  We present recent improvements in the perturbative treatment of particle
interactions in Kinetic Field Theory (KFT) for inertial Zel'dovich
trajectories. KFT has been developed for the systematic analytical calculation
of non-linear cosmic structure formation on the basis of microscopic
phase-space dynamics. We improve upon the existing treatment of the interaction
operator by deriving a more rigorous treatment of phase-space trajectories of
particles in an expanding universe. We then show how these results can be
applied to KFT perturbation theory by calculating corrections to the late-time
dark matter power spectrum at second order in the interaction operator. We find
that the modified treatment of interactions w.r.t. inertial Zel'dovich
trajectories improves the agreement of KFT with simulation results on
intermediate scales compared to earlier results. Additionally, we illustrate
that including particle interactions up to second order leads to a systematic
improvement of the non-linear power spectrum compared to the first-order
result.

Cosmology and Nongalactic Astrophysics