million pion pairs are considered). For the cascade calculations, we take the results from our previous publications as reference [@Li:2006gb; @Li:2007yd]. We found that the residual Coulomb effect after the hadron freeze-out on the HBT radii of the pion source is small [@Li:2008ge], therefore we omit it in the present analysis. Finally, we choose the longitudinal comoving system (LCMS) frame of the pair (also called the “Out-Side-Long” system, in which the longitudinal component of the pair velocity vanishes), which is frequently adopted in recent years, and fit the correlator by a 3D Gaussian distribution

$$\begin{aligned}
&&C(q_O,q_S,q_L)= K[1+\lambda \nonumber \\
&&\times {\rm
exp}(-R_L^2q_L^2-R_O^2q_O^2-R_S^2q_S^2-2R_{OL}^2q_Oq_L)].
\label{fit1}\end{aligned}$$

Here $K$ is the overall normalization factor, the $q_x$ and $R_x$ are the components of the pair relative momentum and homogeneity length (HBT radius) in the $x$ direction, respectively. The $\lambda$ parameter is called as the incoherence factor or, more correctly, the intercept parameter and lies for Bose-Einstein statistics between 0 and 1 for two-boson correlations in realistic HICs. Because the parameter $\lambda$ might be influenced by many additional factors, such as contamination, long-lived resonances, or the details of the residual Coulomb modification, we regard it as a free parameter and do not show it in this 