Relativistic hydrodynamics for ideal and viscous fluids is discussed as a tool to describe relativistic heavy-ion collisions and to extract transport properties of the quark-gluon plasma from experimentally measured hadron momentum spectra.
We numerically solve fully (3+1)-dimensional relativistic hydrodynamical equation with the baryon number conservation law. For realistic initial conditions we adopt the results from the event generator (URASiMA). Using this model we discuss collective flow.
We present calculations of the hydrodynamics of the inner crust of neutron stars, where a superfluid neutron liquid coexists with a lattice of neutron-rich nuclei. The long-wavelength collective oscillations are combinations of phonons in the lattice and phonons in the superfluid neutrons. Velocities of collective modes are calculated from information about effective nucleon-nucleon interactions derived from Lattimer and Swesty's microscopic calculations based on a compressible liquid drop picture of the atomic nuclei and the surrounding neutrons.
We investigate the effect of early chemical freeze-out on hydrodynamic flow and HBT radii at the RHIC energy with a genuinely three dimensional hydrodynamic model. It is found that the property of early chemical freeze out reduces radial flow, elliptic flow, R_out and R_long.
Hydrodynamics is applied to describe the dynamics of relativistic heavy-ion collisions. The focus of the present study is the influence of a possible (phase) transition to the quark-gluon plasma in the nuclear matter equation of state on collective observables, such as the lifetime of the system and the transverse directed flow of matter. It is shown that such a transition leads to a softening of the equation of state, and consequently to a time-delayed expansion which is in principle observable via two-particle correlation functions. Moreover, the delayed expansion leads to a local minimum in…
We investigate the interplay between hydrodynamic fluctuations and initial longitudinal fluctuations for their effects on the rapidity decorrelation of collective flow in high-energy nuclear collisions. We use a (3+1)-dimensional integrated dynamical model in which we combine initial conditions with longitudinal fluctuations, fluctuating hydrodynamics and hadronic cascades. We analyse the factorisation ratio in the longitudinal direction to study the effect of these fluctuations on the rapidity decorrelation. We find an essential difference between the effects of the hydrodynamic fluctuations …
We review the basic concepts of perturbative quantum chromodynamics (QCD) and relativistic hydrodynamics, and their applications to hadron production in high energy nuclear collisions. We discuss results from the Relativistic Heavy Ion Collider (RHIC) in light of these theoretical approaches. Perturbative QCD and hydrodynamics together explain a large amount of experimental data gathered during the first decade of RHIC running, although some questions remain open. We focus primarily on practical aspects of the calculations, covering basic topics like perturbation theory, initial state nuclear …
The presence of a critical point in the QCD phase diagram can deform the trajectories describing the evolution of the expanding fireball in the QCD phase diagram. The deformation of the hydrodynamic trajectories will change the transverse velocity dependence of the proton-antiproton ratio when the fireball passes in the vicinity of the critical point. An unusual transverse velocity dependence of the anti-proton/proton ratio in a narrow beam energy window would thus signal the presence of the critical point.
Collective excitations of a hot QCD medium are the main focus of the present article. The analysis is performed within semi-classical transport theory with isotropic and anisotropic momentum distribution functions for the gluonic and quark-antiquark degrees of freedom that constitutes the hot QCD plasma. The isotropic/equilibrium momentum distributions for gluons and quarks are based on a recent quasi-particle description of hot QCD equations of state. The anisotropic distributions are just the extensions of isotropic ones by stretching or squeezing them in one of the directions. The hot QCD m…
Collective modes of an anisotropic hot QCD medium have been studied within the semi-classical transport theory employing Bhatnagar-Gross-Krook (BGK) collisional kernel. The modeling of the isotropic medium is primarily based on a recent quasi-particle description of hot QCD equation of state where the medium effects have been encoded in effective gluon and quark/anti-quark momentum distributions that posses non-trivial energy dispersions. The anisotropic distribution functions are obtained in a straightforward the way by stretching or squeezing the isotropic ones along one of the directions. T…
We investigate hadronic particle spectra and flow characteristics of heavy-ion reactions in the FAIR/NICA energy range of $1\ A{\rm GeV} \le E_{\rm lab} \le 10\ A{\rm GeV}$ within a relativistic ideal hydrodynamic one-fluid approach. The particlization is realized by sampling the Cooper-Frye distribution for a grand canonical hadron gas on a hypersurface of constant energy density. Results of the hydrodynamic calculations for different underlying equations of state are presented and compared with experimental data and microscopic transport simulations. The sensitivity of the approach to physic…
