State-of-the-art time-resolved probes offer unprecedented access to the dynamics of
interacting quasiparticles in solids on their natural time scales. While extensive research has
been conducted on the ultrafast dynamics of electrons and phonons, much less is known about the ultrafast response of magnetic moments. This thesis investigates pump-induced out-of-equilibrium magnetism in the Mott insulator CuO by combining resonant magnetic X-ray scattering experiments with quantum-kinetic simulations. In particular, time-dependent diffuse scattering measurements reveal evolving magnetic correlations, which are interpreted in terms of interacting magnons. To gain quantitative insight, the time-dependent quantum Boltzmann equation is solved, enabling the study of magnon–magnon scattering processes. For this purpose, Linear Spin Wave Theory is extended to second order using a novel and efficient matrix formalism that captures magnon interactions in greater detail. The resulting simulations are in good agreement with experimental observations and offer a comprehensive description of the magnetic dynamics, highlighting the role of non-thermal magnons and their progression toward thermal equilibrium