Elliptic flow of π⁰ mesons in Cu$+$Au collisions at √s_NN=200 GeV and U$+$U at √s_NN=193 GeV

The second-order azimuthal anisotropy coefficients (v₂) of neutral $π$ mesons (π⁰) have been measured as a function of the transverse momentum (pT) and centrality of Cu$+$Au collisions at √s_NN=200~GeV and U$+$U at √s_NN=193 GeV at the Relativistic Heavy Ion Collider. The analysis used experimental data collected by the PHENIX experiment at midrapidity $|η|<0.35$ over a broad pT range up to ≈10~GeV/c, and the obtained results are compared with previous PHENIX measurements in Au$+$Au collisions at √s_NN=200~GeV. In all three collision systems, the π⁰~v₂ values follow the scaling with the second-order participant eccentricity and the cube root of the number of participating nucleons (ε₂ Nₚₐᵣₜ1/3) up to ≈4~GeV/c. Furthermore, the behavior of the azimuthal-dependent π⁰ nuclear-modification factors and associated fractional parton-energy losses are evaluated from measured nonzero v₂ values of π⁰ at pT>5 GeV/c and found to be approximately the same for similar values of Nₚₐᵣₜ1/3 in these collision systems. These findings demonstrate that the mechanism of π⁰ v₂ generation exhibits a high degree of universality across different initial geometries of heavy-ion collisions.