Bayesian calibration of viscous anisotropic hydrodynamic simulations of heavy-ion collisions

Abstract

Due to large pressure gradients at early times, standard hydrodynamic model simulations of relativistic heavyion collisions do not become reliable until O(1) fm/c after the collision. To address this one often introduces a pre-hydrodynamic stage that models the early evolution microscopically, typically as a conformal, weakly interacting gas. In such an approach the transition from the pre-hydrodynamic to the hydrodynamic stage is discontinuous, introducing considerable theoretical model ambiguity. Alternatively, fluids with large anisotropic pressure gradients can be handled macroscopically using the recently developed Viscous Anisotropic Hydrodynamics (VAH). In high-energy heavy-ion collisions VAH is applicable already at very early times, and at later times transitions smoothly into conventional second-order viscous hydrodynamics (VH). We present a Bayesian calibration of the VAH model with experimental data for Pb-Pb collisions at the LHC at √ sNN = 2.76 TeV. We find that the VAH model has the unique capability of constraining the specific viscosities of the QGP at higher temperatures than other previously used models.1 A very recent study [18] employed the Color-Glass Condensate based IP-Glasma model to dynamically evolve the pre-hydrodynamic stage. While this is a significant conceptual improvement over free-streaming partons, it shares with the latter approach that, being rooted in Classical Yang-Mills dynamics for the interacting gluon fields, it keeps the system from naturally approaching local thermal equilibrium.