At finite temperature, the hydrodynamic collective modes of superfluid trapped Fermi gases are coupled to the motion of the normal component, which in the BCS limit behaves like a collisionless normal Fermi gas. The coupling between the superfluid and the normal components is treated in the framework of a semiclassical transport theory for the quasiparticle distribution function, combined with a hydrodynamic equation for the collective motion of the superfluid component. We develop a numerical test-particle method for solving these equations in the linear response regime. As a first applicatio…
We perform 3+1D viscous hydrodynamic calculations of proton-lead and lead-lead collisions at top LHC energy. We show that existing data from high-multiplicity p-Pb events can be well described in hydrodynamics, suggesting that collective flow is plausible as a correct description of these collisions. However, a more stringent test of the presence of hydrodynamic behavior can be made by studying the detailed momentum dependence of two-particle correlations. We define a relevant observable, $r_n$, and make predictions for its value and centrality dependence if hydrodynamics is a valid descriptio…
Utilizing a second-order hydrodynamics formalism, the dispersion relations for the frequencies and damping rates of collective oscillations as well as spatial structure of these modes up to the decapole oscillation in both two- and three- dimensional gas geometries are calculated. In addition to higher-order modes, the formalism also gives rise to purely damped "non-hydrodynamic" modes. We calculate the amplitude of the various modes for both symmetric and asymmetric trap quenches, finding excellent agreement with an exact quantum mechanical calculation. We find that higher-order hydrodynamic …
In this article, we briefly review the recent progress on collective flow and hydrodynamics in large and small systems at the Large Hadron Collider (LHC), which includes the following topics: extracting the QGP viscosity from the flow data, initial state fluctuations and final state correlations in 2.76 A TeV Pb--Pb collisions, correlations and collective flow in high energy p--Pb and p--p collisions.
A short review of the two recently analyzed collective effects in dense non-Abelian matter, the photon and dilepton production in nonequilibrium glasma and polarization properties of turbulent Abelian and non-Abelian plasmas, is given.
We describe fireballs that rehadronize from a perfectly fluid quark matter to a chemically frozen, multi-component hadron gas. In the hydrodynamics of these fireballs, we utilize the lattice QCD equation of state, however, we also apply non-relativistic kinematics for simplicity and clarity. Two new classes of exact, analytic solutions of fireball hydrodynamics are presented: the first class describes triaxially expanding, non-rotating ellipsoidal fireballs, while the second class of exact solutions corresponds to spheroidally symmetric, rotating fireballs. In both classes of solutions, we fin…
We discuss the present collective flow signals for the phase transition to quark-gluon plasma (QGP) and the collective flow as a barometer for the equation of state (EoS). A study of Mach shocks induced by fast partonic jets propagating through the QGP is given. We predict a significant deformation of Mach shocks in central Au+Au collisions at RHIC and LHC energies as compared to the case of jet propagation in a static medium. Results of a hydrodynamical study of jet energy loss are presented.
We generalize a previously known class of exact analytic solutions of relativistic perfect fluid hydrodynamics for the first time to arbitrary temperature-dependent Equation of State. We investigate special cases of this class of solutions, in particular, we present hydrodynamical solutions with the Equation of State determined from lattice QCD calculations. We discuss the phenomenological relevance of these solutions as well.
We show that the centrality and system-size dependence of elliptic flow measured at RHIC are fully described by a simple model based on eccentricity scaling and incomplete thermalization. We argue that the elliptic flow is at least 25% below the (ideal) ``hydrodynamic limit'', even for the most central Au-Au collisions. This lack of perfect equilibration allows for estimates of the effective parton cross section in the Quark-Gluon Plasma and of its viscosity to entropy density ratio. We also show how the initial conditions affect the transport coefficients and thermodynamic quantities extracte…
We consider the experimental implications of the axial current triangle diagram anomaly in a hydrodynamic description of high density QCD. We propose a signal of an enhanced production of spin-excited hadrons in the direction of the rotation axis in off-central heavy ion collisions.
Heavy-ion collisions are well described by a dynamical evolution with a long hydrodynamical phase. In this phase the properties of the strongly coupled quark-gluon plasma are reflected in the equation of state (EoS) and the transport coefficients, most prominently by the shear and bulk viscosity over entropy density ratios $η$/s(T) and $ζ$/s(T), respectively. While the EoS is by now known to a high accuracy, the transport coefficients and in particular their temperature and density dependence are not well known from first-principle computations yet, as well as the possible influence they can h…
We compute the longitudinal hydrodynamic flow in ultrarelativistic heavy ion collisions at $\sqrt{s}$ = 5500 GeV by using boost non-invariant initial conditions following from perturbative QCD. The transfer of entropy and energy from the central region to larger rapidities caused by boost non-invariance is determined and the associated decrease in the lifetime of the system is estimated.
