System size dependence of the non-monotonous pion freeze-out volume excitation function

Hanburry-Brown-Twiss (HBT) correlation functions and radii of negatively charged pions from C+C, Si+Si, Cu+Cu, and In+In at lower RHIC/SPS energies are calculated with the UrQMD transport model and the CRAB analyzing program. We find a minimum in the excitation function of the pion freeze-out volume at low transverse momenta and around E lab ∼ 20–30AGeV which can be related to the change from initial string emission to bulk emission from the created resonance matter. For small systems, we predict a local minimum in the excitation function of the HBT (freeze-out) volume which is explained by the competition of two mechanisms of the particle production, namely resonance decays and string formation/fragmentation.

HBT CORRELATION AS A PROBE OF EoS

Based on the UrQMD and the CRAB analyzing program, HBT correlators of nucleons and pions are calculated with beam energies ranging from SIS up to RHIC. It is found that the HBT correlator and the ratio between the HBT radii RO and RS are sensitive to the stiffness of the EoS. A relatively stiff EoS is favored from the RO/RS ratio of pions. For the two-nucleon correlation at SIS beam energies, it is found that both the symmetry potential energy and conditions of constructing clusters at the late stage influence the correlator with the same power.