A brief look in the literature (eg, in the index to Chao & Tigner)
turns up many well known "popular" diffusion mechanisms, including ground
motion, power supply noise, tune modulation, Intra Beam Scattering -- et
cetera. Lesser known long term mechanisms, such as "modulational diffusion"
(see Lichtenberg & Liebermann), are much less studied, and are ill understood.
Theory, simulation, and experiment are not in good agreement, even in the
most controlled of circumstances. This limits the confidence with which
we can predict the fundamental performances of future (Very) Large Hadron
Colliders which operate in unfamiliar parameter regimes. A better theoretical
and experimental understanding of diffusion is therefore a major challenge
to Accelerator Physics.