Measurement of the distributions of event-by-event flow harmonics in lead–lead collisions at √ sNN = 2 .76 TeV with the ATLAS detector at the LHC

Aad, G. and Abajyan, T. and Abbott, B. and Abdallah, J. and Abdel, Khalek S. and Abdelalim, A. A. and Abdinov, O. and Aben, R. and Abi, B. and Abolins, M. and AbouZeid, O. S. and Abramowicz, H. and Abreu, H. and Abulaiti, Y. and Acharya, B. S. and Adamczyk, L. and Adams, D. L. and Addy, T. N. and Adelman, J. and Adomeit, S. and Adye, T. and Aefsky, S. and Aguilar-Saavedra, J. A. and Agustoni, M. and Ahlen, S. P. and Ahles, F. and Ahmad, A. and Ahsan, M. and Aielli, G. and Åkesson, T. P. A. and Krasznahorkay, Attila and Nagai, Yoshikazu and Pásztor, Gabriella and Tóth, József (2013) Measurement of the distributions of event-by-event flow harmonics in lead–lead collisions at √ sNN = 2 .76 TeV with the ATLAS detector at the LHC. JOURNAL OF HIGH ENERGY PHYSICS (11). ISSN 1126-6708

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Abstract

The distributions of event-by-event harmonic flow coefficients v n for n =2–4 are measured in √ sNN = 2 .76 TeV Pb+Pb collisions using the ATLAS detector at the LHC. The measurements are performed using charged particles with transverse momentu m p T > 0 . 5 GeV and in the pseudorapidity range |η| < 2.5 in a dataset of approximately 7 µb−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 v2 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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