Collective Flow signals the Quark Gluon Plasma

TL;DR

This paper investigates collective flow across SPS to RHIC energies as a probe of the QCD phase diagram and the quark–gluon plasma (QGP) equation of state. It combines lattice QCD predictions with transport and hydrodynamic analyses to interpret flow observables ($v_1$, $v_2$) and jet quenching, highlighting a first-order phase transition at high baryon density ($μ_B$) near $μ_B^c ≈ 400$ MeV and strong early-QGP pressure at RHIC. It argues that hadronic rescattering accounts for only a portion of the observed flow and jet quenching, implying substantial partonic dynamics and rapid thermalization, with jet wake and bow-shock phenomena offering further EoS constraints. The work advocates upgrades and second-generation experiments at RHIC and FAIR to explore the fragmentation region ($y ≈ 4-5$, $μ_B ≈ 400$ MeV) and to refine measurements of the QGP's transport properties.

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$\cdot$GeV: here the hydrodynamic model has predicted the collapse of the $v_1$-flow and of the $v_2$-flow at $\sim 10$ A$\cdot$GeV; at 40 A$\cdot$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 potential 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. 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 $μ_B \approx 400$ MeV ($y \approx 4-5$), where the collapse of the proton flow should be seen in analogy to the 40 A$\cdot$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).

Figures (22)

• Figure 1: The new phase diagram with the critical end point at $\mu_B \approx 400 \hbox{MeV}, T \approx 160 \hbox{MeV}$ as predicted by Lattice QCD. In addition, the time evolution in the $T-\mu$-plane of a central cell in UrQMD calculations Bravina is depicted for different bombarding energies. Note, that the calculations indicate that bombarding energies $E_{LAB} \stackrel{<}{\sim} 40$ A$\cdot$GeV are needed to probe a first order phase transition. At RHIC (see insert at the $\mu_B$ scale) this point is accessible in the fragmentation region only (taken from Bratkov04).
• Figure 2: The $J/\Psi$ suppression as a function of the transverse energy $E_T$ in $Pb~+~Pb$ collisions at 160 A$\cdot$GeV. The solid line shows the HSD result within the comover absorption scenario Brat03. The different symbols stand for the NA50 data NA50_QM02 from the year 2000 (ana-lysis A,B,C) while the dashed histogram is the UrQMD result Spieles.
• Figure 3: The calculated $J/\Psi$ multiplicity per binary collision -- multiplied by the branching to dileptons -- as a function of the number of participating nucleons, $N_{part}$, in comparison to the preliminary data from the PHENIX collaboration PHENIX2 for $Au+Au$ and $pp$ reactions (taken from Ref. Brat03).
• Figure 4: Left: Sideward flow $p_x=v_1 \cdot p_T$ of K, $\Lambda$ and p's at 6 A$\cdot$GeV as measured by E895 in semi-central collisions at the AGS. Right: The same directed flow data for $p$ and $\Lambda$ compared to UrQMD calculations for $b < 7$ fm Soff99 .
• Figure 5: Directed flow from ideal hydrodynamics with a QGP phase (open symbols) and from the Quark Gluon String Model without QGP phase (full symbols) Csernai99 .