Energy and system size dependence of φ meson production in Cu+Cu and Au+Au collisions

We study the beam-energy and system-size dependence of φ meson production (using the hadronic decay mode φ→ K+K−) by comparing the new results from Cu + Cu collisions and previously reported Au+Au collisions at √ sNN = 62.4 and 200 GeV measured in the STAR experiment at RHIC. Data presented in this Letter are from mid-rapidity (|y| <0.5) for 0.4 <pT <5 GeV/c. At a given beam energy, the transverse momentum distributions for φ mesons are observed to be similar in yield and shape for Cu + Cu and Au + Au colliding systems with similar average numbers of participating nucleons. The φ meson yields in nucleus–nucleus collisions, normalized by the average number of participating nucleons, are found to be enhanced relative to those from p + p collisions. The enhancement for φ mesons lies between strange hadrons having net strangeness = 1 (K− and ¯Λ) and net strangeness = 2 (Ξ). The enhancement for φ mesons is observed to be higher at √ sNN = 200 GeV compared to 62.4 GeV. These observations for the produced φ(s¯s) mesons clearly suggest that, at these collision energies, the source of enhancement of strange hadrons is related to the formation of a dense partonic medium in high energy nucleus–nucleus collisions and cannot be alone due to canonical suppression of their production in smaller systems.