After reviewing the evidence that QCD matter at ultrarelativistic energies behaves as a very good fluid, we describe the connection of QCD fluidity to heavy quark observables. We review the way in which heavy quark spectra can place tighter limits on the viscosity of QCD matter. Finally, we show that correlations between flow observables and the event-by-event charm quark abundance ("flavoring") can shed light on the system's equation of state.
Using relativistic conformal hydrodynamics coupled to the linear $σ$ model we study the evolution of matter created in heavy--ion collisions. We focus the study on the influence of the dynamics of the chiral fields on the charged-hadron elliptic flow $v_2$ for a temperature--independent as well as for a temperature--dependent $η/s$ that is calculated from kinetic theory. We find that $v_2$ is not very sensitive to the coupling of chiral fields to the hydrodynamic evolution, but the temperature dependence of $η/s$ plays a much bigger role on this observable.
The hydrodynamic description of the Fermi arc surface states is proposed. In view of the strong suppression of scattering on impurities, the hydrodynamic regime for Fermi arc states should be, in principle, plausible. By using the kinetic theory, the Fermi arc hydrodynamics is derived and the corresponding effects on the bulk flow and surface collective modes are studied. For the bulk flow, the key effect of the proposed Fermi arc hydrodynamics is the modification of the corresponding boundary conditions. In a slab geometry, it is shown that, depending on the transfer rates between the surface…
The large density of matter in the interaction region of the proton-nucleus or deuteron-nucleus collisions enables the collective expansion of the fireball. Predictions of the hydrodynamic model for the asymmetric transverse flow are presented and compared to experimental data.
We compute the hydrodynamic relaxation times $τ_π$ and $τ_j$ for hot QCD at next-to-leading order in the coupling with kinetic theory. We show that certain dimensionless ratios of second-order to first-order transport coefficients obey bounds which apply whenever a kinetic theory description is possible; the computed values lie somewhat above these bounds. Strongly coupled theories with holographic duals strongly violate these bounds, highlighting their distance from a quasiparticle description.
High-energy nuclear collisions produce a nonequilibrium plasma of quarks and gluons which thermalizes and exhibits hydrodynamic flow. There are currently no practical frameworks to connect the early particle production in classical field simulations to the subsequent hydrodynamic evolution. We build such a framework using nonequilibrium Green's functions, calculated in QCD kinetic theory, to propagate the initial energy-momentum tensor to the hydrodynamic phase. We demonstrate that this approach can be easily incorporated into existing hydrodynamic simulations, leading to stronger constraints …
We present results on the evolution of <p_t> with energy, for hadrons emerging from hadronization of a quark-gluon plasma possibly produced in high-energy heavy-ion collisions. We find that BNL-RHIC energy corresponds to the previously predicted plateau of <p_t>, reminiscent of the assumed first-order QCD phase transition. Heavy hadrons are the best messengers of the transverse flow stall.
We investigate the effect of early chemical freeze-out on radial flow, elliptic flow and HBT radii by using a fully three dimensional hydrodynamic model. When we take account of the early chemical freeze-out, the space-time evolution of temperature in the hadron phase is considerably different from the conventional model in which chemical equilibrium is always assumed. As a result, we find that radial and elliptic flows are suppressed and that the lifetime and the spatial size of the fluid are reduced. We analyze the p_t spectrum, the differential elliptic flow, and the HBT radii at the RHIC e…
We compute the initial energy densities produced in ultrarelativistic heavy-ion collisions from NLO perturbative QCD using a saturation conjecture to control soft particle production, and describe the subsequent space-time evolution of the system with hydrodynamics, event by event. The resulting centrality dependence of the low-$p_T$ observables from this pQCD + saturation + hydro ("EKRT") framework are then compared simultaneously to the LHC and RHIC measurements. With such an analysis we can test the initial state calculation, and constrain the temperature dependence of the shear viscosity-t…
We analyze the behavior of (net-)proton number cumulants in central collisions of heavy ions across a broad collision energy range by utilizing hydrodynamic simulations. The calculations incorporate essential non-critical contributions to proton fluctuations such as repulsive baryonic core and exact baryon number conservation. The experimental data are consistent with non-critical physics at collision energies $\sqrt{s_{\rm NN}} \geq 20$ GeV. The data from STAR and HADES Collaborations at lower collision energies indicate an excess of (multi-)proton correlations over the non-critical reference…
Finding the existence and the location of the QCD critical point is one of the main goals of the RHIC beam energy scan program. To make theoretical predictions and corroborate with the experimental data requires modeling the space-time evolution of the matter created in heavy-ion collisions by dynamical models such as the relativistic hydrodynamics with an appropriate Equation of State (EoS). In the present exploratory study, we use a viscous 2+1 dimensional event-by-event (e-by-e) hydrodynamic code at finite baryon densities with two different EoSs (i) Lattice QCD + HRG with a crossover trans…
The string breaking model of particle production is extended in order to help explain the transverse momentum distribution in elementary collisions. Inspired by an idea of Bialas', we treat the string using a collective coordinate approach. This leads to a chromo-electric field strength which fluctuates, and in turn implies that quarks are produced according to a thermal distribution.
