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

The distributions of event-by-event harmonic flow coefficientsv_nforn= 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 momentump_T>0.5 GeV and in the pseudorapidity range |η|<2.5 in a dataset of approximately 7μb⁻¹recorded in 2010. The shapes of thev_ndistributions suggest that the associated flow vectors are described by a two-dimensional Gaussian function in central collisions forv₂and over most of the measured centrality range forv₃andv₄. Significant deviations from this function are observed forv₂in mid-central and peripheral collisions, and a small deviation is observed forv₃in 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. Thev_ndistributions are also measured independently for charged particles with 0.5< p_T<1 GeV andp_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 twop_Tranges. Thev_ndistributions 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.