  Wave-particle interaction is a key process in particle diffusion in
collisionless plasmas. We look into the interaction of single plasma waves with
individual particles and discuss under which circumstances this is a chaotic
process, leading to diffusion. We derive the equations of motion for a particle
in the fields of a magnetostatic, circularly polarized, monochromatic wave and
show that no chaotic particle motion can arise under such circumstances. A
novel and exact analytic solution for the equations is presented. Additional
plasma waves lead to a breakdown of the analytic solution and chaotic particle
trajectories become possible. We demonstrate this effect by considering a
linearly polarized, monochromatic wave, which can be seen as the superposition
of two circularly polarized waves. Test particle simulations are provided to
illustrate and expand our analytical considerations.