Understanding the origins of collective, fluid-like behavior in ultrarelativistic nuclear collisions constitutes one of the biggest open challenges in the field. In this Letter, it is argued that certain features in the multiplicity dependence of the source sizes extracted using pion interferometry may be understood quite naturally if small systems evolve hydrodynamically at sufficiently large multiplicities. Interferometry may therefore provide a baseline for probing and constraining the nature of collective behavior in nuclear collisions.
I review recent developments in QCD thermodynamics and collective excitations from the hard-thermal-loop effective theory. I begin by motivating the discussion with open questions from heavy-ion collisions. I then discuss a finite-temperature and -density calculation of QCD thermodynamics at NNLO from the hard-thermal-loop perturbation theory. Finally I discuss a recent exploration of generalizing the hard-thermal-loop framework to the (chromo)magnetic scale $g^2T$, from which a novel massless mode is uncovered.
Motivated by recent interest in collectivity in small systems, we calculate the harmonic flow response to initial geometry deformations within weakly coupled QCD kinetic theory using the first correction to the free-streaming background. We derive a parametric scaling formula that relates harmonic flow in systems of different sizes and different generic initial gluon distributions. We comment on similarities and differences between the full QCD effective kinetic theory and the toy models used previously. Finally we calculate the centrality dependence of the integrated elliptic flow $v_2$ in ox…
We introduce an event-by-event pQCD + saturation + hydro ("EKRT") framework for high-energy heavy-ion collisions, where we compute the produced fluctuating QCD-matter energy densities from next-to-leading order (NLO) perturbative QCD (pQCD) using saturation to control soft particle production, and describe the space-time evolution of the QCD matter with viscous hydrodynamics, event by event (EbyE). We compare the computed centrality dependence of hadronic multiplicities, p_T spectra and flow coefficients v_n against LHC and RHIC data. We compare also the computed EbyE probability distributions…
Very different strongly interacting quantum systems such as Fermi gases, quark-gluon plasmas formed in high energy ion collisions and black holes studied theoretically in string theory are known to exhibit quantitatively similar damping of hydrodynamic modes. It is not known if such similarities extend beyond the hydrodynamic limit. Do non-hydrodynamic collective modes in Fermi gases with strong interactions also match those from string theory calculations? In order to answer this question, we use calculations based on string theory to make predictions for novel types of modes outside the hydr…
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…
In the first part I discuss the sensitivity of collective matter expansion in ultrarelativistic heavy-ion collisions to the transition between quark and hadronic matter (physics of the softest point of the Equation of State). A kink in the centrality dependence of elliptic flow has been suggested as a signature for the phase transition in hot QCD matter. Indeed, preliminary data of NA49 presented at this conference show first indications for the predicted kink. In the second part I have a look at the present theories of heavy-ion reactions. These remarks may also be seen as a critical comment …
Relativistic Hydrodynamics and Vorticity Dynamics in High-Energy Heavy-Ion Collisions: A Collective Flow Perspective
Mohammed V University in Rabat, Faculty of Sciences(拉巴特穆罕默德五世大学理学院);School of Physics Applied and Engineering, Mohammed VI Polytechnic University(穆罕默德六世理工大学应用物理工程学院);Irfu, CEA, Université Paris-Saclay(巴黎萨克雷大学CEA IRFU)
QCD strings originate from high-energy scattering in the form of Reggeons and Pomerons, and have been studied in some detail in lattice numerical simulations. Production of multiple strings, with their subsequent breaking, is now a mainstream model of high energy $pp$ and $pA$ collisions. Recent LHC experiments revealed that high multiplicity end of such collisions show interesting collective effects. This ignited an interest in the interaction of QCD strings and multi-string dynamics. Holographic models, collectively known as AdS/QCD, developed in the last decade, describe both hadronic spect…
Collective phenomena in quantum many-body systems are often described in terms of hydrodynamics, an appropriate framework when the involved particle numbers are effectively macroscopic. We propose to use experiments on expanding clouds of few and many interacting cold atoms to investigate the emergence of hydrodynamics as a function of particle number. We consider gases confined in two-dimensional elliptically-deformed traps, and we employ the manifestation of elliptic flow as an indicator of collective behavior. We quantify the response of the gas to the deformation of the trapping potential,…
