Equation of state and sound velocity of a hadronic gas with a hard-core interaction

Thermodynamic properties of hot and dense hadronic systems with a hard-sphere interaction are calculated in the Boltzmann approximation. Two parametrizations of pressure as a function of density are considered: the first one, used in the excluded-volume model and the second one, suggested earlier by Carnahan and Starling. The results are given for one-component systems containing only nucleons or pions, as well as for chemically equilibrated mixtures of different hadronic species. It is shown that the Carnahan-Starling approach can be used in a much broader range of hadronic densities as compared to the excluded-volume model. In this case the superluminal sound velocities appear only at very high densities, in the region where the deconfinement effects should be already important.