Hydrodynamization and thermalization in heavy-ion collisions: A kinetic theory perspective

¹ University of Jyväskylä, Department of Physics, P.O.B. 35, FI-40014 University of Jyväskylä, Finland
² Helsinki Institute of Physics, P.O.B. 64, FI-00014 University of Helsinki, Finland
³ Instituto de Física, Universidade Federal Fluminense, Av. Gal. Milton Tavares de Souza, S/N, 24210-346, Gragoatá, Niterói, Rio de Janeiro, Brazil

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Published online: 17 April 2026

Abstract

Understanding the applicability of fluid-dynamical models to describe the hot and dense matter produced in the early stages of hadronic collisions is a fundamental problem in the field. In particular, it is not clear to what degree this hydrodynamization process requires proximity to a local equilibrium state. In this contribution, we study this problem in kinetic theory considering an ultrarelativistic gas undergoing strong longitudinal expansion, assuming Bjorken flow. We solve the Boltzmann equation and verify that the system displays considerable deviations from local equilibrium, even though the energy-momentum tensor is well described by fluid dynamics. We further quantify this effect computing the emission of photons in the quark-gluon plasma and verify whether this deviation from equilibrium can be observed.