Evidence for non-hadronic interactions of charm degrees of freedom in heavy-ion collisions at relativistic energies

Abstract

Within the Hadron-String Dynamics (HSD) transport approach we study the suppression pattern of charmonia at RHIC with respect to centrality and rapidity employing various model concepts, such as variants of the ‘comover absorption’ model or the ‘charmonium melting’ scenario. We find that especially the ratio of the forward to mid-rapidity nuclear modification factors of J / Ψ ( R A A forward ( J / Ψ ) / R A A mid ( J / Ψ ) ) cannot be explained by the interactions with ‘formed’ comoving mesons or by the ‘color screening mechanism’ alone. Only when incorporating interactions of the c or c ¯ quark with a pre-hadronic medium satisfactory results are obtained. A detailed comparison to the PHENIX data demonstrates that non-hadronic interactions are mandatory to describe the narrowing of the J / Ψ rapidity distribution from pp to central Au + Au collisions. The Ψ ′ to J / Ψ ratio is found to be crucial in disentangling the different charmonium absorption scenarios, especially in the RHIC energy range. Furthermore, a comparison of the transport calculations to the statistical model of Gorenstein and Gazdzicki as well as the statistical hadronization model of Andronic et al. shows differences in the energy dependence as well as centrality dependence of the J / Ψ to pion ratio which may be exploited experimentally to disentangle different concepts. We find additionally that the collective flow of charm in the HSD transport appears compatible with the data at SPS energies but substantially underestimates the data at top RHIC energies, such that the large elliptic flow v 2 of charm seen experimentally has to be attributed to early interactions of non-hadronic degrees of freedom.