is adopted [@Pratthome], which is based on the formula: $$C({\bf k},{\bf q}) = \frac {\int d^4x_i d^4x_j g(x_i,p_i) g(x_j,
p_j) |\phi({\bf r}^\prime, {\bf q}^\prime)|^2} {{\int d^4x_i
g(x_i,p_i)} {\int d^4x_j g(x_j,p_j)}}. \label{cpq}$$ Here $g(x_i,p_i)$ is an effective probability for emitting a particle $i$ with 4-momentum $p_i=({E_i, \bf p}_i)$ from the space-time point $x_i = (t_i, {\bf r}_i)$. $\phi({\bf r}^\prime,
{\bf q}^\prime)$ is the relative wave function with ${\bf r}^\prime$ being the relative position in the pair’s rest frame. ${\bf q}={\bf
p}_i-{\bf p}_j$ and $\textbf{k}=(\textbf{p}_{i}+\textbf{p}_{j})/2$ are the relative momentum and the average momentum of the two particles $i$ and $j$.

In this work, we select central ($<7.2\%$ of the total cross section $\sigma_T$) Pb+Pb collisions at SPS energies: $E_b=20A$, $30A$, $40A$, $80A$ and $158A$ GeV, with a pair rapidity cut $|Y_{\pi\pi}|<0.5$ ($Y_{\pi\pi}={\rm log}((E_1+E_2+p_{\parallel
1}+p_{\parallel 2})/(E_1+E_2-p_{\parallel 1}-p_{\parallel 2}))/2$ is the pair rapidity with pion energies $E_1$ and $E_2$ and longitudinal momenta $p_{\parallel 1}$ and $p_{\parallel 2}$ in the center of mass system). For each EoS about $2500$ events are calculated. All particles with their phase space coordinates at freeze-out are then given into the CRAB analyzing program. Only the negatively charged pions are considered during the analyzing process (for each analysis, one hundred 