Abstract
We introduce a model for heavy ion collisions named Trajectum, which includes an expanded initial stage with a variable free streaming velocity $v_{\rm fs}$ and a hydrodynamic stage with three varying second order transport coefficients. We describe how to obtain a Gaussian Emulator for this 20-parameter model and show results for key observables. This emulator can be used to obtain Bayesian posterior estimates on the parameters, which we test by an elaborate closure test as well as a convergence study. Lastly, we employ the optimal values of the parameters found in [1] to perform a detailed comparison to experimental data from PbPb and $p$Pb collisions. This includes both observables that have been used to obtain these values as well as wider transverse momentum ranges and new observables such as correlations of event-plane angles.
Highlights
IntroductionThe most convincing evidence for an almost perfect fluid comes from the anisotropies of the particle spectra at low momentum in combination with elaborate hydrodynamical modeling that can explain these nontrivial correlations [2]
The collisions of heavy ions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven and the Large Hadron Collider (LHC) at CERN have led to an accepted picture of a short prehydrodynamic phase followed by a relativistic fluid composed of quark-gluon plasma (QGP) with remarkably small shear viscosity [1].The most convincing evidence for an almost perfect fluid comes from the anisotropies of the particle spectra at low momentum in combination with elaborate hydrodynamical modeling that can explain these nontrivial correlations [2]
This started with ideal hydrodynamics [3], after which simulations including viscosity pointed to a surprisingly small value of the specific viscosity of η/s ≈ 0.08, with s the entropy density [4]
Summary
The most convincing evidence for an almost perfect fluid comes from the anisotropies of the particle spectra at low momentum in combination with elaborate hydrodynamical modeling that can explain these nontrivial correlations [2] This started with ideal hydrodynamics (effectively having shear viscosity η = 0) [3], after which simulations including viscosity pointed to a surprisingly small value of the specific viscosity of η/s ≈ 0.08, with s the entropy density [4]. It was assumed early on that the exploding debris left after the collision interacts so strongly that starting at a time as early as 1 fm/c (as in [4]) a fluid would be formed that can be described by viscous hydrodynamics
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