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Centre d'études et d'expertise sur les risques, l'environnement, la mobilité et l'aménagement

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In ultrarelativistic heavy-ion collisions, the Fourier decomposition of the relative azimuthal angle, \Delta \phi, distribution of particle pairs yields a large cos(3\Delta \phi) component, extending out to large rapidity separations \Delta \eta >1. This component captures a significant portion of the ridge and shoulder structures in the \Delta \phi distribution, which have been observed after contributions from elliptic flow are subtracted. An average finite triangularity due to event-by-event fluctuations in the initial matter distribution, followed by collective flow, naturally produces a cos(3\Delta \phi) correlation. Using ideal and viscous hydrodynamics, and transport theory, we study the physics of triangular (v_3) flow in comparison to elliptic (v_2), quadrangular (v_4) and pentagonal (v_5) flow. We make quantitative predictions for v_3 at RHIC and LHC as a function of centrality and transverse momentum. Our results for the centrality dependence of v_3 show a quantitative agreement with data extracted from previous correlation measurements by the STAR collaboration. This study supports previous results on the importance of triangular flow in the understanding of ridge and shoulder structures. Triangular flow is found to be a sensitive probe of initial geometry fluctuations and viscosity.Comment: 10 pages, 12 figures. minor changes, and results for $v_5$ added (fig.12

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In this work it is shown that the use of a hydrodynamical model of heavy ion collisions which incorporates recent developments, together with updated photon emission rates, greatly improves agreement with both ALICE and PHENIX measurements of direct photons, supporting the idea that thermal photons are the dominant source of direct photon momentum anisotropy. The eventby-event hydrodynamical model uses IP-Glasma initial states and includes, for the first time, both shear and bulk viscosities, along with second order couplings between the two viscosities. The effect of both shear and bulk viscosities on the photon rates is studied, and those transport coefficients are shown to have measurable consequences on the photon momentum anisotropy. arXiv:1509.06738v3 [hep-ph]

A new set of equations for relativistic viscous hydrodynamics that captures both weak-coupling and strongcoupling physics to second order in gradients has been developed recently. We apply this framework to bulk physics at the Relativistic Heavy Ion Collider (RHIC), both for standard (Glauber-type) as well as for colorglass-condensate (CGC) initial conditions and show that the results do not depend strongly on the values for the second-order transport coefficients. Results for multiplicity, radial flow and elliptic flow are presented, and we quote the ratio of viscosity over entropy density for which our hydrodynamic model is consistent with experimental data. For CGC initial conditions, early thermalization does not seem to be required in order for hydrodynamics to describe charged hadron elliptic flow.

We investigate how the initial geometry of a heavy-ion collision is transformed into final flow observables by solving event-by-event ideal hydrodynamics with realistic fluctuating initial conditions. We study quantitatively to what extent anisotropic flow (v n ) is determined by the initial eccentricity ε n for a set of realistic simulations, and we discuss which definition of ε n gives the best estimator of v n . We find that the common practice of using an r 2 weight in the definition of ε n in general results in a poorer predictor of v n than when using r n weight, for n > 2. We similarly study the importance of additional properties of the initial state. For example, we show that in order to correctly predict v 4 and v 5 for noncentral collisions, one must take into account nonlinear terms proportional to ε 2 2 and ε 2 ε 3 , respectively. We find that it makes no difference whether one calculates the eccentricities over a range of rapidity or in a single slice at z = 0, nor is it important whether one uses an energy or entropy density weight. This knowledge will be important for making a more direct link between experimental observables and hydrodynamic initial conditions, the latter being poorly constrained at present.