Excitation function of femtoscopic Lévy source parameters of pion pairs in EPOS4

Three-dimensional (3D) femtoscopic source parameters of pions provide a sensitive probe of the space-time structure of particle-emitting sources in high-energy heavy-ion collisions. Compared to one-dimensional measurements, three-dimensional femtoscopy not only provides a valuable cross-check but also offers a more complete characterization of the source geometry and its dynamical evolution. Particularly, differences between the “out” and “side” directions are sensitive to signals of a strong first-order phase transition, while the collision-energy dependence of Lévy radii may reveal non-monotonic features related to the equation of state. In this work, we systematically investigate the transverse mass ( m T ) and collision-energy ( s N N ) dependence of the three-dimensional femtoscopic parameters of pion pairs with Lévy-type sources in the STAR Beam Energy Scan (BES) range from s N N = 7.7 to 200 GeV using the EPOS4 model. The analyzed parameters include the Lévy index α , the correlation strength λ , and the three-dimensional radii R out , R side and R long , corresponding to the outward, sideward, and longitudinal (beam) directions. Derived quantities such as the radius difference R diff = R out 2 − R side 2 and the ratio R out / R side are also investigated. The results show that the extracted radii R side and R long decrease with increasing transverse mass and increase gradually with collision energy, while R out shows little energy dependence. The Lévy index α exhibits only a mild dependence on m T and collision energy, whereas the correlation strength λ shows a clear m T dependence and generally decreases with increasing collision energy. A comparison with EPOS3 results indicates general agreement within approximately 2 σ , with the notable exception of R side , which is systematically smaller in EPOS4.