First proposed by Israel and Stewart, transient hydrodynamic models emerged from the necessity to replace Navier-Stokes (NS) theory in relativistic scenarios. In this regime, NS leads to equations of motion that are acausal and that are unstable against perturbations around global equilibrium. In transient models, dissipative currents obey relaxation-type equations implementing a delayed response of the fluid to inhomogeneities in, e.g., fluid velocity. In Heavy-Ion collisions, the evolution of the expanding matter is described through hybrid codes, which also contain, i.a., models for the initial conditions, for the evolution of the so-called Quark-Gluon Plasma (QGP), for the transition from fluid to particles, whose correlations can be compared with experiments. In such a context, transient hydrodynamic models describe the evolution of the QGP. In codes solving the transient hydrodynamics, such as MUSIC, the corresponding transport coefficients are computed from Kinetic Theory assuming a single-species system in a high temperature limit, and employing the relaxation time approximation (RTA) for the collision term. In the present talk, in order to assess the effect of more realistic degrees of freedom, I shall revisit the computation of these transport coefficients in Kinetic Theory employing two models: a multi-species hadron-resonance gas model and a thermal-mass quasiparticle model, whose temperature-dependent mass is tuned to recover QCD thermodynamics.
