/ hep-ex arXiv:1305.2942

Measurement of the distributions of event-by-event flow harmonics in lead--lead collisions at $\sqrt{s_{NN}}$=2.76 TeV with the ATLAS detector at the LHC

Pages: 41

Abstract: The distributions of event-by-event harmonic flow coefficients $v_n$ for n=2-4 are measured in $\sqrt{s_{NN}}$=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 are described by a two-dimensional Gaussian function for the underlying flow vector 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. It is shown that the commonly used multi-particle cumulants are insensitive to the deviations for $v_2$. 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.

Note: 41 pages plus author list (62 pages total), 33 figures, submitted to JHEP, All figures including auxiliary figures can be found at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2012-10/

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