The expanding pion liquid and the two-pion correlation function

We examine the effects that a confining boundary together with hydrodynamical expansion play on two-pion distributions in relativistic heavy-ion collisions. We show that the effects arise from the introduction of further correlations due both to collective motion and the system's finite size. We study the behavior of the correlation function intercept with the system's density. We show that as the density increases, the intercept decreases. The effect is due to the fact that, as the ground state population becomes a significant fraction of the total multiplicity, pions coming from this single quantum state do not partake in the HBT effect, leading to a reduction in the intercept. We also show that, though the effects coming from hydrodynamical expansion lead to a reduction in the apparent source radius with increasing and large average pair momentum K, for small K, the presence of the boundary leads also to a decrease of the apparent source radius with decreasing K. These two competing effects produce a maximum for the effective source radius as a function of K.