Hydrodynamic Attractors, Initial State Energy, and Particle Production in Relativistic Nuclear Collisions

Abstract

We exploit the concept of hydrodynamic attractors to establish a general relation between the initial state energy and the produced particle multiplicities in high-energy nuclear collisions. When combined with an ab initio model of energy deposition, the entropy production during the preequilibrium phase naturally explains the universal centrality dependence of the measured charged particle yields in nucleus-nucleus collisions. We further estimate the energy density of the farfrom-equilibrium initial state and discuss how our results can be used to constrain non-equilibrium properties of the quark-gluon plasma.

Entropy production in pp and Pb-Pb collisions at energies available at the CERN Large Hadron Collider

“…The precise knowledge of the total produced entropy in heavy ion collisions and the entropy per final-state charged hadron is important for constraining the bulk properties of the initial-state from the final-state observables [54,62,66]. To determine the initial medium properties for high-multiplicity pp and Pb-Pb collisions, we performed simulations of averaged initial conditions starting at τ 0 = 0.1 fm/c with kinetic pre-equilibrium model KøMPøST [62,63,70] and viscous relativistic hydrodynamics code FLUIDUM [64].…”

“…To determine the initial medium properties for high-multiplicity pp and Pb-Pb collisions, we performed simulations of averaged initial conditions starting at τ 0 = 0.1 fm/c with kinetic pre-equilibrium model KøMPøST [62,63,70] and viscous relativistic hydrodynamics code FLUIDUM [64]. Importantly, these calculations take into account the produced entropy and work done in both the pre-equilibrium and hydrodynamic phases of the expansion [50][51][52][53][54]. We find that for simulations with the specific shear viscosity value η/s = 0.08 the initial pre-equilibrium energy per unity rapidity is about three times larger than at the final state in 0%-10% most central Pb-Pb collisions at √ s NN = 2.76 TeV, and approximately twice larger in high-multiplicity pp collisions at √ s = 7 TeV.…”

“…For initial energy estimates at even earlier times, see a recent publication[54], where a generalized constitutive equation of a hydrodynamic attractor was considered.…”

Nonequilibrium Attractor in High-Temperature QCD Plasmas

“…1(a) where the evolutions of all the various different initial conditions eventually converge onto a universal curve-the late-time attractor (see also Refs. [51,52]). However, as observed also in other models [13,17,18,27,30,32,53], this collapse takes place before the system is well described by the hydrodynamic gradient expansion, the first order of which is shown in Fig.…”

Hydrodynamic Attractors in Phase Space

“…It was also observed there that a "slow-roll" condition akin to what is used in inflationary cosmology [43,44] leads to an accurate approximation of this attractor. Subsequent works have lead to many interesting phenomenological applications [2,5,16,45,46]. Despite these developments, there are three important yet largely unexplored issues.…”