Examination of the universal behavior of the η -to- π0 ratio in heavy-ion collisions

We demonstrate that the pT dependence of the η/π0 ratio at midrapidity is universal within a few percent for high-energy p+p, p+A, and d+A collisions, over a broad range of collision energies. The η/π0 ratio increases with pT up to 4 to 5 GeV/c where it saturates at a nearly constant value of 0.487±0.024. Above pT=5 GeV/c, the same constant value is also observed in A+A collisions independent of collision system, energy, and centrality. At lower pT, where accurate η/π0 data are absent for A+A collisions, we estimate possible deviations from the universal behavior, which could arise due to the rapid radial hydrodynamic expansion of the A+A collision system. For A+A collisions at the BNL Relativistic Heavy Ion Collider we find that possible deviations are limited to the pT range from 0.4 to 3 GeV/c and remain less than 20% for the most central collisions.