Abstract
We investigate the chaoticity parameter $\lambda$ in two-pion
interferometry in an expanding boson gas model. The degree of
Bose-Einstein condensation of identical pions, density distributions,
and Hanbury-Brown-Twiss (HBT) correlation functions are calculated for
the expanding gas within the mean-field description with a harmonic oscillator
potential. The results indicate that a sources with thousands of
identical pions may exhibit a degree of Bose-Einstein condensation at
the temperatures during the hadronic phase in relativistic heavy-ion
collisions. This finite condensation may decrease the chaoticity
parameter $\lambda$ in the two-pion interferometry measurements at low
pion pair momenta, but influence only slightly the $\lambda$ value at
high pion pair momentum.