We propose a new strategy for the experimental search of the QCD phase transition in heavy ion collisions: One may tune collision energy around the point where the lifetime of the fireball is expected to be longest. We demonstrate that the hydrodynamic evolution of excited nuclear matter does change dramatically as the initial energy density goes through the "softest point" (where the pressure to energy density ratio reaches its minimum). For our choice of equation of state, this corresponds to epsilon_i approx. = 1.5 GeV/fm^3 and collision energy E_lab/A approx. = 30 GeV (for Au+Au). Various …
A hybrid transport approach for the bulk evolution of viscous QCD matter produced in ultra-relativistic heavy-ion collisions is presented. The expansion of the dense deconfined phase of the reaction is modeled with viscous hydrodynamics while the dilute late hadron gas stage is described microscopically by the Boltzmann equation. The advantages of such a hybrid approach lie in the improved capability of handling large dissipative corrections in the late dilute phase of the reaction, including a realistic treatment of the non-equilibrium hadronic chemistry and kinetic freeze-out. By varying the…
We report the continuum limits of the speed of sound, c_s, and the specific heat at constant volume, c_v, in quenched QCD at temperatures of 2Tc and 3Tc. At these temperatures, c_s is within 2-sigma of the ideal gas value, whereas c_v differs significantly from the ideal gas. However, both are compatible with results expected in a theory with conformal symmetry. We investigate effective measures of conformal symmetry in the high temperature phase of QCD.
We evaluate the effects of preequilibrium dynamics on observables in ultrarelativistic heavy-ion collisions. We simulate the initial nonequilibrium phase within A MultiPhase Transport (AMPT) model, while the subsequent near-equilibrium evolution is modeled using (2+1)-dimensional relativistic viscous hydrodynamics. We match the two stages of evolution carefully by calculating the full energy-momentum tensor from AMPT and using it as input for the hydrodynamic evolution. We find that when the preequilibrium evolution is taken into account, final-state observables are insensitive to the switchin…
We discuss the use of the hydrodynamic model for the description of the evolution of dense matter formed in ultrarelativistic heavy-ion collisions. The collective flow observed in heavy-ion collisions at the BNL Relativistic Heavy Ion Collider and at the CERN Large Hadron Collider is consistent with the assumption that a fireball of strongly interacting matter is formed. Experimental results from p-Pb and d-Au collisions show similar phenomena, which suggests that collective expansion appears in small systems as well. We review the recent application of the hydrodynamic model to small systems …
We bring the EKRT framework, which combines perturbative QCD (pQCD) minijet production with gluon saturation and hydrodynamics, to next-to-leading order (NLO) in pQCD as rigorously as possible. We chart the model uncertainties, and study the viability and predictive power of the model in the light of the RHIC and LHC measurements in central $A+A$ collisions. In particular, we introduce a new set of measurement functions to define the infrared- and collinear-safe minijet transverse energy, $E_T$, in terms of which we formulate the saturation. We update the framework with the EPS09 NLO nuclear p…
We apply the hydrodynamic model for the dynamics of matter created in p-Pb collisions at 4.4TeV and d-Pb collisions at 3.11TeV. The fluctuating initial conditions are calculated in the Glauber Monte-Carlo model for several centrality classes. The expansion is performed event by event in 3+1-dimensional viscous hydrodynamics. Noticeable elliptic and triangular flows appear in the distributions of produced particles.
The formation and collective expansion of the fireball formed in ultrarelativistic p-A and d-A collisions is discussed. Predictions of the hydrodynamic model are compared to recent experimental results. The presence of strong final state interaction effects in the small dense systems is consistent with the observed azimuthal anisotropy of the flow and with the mass dependence of the average transverse momentum and of the elliptic flow. This raises the question of the mechanism explaining such a rapid build up of the collective flow and the large degree of local equilibration needed to justify …
We solve the relativistic Navier-Stokes equations with homogeneous boost-invariant boundary conditions, and perform a stability analysis of the solution. We show that, if the bulk viscosity has a peak around $T_c$ as inferred from QCD-based arguments, the background solution "freezes" at $T_c$ to a nearly constant temperature state. This state is however highly unstable with respect to certain inhomogeneous modes. Calculations show that these modes have enough time to blow up and tear the system into droplets. We conjecture that this is how freeze-out occurs in the QGP created in heavy ion col…