  We present a detailed experimental study of the velocity distribution of
atoms cooled in an optical lattice. Our results are supported by full-quantum
numerical simulations. Even though the Sisyphus effect, the responsible cooling
mechanism, has been used extensively in many cold atom experiments, no detailed
study of the velocity distribution has been reported previously. For the
experimental as well as for the numerical investigation, it turns out that a
Gaussian function is not the one that best reproduce the data for all
parameters. We also fit the data to alternative functions, such as Lorentzians,
Tsallis functions and double Gaussians. In particular, a double Gaussian
provides a more precise fitting to our results.
