Rescattering Effects on HBT Interferometry

We derive a general formula for the two-particle correlation function with
the inclusion of elastic rescattering effects in the medium after the two
particles are produced. We apply this formula to the special case where the
scatterers are static, localized potentials. In three dimensions both
$R^2_{\rm side}$ and $R^2_{\rm out}$ are increased by an amount proportional
to the square of the spatial density of scatterers and to the forward
differential cross section. Specific numbers are used to illustrate kaon
interferometry.