Publication : t20/146

Abstract:
We show that the momentum distributions calculated in ideal hydrodynamic simulations of nucleus-nucleus collisions are determined, to a good approximation, by the distribution of the transverse velocity of the fluid. We compute the fluid velocity distribution that gives the best fit to experimental data on Pb+Pb collisions at sNN−−−√=2.76 TeV. We obtain reasonable fits up to pt∼6 GeV, much beyond the range where hydrodynamics is usually applied. However, the fit is not perfect, even at low pt. We actually argue that an ideal hydrodynamic calculation cannot fit simultaneously all identified particle spectra, irrespective of the specific implementation. In particular, data display a significant excess of pions at low pt, whose physical interpretation is discussed. Data also show that the distribution of the fluid velocity becomes broader as the collision becomes less central. This broadening is explained by event-by-event hydrodynamic calculations, where it results from the centrality dependence of initial state fluctuations.

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