Chaoticity parameter $\lambda$ in two-pion interferometry in an   expanding boson gas model
  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. 
Jie Liu, Peng Ru, Wei-Ning Zhang, Cheuk-Yin Wong
nucl-th