Measurement of the distributions of event-by-event flow harmonics in lead-lead collisions at root s(NN)=2.76 TeV with the ATLAS detector at the LHC

Abstract
The distributions of event-by-event harmonic flow coefficients v (n) for n = 2- 4 are measured in = 2.76 TeV Pb + Pb collisions using the ATLAS detector at the LHC. The measurements are performed using charged particles with transverse momentum p (T) > 0.5 GeV and in the pseudorapidity range |eta| < 2.5 in a dataset of approximately 7 mu b(-1) recorded in 2010. The shapes of the v (n) distributions suggest that the associated flow vectors are described by a two-dimensional Gaussian function in central collisions for v (2) and over most of the measured centrality range for v (3) and v (4). Significant deviations from this function are observed for v (2) in mid-central and peripheral collisions, and a small deviation is observed for v (3) in mid-central collisions. In order to be sensitive to these deviations, it is shown that the commonly used multi-particle cumulants, involving four particles or more, need to be measured with a precision better than a few percent. The v (n) distributions are also measured independently for charged particles with 0.5 < p (T) < 1 GeV and p (T) > 1 GeV. When these distributions are rescaled to the same mean values, the adjusted shapes are found to be nearly the same for these two p (T) ranges. The v (n) distributions are compared with the eccentricity distributions from two models for the initial collision geometry: a Glauber model and a model that includes corrections to the initial geometry due to gluon saturation effects. Both models fail to describe the experimental data consistently over most of the measured centrality range.
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ATLAS Collaboration Cabrera Urbán, Susana Castillo Giménez, María Victoria Costa Mezquita, María José Fassi, Farida Ferrer Soria, Antonio Fiorini, Luca Fuster Verdú, Juan A. García García, Carmen García Navarro, José Enrique González de la Hoz, Santiago Hernández Jiménez, Yesenia Higón Rodríguez, Emilio Irles Quiles, Adrián Kaci, Mohammed Lacasta Llácer, Carlos Lacuesta Miquel, Vicente March Ruiz, Luis Martí García, Salvador Miñano Moya, Mercedes Mitsou, Vasiliki Moles Valls, Regina Moreno Llácer, María Oliver García, Elena Pedraza López, Sebastián Pérez García-Estañ, María Teresa Romero Adam, E. Ros Martínez, Eduardo Salt Cairols, José Sánchez Martínez, V. Soldevila, U. Sánchez, J. Torró Pastor, Emma Valero Biot, Alberto Valladolid Gallego, E. Valls Ferrer, Juan Antonio Villaplana Pérez, Miguel Vos, Marcel 2013 Measurement of the distributions of event-by-event flow harmonics in lead-lead collisions at root s(NN)=2.76 TeV with the ATLAS detector at the LHC Journal of High Energy Physics 11 11 183-1 183-57
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