Cosmic rays propagating through magnetized turbulence scatter off magnetic fluctuations, changing their pitch angle — the angle between their velocity and the local magnetic field. Standard transport theory treats this as diffusion: each scattering event is independent, and the pitch angle executes a random walk. The Markov assumption.
This paper shows the assumption breaks at large pitch angles. Particles moving nearly perpendicular to the field (pitch angles near 90°) become trapped in magnetic wells created by compressions in the turbulence. While trapped, they oscillate back and forth, their pitch angle evolution correlated across multiple bounces. The transport is anomalous — subdiffusive — because the particle remembers where it has been.
The escape mechanism is gyroresonant scattering: occasional resonant interactions with small-scale fluctuations kick the particle out of the well. The interplay between mirror trapping (which creates memory) and gyroresonant scattering (which destroys it) produces a transition from anomalous to normal diffusion as scattering strengthens. At weak scattering, particles remember; at strong scattering, they forget fast enough to look Markovian.
The result reframes cosmic ray transport as a competition between two timescales: the trapping time in magnetic wells and the escape time via resonant scattering. Small-pitch-angle particles are always in the escape-dominated regime. Large-pitch-angle particles are in the trapping-dominated regime unless scattering is strong enough to overwhelm the wells. The same turbulence produces qualitatively different transport for particles at different angles — memory for the perpendicular, diffusion for the parallel.