Quark-gluon plasma formation in heavy-ion collisions
Specific indications for the formation of a quark-gluon plasma in relativistic heavy-ion collisions are investigated theoretically from AGS to SPS energies (from several GeV to 17.3 GeV per particle pair).
In a qcd-based model we study possible irregularities in the net-baryon distributions as a function of energy. It has recently been claimed in the literature that the available stopping data from central Au + Au at AGS, and Pb + Pb at SPS exhibit a non-monotonous behaviour as function of energy in the region sqrt(s_NN) = 5 - 8 GeV. This would then indicate an intermediary transition (cross-over) of the hadronic system to the quark-gluon phase.
Indications for a phase transition in this region had previously been found through a local maximum in the K+/pi+ ratio. This effect is, however, still controversial: The local maximum is less pronounced now as compared to 2002 when the original data appeared, and it can be understood qualitatively using straightforward thermal models.
For net baryons, very specific equations of state are required to produce an irregular behaviour using hydrodynamic calculations. An independent microscopic investigation based on a partonic approach is clearly required.