Limiting Temperature, Phase Transition(s), Crossover, ...

I present a short review of two physical models devoted to the equation of state at high energy densities: The Hagedorn concept of limiting temperature \(T=T_H\) and the statistical bag model of phase transitions at \(T=T_c\). The statistical bag model admits the different orders of phase transitions between hadrons (small bags) and quark gluon plasma (infinitely large bags). The crossover transition between hadron resonance gas and cluster quark gluon plasma at \(T=T_\mathrm{cr}\) is also possible within the statistical bag model. For all these different phenomena rather similar values of the temperatures, \(T_H\sim T_c \sim T_\mathrm{cr} =\) 150–160 MeV, have been assumed.