  We study the dynamics of the cooling of a gas of caesium atoms in an optical
lattice, both experimentally and with 1D full-quantum Monte Carlo simulations.
We find that, contrary to the standard interpretation of the Sisyphus model,
the cooling process does not work by a continuous decrease of the average
kinetic energy of the atoms in the lattice. Instead, we show that the momentum
of the atoms follows a bimodal distribution, the atoms being gradually
transferred from a hot to a cold mode. We suggest that the cooling mechanism
should be depicted in terms of a rate model, describing the transfer between
the two modes along with the processes occurring within each mode.

Atomic Physics;Quantum Physics