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Chaoticity parameter $λ$ in two-pion interferometry in an expanding boson gas model

We investigate the chaoticity parameter $λ$ 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 $λ$ in the two-pion interferometry measurements at low pion pair momenta, but influence only slightly the $λ$ value at high pion pair momentum.