Thermodynamically anomalous regions and possible new signals of mixed-phase formation
Using an advanced version of the hadron resonance gas model we have found indications for irregularities in data for hadrons produced in relativistic heavy-ion collisions. These include an abrupt change of the effective number of degrees of freedom, a change of the slope of the ratio of lambda hyperons to protons at laboratory energies 8.6-11.6A GeV, as well as a plateau in the collision-energy dependence of the thermal pion number per baryon at laboratory energies 6.9-11.6A GeV. We also find hints for the existence of plateaus in the collision-energy dependence of the entropy per baryon and the total pion number per baryon, which are correlated to the one of the thermal pion number per baryon at the same collision-energy range. Also, we observe a sharp peak in the dimensionless trace anomaly at a laboratory energy of 11.6A GeV. On the basis of the generalized shock-adiabat model we demonstrate that these observations give evidence for the anomalous thermodynamic properties of the mixed phase at its boundary to the quark-gluon plasma. We argue that the trace-anomaly peak and the local minimum of the generalized specific volume observed at a laboratory energy of 11.6A GeV provide a signal for the formation of a mixed phase between the quark-gluon plasma and the hadron phase. This naturally explains the change of slope in the energy dependence of the yield of lambda hyperons per proton at a laboratory energy of 8.6A GeV.