Behavior of matter at superhigh density

Abstract

The behavior of matter at densities higher than ${10}^{16}$ g/${\mathrm{cm}}^{3}$ is investigated. It is found that little information can be gained from the data on neutron stars, whose properties are essentially determined by the behavior of $p={{c}_{s}}^{2}\ensuremath{\epsilon}$ in the region ${10}^{14}\ensuremath{\le}\ensuremath{\rho}=\frac{\ensuremath{\epsilon}}{{c}^{2}}\ensuremath{\le}5\ifmmode\times\else\texttimes\fi{}{10}^{15}$ g/${\mathrm{cm}}^{3}$. On the contrary, the study of the properties of the $p\ensuremath{-}p$ collision in the GeV region seems to provide more stringent conditions. The Landau model is used to analyze data concerning the average multiplicity, the constancy of transverse momenta, and the behavior of the energy per particle. Using a newly derived relation for the hadronic viscosity $\ensuremath{\eta}\ensuremath{\sim}{T}^{\frac{1}{{{c}_{s}}^{2}}}$, we show that the Landau hydrodynamic model reproduces the same results as the multiperipheral model without the need of any extra hypothesis, if the value ${{c}_{s}}^{2}=1$ is adopted. Once this conclusion is reached, several pion-pion Lagrangians are analyzed and their predictions for ${{c}_{s}}^{2}$ investigated. The fundamental assumption of the Landau hydrodynamical model, i.e., the laminar nature of the flow, is also investigated. The result is found that the pion fluid can become turbulent in the later stages of evolution, thus giving rise to eddies or clumps of matter tentatively identified with the clusters found in the $p\ensuremath{-}p$ scattering.