  Cooperative spontaneous emission of a single photon from a cloud of N atoms
modifies substantially the radiation pressure exerted by a far-detuned laser
beam exciting the atoms. On one hand, the force induced by photon absorption
depends on the collective decay rate of the excited atomic state. On the other
hand, directional spontaneous emission counteracts the recoil induced by the
absorption. We derive an analytical expression for the radiation pressure in
steady-state. For a smooth extended atomic distribution we show that the
radiation pressure depends on the atom number via cooperative scattering and
that, for certain atom numbers, it can be suppressed or enhanced.
