  The leaky box model is now ruled out by measurements of a cosmic ray gradient
throughout the galactic disk. It needs to be replaced by a more refined
treatment which takes into account the diffusion of cosmic rays in the magnetic
fields of the Galaxy. We have estimated the flux of antiprotons on the Earth in
the framework of a two-zone diffusion model. Those species are created by the
spallation reactions of high-energy nuclei with the interstellar gas. Another
potential source of antiprotons is the annihilation of supersymmetric particles
in the dark halo that surrounds our Galaxy. In this letter, we investigate both
processes. Special emphasis is given to the antiproton signature of
supersymmetric dark matter. The corresponding signal exceeds the conventional
spallation flux below 300 MeV, a domain that will be thoroughly explored by the
Antimatter Spectrometer experiment. The propagation of the antiprotons produced
in the remote regions of the halo back to the Earth plays a crucial role.
Depending on the energy, the leaky box estimates are wrong by a factor varying
from 0.5 up to 3.
