  Circular particle accelerators at the energy frontier are based on
superconducting magnets that are extremely sensitive to beam losses as these
might induce quenches, i.e.\ transitions to the normal-conducting state.
Furthermore, the energy stored in the circulating beam is so large that
hardware integrity is put in serious danger, and machine protection becomes
essential for reaching the nominal accelerator performance. In this challenging
context, the beam halo becomes a potential source of performance limitations
and its dynamics needs to be understood in detail to assess whether it could be
an issue for the accelerator. In this paper, we discuss in detail a novel
framework, based on a diffusive approach, to model beam-halo dynamics. The
functional form of the optimal estimate of the perturbative series, as given by
Nekhoroshev's theorem, is used to provide the functional form of the action
diffusion coefficient. The goal is to propose an effective model for the
beam-halo dynamics and to devise an efficient experimental procedure to obtain
an accurate measurement of the diffusion coefficient.
