  We consider a two-dimensional dilute Bose gas above its superfluid transition
temperature. We show that the t-matrix approximation corresponds to the leading
set of diagrams in the dilute limit, provided the temperature is sufficiently
larger than the superfluid transition temperature. Within this approximation,
we give an explicit expression for the wave vector and frequency dependence of
the self-energy, and calculate the corrections to the chemical potential and
the effective mass arising from the interaction. We also argue that the
breakdown of the t-matrix approximation, which occurs upon lowering the
temperature, provides a simple criterion to estimate the superfluid critical
temperature for the two-dimensional dilute Bose gas. The critical temperature
identified by this criterion coincides with earlier results obtained by Popov
and by Fisher and Hohenberg using different methods. Extension of this
procedure to the three-dimensional case gives good agreement with recent Monte
Carlo data.
