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

TL;DR

This study uses the HSD transport model to analyze charmonium suppression in heavy-ion collisions at RHIC, revealing that non-hadronic interactions of charm quarks are essential to explain experimental observations, especially the rapidity narrowing and elliptic flow.

Contribution

It demonstrates the necessity of including non-hadronic charm interactions in transport models to accurately describe RHIC charmonium data, advancing understanding of charm dynamics in quark-gluon plasma.

Findings

Non-hadronic interactions are crucial for explaining J/Psi rapidity narrowing.

The model reproduces the elliptic flow of charm at SPS but underestimates at RHIC.

Differences between transport and statistical models suggest experimental ways to distinguish charmonium suppression mechanisms.

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/Psi (R_AA (forward) / R_AA (mid)) 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-bar 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/Psi rapidity distribution from p+p to central Au+Au collisions. The Psi' to J/Psi 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/Psi 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.