016/j.physletb.2012.01.060⟩. ⟨in2p3-00622891⟩

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