  Motivated by isotropization of QCD matter in the initial stages of heavy-ion
collisions, we consider a system of scalar fields that undergoes a boost
invariant longitudinal expansion. We use the framework of the two-particle
irreducible (2PI) effective action, which is close to the underlying quantum
field theory, and resum self-energy corrections up to three loops. The
resulting 2PI equations of motion are expressed in terms of the Milne
coordinates to account for longitudinal expansion. By solving numerically these
equations of motion, we can extract the occupation density and the effective
mass generated by in-medium interactions. At the largest values of the coupling
considered in this study, we observe the onset of isotropization both in the
occupation number and in the momentum dependence of the effective mass.
