Pre-hydrodynamic evolution and its signatures in final-state heavy-ion observables

We investigate the effects of pre-hydrodynamic evolution on final-state observables in heavy-ion collisions, including results in small systems, using state-of-the art event simulations coupled to different pre-hydrodynamic scenarios, which include the recently-developed effective kinetic transport theory evolution model KoMPoST. While we found flow observables to be insensitive to the details of pre-hydrodynamic evolution, we observe an effect in the transverse momentum spectra. We point out that at least part of this effect is a consequence of the underlying conformal invariance assumption currently present in pre-equilibrium approaches, which leads to an artificially large out-of-equilibrium bulk pressure when switching from (conformal) pre-hydrodynamic evolution to hydrodynamics (using the non-conformal QCD equation of state). These results indicate that a consistent treatment of pre-hydrodynamic evolution in heavy-ion collisions requires the use of non-conformal models of early-time dynamics.