Negative elliptic flow of J/ψ’s: A qualitative signature for charm collectivity at RHIC

Abstract

We discuss one of the most prominent features of the very recent preliminary elliptic flow data of J/ψ-mesons from the PHENIX Collaboration (PHENIX Collaboration (C. Silvestre), arXiv:0806.0475 [nucl-ex]). Even within the rather large error bars of the measured data a negative elliptic flow parameter (v2) for J/ψ in the range of p T = 0.5-2.5 GeV/c is visible. We argue that this negative elliptic flow at intermediate pT is a clear and qualitative signature for the collectivity of charm quarks produced in nucleus-nucleus reactions at RHIC. Within a parton recombination approach we show that a negative elliptic flow puts a lower limit on the collective transverse velocity of heavy quarks. The numerical value of the transverse flow velocity $ \beta_{T}^{}$ for charm quarks that is necessary to reproduce the data is $ \beta_{T}^{}$ (charm) ∼ 0.55-0.6c and therefore compatible with the flow of light quarks.