Collective flow without hydrodynamics: simulation results for relativistic ion collisions

Flow signatures in experimental data from relativistic ion collisions are usually interpreted as a fingerprint of the presence of a hydrodynamic phase during the evolution of these systems. In this work, flow signatures arising from event-by-event viscous hydrodynamics are compared to those arising from event-by-event non-interacting particle dynamics (free-streaming), both followed by a late-stage hadronic cascade, in d+Au, 3He+Au at sqrt(s)=200 GeV and p+Pb collisions at sqrt(s)=5 TeV, respectively. For comparison, also Pb+Pb collisions at sqrt(s)=2.76 TeV are simulated. It is found that non-hydrodynamic evolution can give rise to equal or larger radial flow than hydrodynamics with eta/s=0.08 in all simulated collision systems. In light-on-heavy-ion collisions, free-streaming gives rise to triangular and quadrupolar flow comparable to or larger than that from hydrodynamics, but it generally leads to considerably smaller elliptic flow. As expected, free-streaming leads to considerably less elliptic, triangular and quadrupolar flow than hydrodynamics in nucleus-nucleus collisions, such as event-by-event Pb+Pb collisions at sqrt(s)=2.76 TeV.Comment: 18 pages; 9 figures; v2: minor errors corrected, HBT radii added; v3: added subsection on radial flow breakdown in peripheral p+Pb, matches published versio

Relativistic Hydrodynamic Attractors with Broken Symmetries: Non-Conformal and Non-Homogeneous

Standard textbooks will state that hydrodynamics requires near-equilibrium to be applicable. Recently, however, out-of-equilibrium attractor solutions for hydrodynamics have been found in kinetic theory and holography in systems with a high degree of symmetry, suggesting the possibility of a genuine out-of-equilibrium formulation of hydrodynamics. This work demonstrates that attractor solutions also occur in non-conformal kinetic theory and spatially non-homogeneous systems, potentially having important implications for the interpretation of experimental data in heavy-ion and proton-proton collisions and relativistic fluid dynamics as a whole.Comment: 13 pages, 2 figures; v2: minor changes (typos, etc.); v3: matches published versio

Azimuthal Anisotropies at High Momentum from Purely Non-Hydrodynamic Transport

In the limit of short mean free path, relativistic kinetic theory gives rise to hydrodynamics through a systematically improvable gradient expansion. In the present work, a systematically improvable expansion in the opposite limit of large mean free path is considered, describing the dynamics of particles which are almost, but not quite, non-interacting. This non-hydrodynamic "eremitic" expansion does not break down for large gradients, and may be useful in situations where a hydrodynamic treatment is not applicable. As applications, azimuthal anisotropies at high transverse momentum in Pb+Pb and p+Pb collisions at $\sqrt{s}=5.02$ TeV are calculated from the first order eremitic expansion of kinetic theory in the relaxation time approximation.Comment: 26 pages, 5 figures; v2: reference to Borghini and Gombeaud added who discussed same setup in 2010, typos corrected; v3: minor changes, matches published versio

Viscosity Information from Relativistic Nuclear Collisions: How Perfect is the Fluid Observed at RHIC?

Relativistic viscous hydrodynamic fits to RHIC data on the centrality dependence of multiplicity, transverse and elliptic flow for sqrt{s}=200 GeV Au+Au collisions are presented. For Glauber-type initial conditions, while data on integrated v_2 is consistent with a ratio of viscosity over entropy density up to eta/s=0.16, data on minimum bias v_2 seems to favor a much smaller viscosity over entropy ratio, below the bound from the AdS/CFT conjecture. Some caveats on this result are discussed.Comment: 4 pages, 5 figures; v2: matches published version, title changed by journa

Light-Heavy Ion Collisions: A window into pre-equilibrium QCD dynamics?

Relativistic collisions of light on heavy ions (p+Au at sqrt(s)=7.7 GeV, p+Au, d+Au,3He+Au at sqrt(s)=62.4 GeV and 200 GeV and p+Pb, 3He+Pb at sqrt(s)=5.02 TeV) are simulated using "superSONIC", a model that includes pre-equilibrium flow, viscous hydrodynamics and a hadronic cascade afterburner. Even though these systems have strong gradients and only consist of at most a few tens of charged particles per unit rapidity, one finds evidence that a hydrodynamic description applies to these systems. Based on these simulations, the presence of a triangular flow component in d+Au collisions at sqrt(s)=200 GeV is predicted to be similar in magnitude to that found in 3He+Au collisions. Furthermore, the v3(p_T) ratio of 3He+Au to d+Au is found to be sensitive to the presence of pre-equilibrium flow. This would imply an experimentally accessible window into pre-equilibrium QCD dynamics using light-heavy ion collisions.Comment: 8 pages plus appendix; 9 figure

Onset of cavitation in the quark-gluon plasma

We study the onset of bubble formation (cavitation) in the quark-gluon plasma as a result of the reduction of the effective pressure from bulk-viscous corrections. By calculating velocity gradients in typical models for quark-gluon plasma evolution in heavy-ion collisions, we obtain results for the critical bulk viscosity above which cavitation occurs. Since present experimental data for heavy-ion collisions seems inconsistent with the presence of bubbles above the phase transition temperature of QCD, our results may be interpreted as an upper limit of the bulk viscosity in nature. Our results indicate that bubble formation is consistent with the expectation of hadronisation in low-temperature QCD.Comment: 12 pages, 2 figure