Properties of Dense Strange Hadronic Matter with Quark Degrees of Freedom

The properties of strange hadronic matter are studied in the context of the modified quark-meson coupling model using two substantially different sets of hyperon-hyperon ($YY$) interactions. The first set is based on the Nijmegen hard core potential model D with slightly attractive $YY$ interactions. The second potential set is based on the recent SU(3) extension of the Nijmegen soft-core potential NSC97 with strongly attractive $YY$ interactions which may allow for deeply bound hypernuclear matter. The results show that, for the first potential set, the $\Sigma$ hyperon does not appear at all in the bulk at any baryon density and for all strangeness fractions. The binding energy curves of the resulting $N\Lambda\Xi$ system vary smoothly with density and the system is stable (or metastable if we include the weak force). However, the situation is drastically changed when using the second set where the $\Sigma$ hyperons appear in the system at large baryon densities above a critical strangeness fraction. We find strange hadronic matter undergoes a first order phase transition from a $N\Lambda\Xi$ system to a $N\Sigma\Xi$ for strangeness fractions $f_S>1.2$ and baryonic densities exceeding twice ordinary nuclear matter density. Furthermore, it is found that the system built of $N\Sigma\Xi$ is deeply bound. This phase transition affects significantly the equation of state which becomes much softer and a substantial drop in energy density and pressure are detected as the phase transition takes place.