Global fluid fits to identified particle transverse momentum spectra from heavy-ion collisions at the Large Hadron Collider

Abstract

Transverse momentum spectra of identified particles produced in heavy-ion collisions at the Large Hadron Collider are described with relativistic fluid dynamics. We perform a systematic comparison of experimental data for pions, kaons and protons up to a transverse momentum of 3 GeV/c with calculations using the FluiduM code package to solve the evolution equations of fluid dynamics, the TrENTo model to describe the initial state and the FastReso code to take resonance decays into account. Using data in five centrality classes at the center-of-mass collision energy per nucleon pair √ s NN = 2.76 TeV, we determine systematically the most likely parameters of our theoretical model including the shear and bulk viscosity to entropy ratios, the initialization time, initial density and freeze-out temperature through a global search and quantify their posterior probability. This is facilitated by the very efficient numerical implementation of FluiduM and FastReso. Based on the most likely model parameters we present predictions for the transverse momentum spectra of multi-strange hadrons as well as identified particle spectra from Pb-Pb collisions at √ s NN = 5.02 TeV.

Transport and hydrodynamics in the chiral limit

“…Currently, there is some evidence for such a soft pion enhancement-see for example Fig. 3 in [46] and Fig. 11 in [47].…”

HIP and HEP

“…This was known for lower collider (RHIC) and fixed target (CERN SPS, BNL AGS) energies. Combined with the phenomenological success of models that include fluid-dynamic expansion [16][17][18][19], it forms the main support for an almost perfect fluid picture of AA collisions. Since the azimuthal asymmetry in Fig 1b provides direct evidence for final state interactions, Bjorken's reasoning is unavoidable.…”

“…HIP and HEP data sets constrain hydrodynamic model parameters [16][17][18][19] that have been related to fundamental thermal properties such as T c and shear (η/s) and bulk (ξ/s) viscous transport coefficients. The methodology with which these quantities are related to data shares close similarities with the analysis of the ΛCDM cosmological concordance model that features in the Particle Data Book.…”

“…In HIP (in cosmology), the propagation of matter perturbations is used to constrain material properties of the smallest (the largest) material physical system studied by mankind. Given that T c , η/s, ξ/s are fundamental properties that can be calculated from first principles of thermal QCD, and given that current phenomenological analyses [15][16][17][18][19][20][21] seem mature, the question arises whether the values derived phenomenologically from LHC and RHIC data could be included in the compilation of the Particle Data Group (PDG), too. In comparison to ΛCDM, the evolution of ultra-relativistic heavy-ion collisions is more complicated since different evolution stages (initial conditions, pre-equilibrium, hydrodynamization, hadronization, freeze-out, ...) are followed with different dynamical concepts, and since modeling is currently needed to bridge between the different stages and between data and QCD-based predictions.…”

“…As a consequence of fluid-dynamic evolution, spatial asymmetries in the initial nuclear overlap (the almond shape of finite impact parameter collisions and event-wise fluctuations) translate into pressure gradients that manifest themselves in measurable "flow" anisotropies v n of the momentum distributions. In this context, a preferred value η/s 1 of the shear viscosity over entropy ratio [16][17][18][19] indicates that interactions may be so efficient that the very notion of particlelike degrees of freedom within the matter becomes questionable. This is theoretically conceivable since we know of classes of strongly coupled quantum field theories (with gravity duals) whose non-abelian plasmas are free of quasi-particle-like excitations [22].…”