Review of QGP Signatures — Ideas Versus Observables

Abstract

We investigate hadron production and transverse hadron spectra in nucleus-nucleus collisions from 2 A·GeV to 21.3 A·TeV within two independent transport approaches (UrQMD and HSD) based on quark, diquark, string and hadronic degrees of freedom. The enhancement of pion production in central Au+Au (Pb+Pb) collisions relative to scaled pp collisions (the 'kink') is described well by both approaches without involving a phase transition. However, the maximum in the K + /π + ratio at 20 to 30 A·GeV (the 'horn') is missed by ∼ 40%. Also, at energies above ∼ 5 A·GeV, the measured K ± mT -spectra have a larger inverse slope than expected from the models. Thus the pressure generated by hadronic interactions in the transport models at high energies is too low. This finding suggests that the additional pressure -as expected from lattice QCD at finite quark chemical potential and temperature -might be generated by strong interactions in the early pre-hadronic/partonic phase of central heavy-ion collisions. Finally, we discuss the emergence of density perturbations in a first-order phase transition and why they might affect relative hadron multiplicities, collective flow, and hadron mean-free paths at decoupling. A minimum in the collective flow v2 excitation function was discovered experimentally at 40 A·GeV -such a behavior has been predicted long ago as signature for a first order phase transition.

Dynamics of strange, charm and high momentum hadrons in relativistic nucleus–nucleus collisions

“…We mention that the excitation functions of the strange hadrons from HSD and UrQMD match rather well with the data available, however, the sharp maximum in the K + /π + ratio at ∼ 20 to 30 A•GeV in central Au+Au (Pb+Pb) collisions is missed [23]. This also holds for the slope of the K ± transverse mass spectra above ∼ 5 A•GeV in central Au+Au (Pb+Pb) collisions [24,25]. The D-mesons with a c are produced more frequently at low energies due to the associated production with Λ c , Σ c , Σ * C similar to the kaon case, where kaon+hyperon production is more frequent than K + K production.…”

“…The exception are the pion rapidity spectra at the highest AGS energy and lower SPS energies, which are overestimated by both models. As a consequence the HSD and UrQMD transport approaches underestimate the experimental maximum of the K + /π + ratio at ∼ 20-30 A·GeV [23,25]. Furthermore, the transport calculations show that the charmonium recreation by D +D → J/Ψ + meson reactions is comparable to the dissociation by 'comoving' mesons at RHIC energies contrary to SPS energies.…”