The order, shape and critical point for the quark–gluon plasma phase transition

The order, shape and critical point for the phase transition between the hadronic matter and quark–gluon plasma are considered in a thermodynamical consistent approach. The hadronic phase is taken as van der Waals gas of all the known hadronic mass spectrum particles mH⩽2.0 GeV as well as Hagedorn bubbles which correspond hadronic states with mass spectrum mH>2.0 GeV. The density of states for Hagedorn bubbles is derived by calculating the microcanonical ensemble for a bag of quarks and gluons with specific internal color-flavor symmetry. The mixed-grand and microcanonical ensembles are derived for massless and massive flavors. We find Hagedorn bubbles are strongly suppressed in the dilute hadronic matter and they appear just below the line of the phase transition. The order of the phase transition depends on Hagedorn bubble's internal color-flavor structure and the volume fluctuation as well. On the other hand, the highly compressed hadronic matter undergoes a smooth phase transition from the gas of known mass spectrum hadrons to another one dominated by Hagedorn bubbles with specific internal color-flavor structure before the phase transition to quark–gluon plasma takes place at last. The phase transition is found a first order for the intermediate and large chemical potentials. The existence of the tri-critical point depends on the modification of the bubble's internal structure specified by a phenomenological parameter γ∝μB in the medium.