relatively soft EoS. It can be understood since in the pure UrQMD model the new particle production is treated either as a resonance decay or a fragmentation of the string, which introduces a finite lifetime and hence leads to a softer EoS. After considering the mean field potentials for both confined and “pre-formed” particles [@Li:2007yd; @Li:2008ge], which gives a strong repulsion at the early stage, the $R_O/R_S$ ratio was seen to decrease in line with results obtained here.

For the full hybrid model calculation the different freeze-out prescriptions do not affect the final results (when comparing the results by dashed lines with open triangles with that by the line with solid squares). The calculation with HG-EoS but without HR (dashed line with open diamonds) seems to provide better description of the data, but, seems clearly unphysical to the authors as a solution to the duration time problem.

Summary and outlook
===================

In summary, the HBT correlation of the negatively charged pion source created in central Pb+Pb collisions at SPS energies was investigated with a hybrid model that incorporates a (3+1)d hydrodynamic evolution in the UrQMD transport approach. We explored different settings, one where the EoS was varied without changing the 