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Physics Letters B 708 (2012) 249-264
Harmonic decomposition of two-particle angular correlations in Pb--Pb collisions at $\mathbf{\sqrt{s_{\rm NN}} = 2.76}$ TeV
K. Aamodt, Sang Un Ahn1, L. Aphecetche2, N. Arbor3, Y.W. Baek1, A. Baldisseri4, A. Baldit1, V. Barret1, N. Bastid1, Guillaume Batigne2, I. Belikov5, H. Borel4, B. Boyer6, M. Bregant2, J. Castillo Castellanos4, J.L. Charvet4, B. Cheynis7, J.-P. Coffin5, G. Conesa Balbastre3, P. Crochet1, H. Delagrange2, O. Driga2, L. Ducroux7, P. Dupieux1, B. Espagnon6, M. Estienne2, J. Faivre3, C. Furget3, M. Germain2, C. Geuna4, J.-Y. Grossiord7, R. Guernane3, M. Guilbaud7, C. Hadjidakis6, B. Hippolyte5, I. Hrivnacova6, R. Ichou2, S. Jangal5, S. Kox3, C. Kuhn5, A. Lardeux2, Y. Le Bornec6, F. Lefevre2, M. Lenhardt2, X. Lopez1, L. Luquin2, A. Maire5, F. Manso1, Y. Mao3, M. Marchisone1, G. Martınez Garcıa2, A. Mas2, L. Massacrier7, A. Michalon5, S. Pal4, P. Pillot2, S. Porteboeuf1, A. Rakotozafindrabe4, J.S. Real3, P. Rosnet1, C. Roy5, D. Stocco2, C. Suire6, R. Tieulent7, A. Uras7, L. Valencia Palomo6, R. Vernet8, B. Vulpescu1, R. Wan5, H. Yang4, X. Zhang1, Y. Zoccarato7
ALICE Collaboration(s)

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.
1 :  LPC - Laboratoire de Physique Corpusculaire [Clermont-Ferrand]
2 :  SUBATECH - Laboratoire SUBATECH Nantes
3 :  LPSC - Laboratoire de Physique Subatomique et de Cosmologie
4 :  IRFU - Institut de Recherches sur les lois Fondamentales de l'Univers (ex DAPNIA)
5 :  IPHC - Institut Pluridisciplinaire Hubert Curien
6 :  IPNO - Institut de Physique Nucléaire d'Orsay
7 :  IPNL - Institut de Physique Nucléaire de Lyon
8 :  CC IN2P3 - Centre de Calcul de l'inst. national de phy. nucléaire et de phy. des particules
Laboratoire de Physique Corpusculaire
Physique/Physique Nucléaire Expérimentale
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