the hydrodynamic phase support the conclusion about the origin of the HBT time-related puzzle. In addition, the effects of the hadronic rescattering and of resonance decays (dubbed as “HR”) after the hydrodynamic freeze-out on the HBT radii and the $R_O/R_S$ ratio deserve more investigation. We have also noticed that some recent progresses of this topic both from an improved hydrodynamic calculation [@Broniowski:2008vp] and from the pion-optical-potential point of view [@Luzum:2008tc] have been published which provides additional new insights.

The paper is arranged as follows. In the next section, the UrQMD+hydrodynamics hybrid model is introduced briefly. The set of different EoS that are employed in the hydrodynamic phase are explained. Two different treatments for the transition process from the hydrodynamic evolution to the final state hadronic cascade are discussed. In Section 3, the analyzing program CRAB for constructing the HBT correlator and the corresponding three dimensional (3D) Gaussian fitting process are introduced. In Section 4, the HBT radii $R_L$, $R_O$, and $R_S$, and the $R_O/R_S$ ratio of the negatively charged pion source from central Pb+Pb collisions at SPS energies are shown and discussed in the context of the experimental data. Finally, in Section 5, a summary and an outlook are given.

