Energy and system size dependence of phi meson production in Cu+Cu and Au+Au collisions
Phys.Lett.B 673 (2009) 183-191, 2009.
The STAR collaboration

Abstract (data abstract)
We study the beam-energy and system-size dependence of $\phi$ meson production (using the hadronic decay mode $\phi \to K^{+}K^{-}$) by comparing the new results from $Cu+Cu$ collisions and previously reported $Au+Au$ collisions at $\sqrt{s_{NN}}=62.4$ and 200 GeV measured in the STAR experiment at RHIC. Data presented in this Letter are from mid-rapidity $(|y| < 0.5)$ for $0.4 < p_{T} < 5$ GeV/$c$. At a given beam energy, the transverse momentum distributions for $\phi$ 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 $\phi$ 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 $\phi$ mesons lies between strange hadrons having net strangeness = 1 ($K^{−}$ and $\bar{\Lambda}$) and net strangeness = 2 ($\Xi$). The enhancement for $\phi$ mesons is observed to be higher at $\sqrt{s_{NN}}=200$ GeV compared to 62.4 GeV. These observations for the produced $\phi (s\bar{s})$ 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.

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