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--11.6 AGeV, as well as highly correlated plateaus in the collision-energy dependence of the entropy per baryon, total pion number per baryon, and thermal pion number per baryon at laboratory energies 6.9-11.6 AGeV. Also, we observe a sharp peak in the dimensionless trace anomaly at a laboratory energy of 11.6 AGeV. 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 11.6 AGeV 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 8.6 GeV.

Thermodynamically Anomalous Regions As A Mixed Phase Signal

Using the most advanced model of the hadron resonance gas we reveal, at chemical freeze-out, remarkable irregularities such as an abrupt change of the effective number of degrees of freedom and plateaus in the collision-energy dependence of the entropy per baryon, total pion number per baryon, and thermal pion number per baryon at laboratory energies 6.9-11.6 AGeV. On the basis of the generalized shock adiabat model we show that these plateaus give evidence for the thermodynamic anomalous properties of the mixed phase at its boundary to the quark-gluon plasma (QGP). A new signal for QGP formation is suggested and justified.

Separate chemical freeze-out of strange particles with conservation laws

The Hadron Resonance Gas Model with two chemical freeze-outs, connected by conservation laws is considered. We are arguing that the chemical freeze-out of strange hadrons should occur earlier than the chemical freeze-out of non-strange hadrons. The hadron multiplicities measured in the heavy ion collisions for the center of mass energy range 2.7 - 200 GeV are described well by such a model. Based on a success of such an approach, a radical way to improve the Hadron Resonance Gas Model performance is suggested. Thus, we suggest to identify the hadronic reactions that freeze-out noticeably earlier or later that most of the others reactions (for different collision energies they may be different) and to consider a separate freeze-out for them.

Bimodality Phenomenon in Finite and Infinite Systems Within an Exactly Solvable Statistical Model

We present a few explicit counterexamples to the widely spread belief about an exclusive role of the bimodal nuclear fragment size distributions as the first order phase transition signal. In thermodynamic limit the bimodality may appear at the supercritical temperatures due to the negative values of the surface tension coefficient. Such a result is found within a novel exactly solvable formulation of the simplified statistical multifragmentation model based on the virial expansion for a system of the nuclear fragments of all sizes. The developed statistical model corresponds to the compressible nuclear liquid with the tricritical endpoint located at one third of the normal nuclear density. Its exact solution for finite volumes demonstrates the bimodal fragment size distribution right inside the finite volume analog of a gaseous phase. These counterexamples clearly demonstrate the pitfalls of Hill approach to phase transitions in finite systems.