The STAR experiment

The femtoscopy method can probe the space-time structure and dynamics of strongly interacting nuclear matter created in high-energy collisions. The correlation of two non-identical particles allows one to extract information about asymmetry in the emission process between two kinds of particles. Such asymmetry arises in a system with a strong radial flow. In this work, femtoscopic analysis of three different systems (pion-kaon, pion-proton, and kaon-proton) in central (0-10\%) Au+Au collisions at $\sqrt{s_{NN}} = 39$ GeV is reported. Source sizes are consistent between different sign combinations and larger in the case of pion-kaon and pion-proton than for kaon-proton pairs. Obtained asymmetries show that lighter particles, on average, are emitted closer to the center of the source and/or later than heavier particles. Additionally, the pion-kaon correlations are analyzed in the centrality dependence in Au+Au collisions at $\sqrt{s_{NN}} = 39$ GeV and for three energies of the Beam Energy Scan program $\sqrt{s_{NN}} = 7.7$, $11.5$, and $39$ GeV. The sizes of pion-kaon sources increase with increasing centrality and energy. These results confirm that collective behavior exists in the lower energies of the BES-I program.