CoMBolt-ITA, a Collective Model via relativistic Boltzmann equation in Isotropization Time Approximation

A new (2+1)-model is developed to investigate the collective behavior of the quark-gluon plasma produced in high-energy heavy-ion collisions. This framework couples pre-equilibrium dynamics with hydrodynamic evolution by solving the Boltzmann equation within the isotropization time approximation. A numerical scheme based on the method of characteristics enables the evolution to begin from a specified initial Boltzmann distribution. In this work, the spatial structure of the initial distribution is modeled using the TrENTo framework. Our results show that a medium initialized at $\tau_0 $ in the order of 1\,[fm/$c$] with a small shear viscosity to entropy density ratio ($\eta/s = 0.008$) evolves consistently with hydrodynamic simulations, such as those performed using the VISH2+1 code, while discrepancies arise for a medium with $\eta/s = 0.8$. Furthermore, when initialized with a highly anisotropic momentum distribution in the longitudinal direction at early times, the system exhibits spatially non-uniform thermalization in the transverse plane, leading to the emergence of a nontrivial hypersurface that marks the onset of hydrodynamic applicability.