Due to the phenomenon of confinement, the hadron cannot be understood as a compound state of independent quarks and gluons but rather represents a strongly bounded many-body parton system. The dynamics of this system gives rise to such fundamental properties of the hadron as spin and mass. How the spin and mass of the hadron arise from the parton dynamics is an outstanding question of modern nuclear science. The parton dynamics can be probed in high-energy scattering experiments using the factorization approach. However, there is a wide class of observables that cannot be described within the conventional factorization schemes due to various reasons, including non-perturbative phenomena. In my talk, I will give several concrete examples and propose a solution based on applying the background field techniques, which provide a unified treatment of parton interactions. I will argue that the application of such techniques allows us to obtain a complete picture of the multi-parton dynamics in QCD and shed light on the origin of the proton spin and mass.
