  We consider many-body effects on particle scattering in one, two and three
dimensional Bose gases. We show that at zero temperature these effects can be
modelled by the simpler two-body T-matrix evaluated off the energy shell. This
is important in 1D and 2D because the two-body T-matrix vanishes at zero energy
and so mean-field effects on particle energies must be taken into account to
obtain a self-consistent treatment of low energy collisions. Using the
off-shell two-body T-matrix we obtain the energy and density dependence of the
effective interaction in 1D and 2D and the appropriate Gross-Pitaevskii
equations for these dimensions. We present numerical solutions of the
Gross-Pitaevskii equation for a 2D condensate of hard-sphere bosons in a trap.
We find that the interaction strength is much greater in 2D than for a 3D gas
with the same hard-sphere radius. The Thomas-Fermi regime is therefore
approached at lower condensate populations and the energy required to create
vortices is lowered compared to the 3D case.
