Collective flow signals the quark–gluon plasma

Abstract

A critical discussion of the present status of the CERN experiments on charm dynamics and hadron collective flow is given. We emphasize the importance of the flow excitation function from 1 to 50 A GeV: here the hydrodynamic model has predicted the collapse of the v 1 -flow and of the v 2 -flow at ∼ 10 A GeV; at 40 A GeV it has been recently observed by the NA49 Collaboration. Since hadronic rescattering models predict much larger flow than observed at this energy we interpret this observation as evidence for a first order phase transition at high baryon density ρ B . A detailed discussion of the collective flow as a barometer for the equation of state (EoS) of hot dense matter at RHIC follows. Here, hadronic rescattering models can explain <30% of the observed elliptic flow, v 2 , for p T > 2 GeV / c . This is interpreted as evidence for the production of superdense matter at RHIC with initial pressure far above hadronic pressure, p > 1 GeV / fm 3 . We suggest that the fluctuations in the flow, v 1 and v 2 , should be measured in future since ideal hydrodynamics predicts that they are larger than 50% due to initial state fluctuations. Furthermore, the QGP coefficient of viscosity may be determined experimentally from the fluctuations observed. The connection of v 2 to jet suppression is examined. It is proven experimentally that the collective flow is not faked by minijet fragmentation. Additionally, detailed transport studies show that the away-side jet suppression can only partially (<50%) be due to hadronic rescattering. We, finally, propose upgrades and second generation experiments at RHIC which inspect the first order phase transition in the fragmentation region, i.e., at μ B ≈ 400 MeV ( y ≈ 4 –5), where the collapse of the proton flow should be seen in analogy to the 40 A GeV data. The study of Jet-Wake-riding potentials and Bow shocks—caused by jets in the QGP formed at RHIC—can give further information on the equation of state (EoS) and transport coefficients of the quark–gluon plasma (QGP).