Isoscalar-vector interaction and hybrid quark core in massive neutron stars

The hadron-quark phase transition in the core of massive neutron stars is studied with a newly constructed two-phase model. For nuclear matter, a nonlinear Walecka type model with general nucleon-meson and meson-meson couplings, recently calibrated by Steiner, Hemper and Fischer, is taken. For quark matter, a modified Polyakov-Nambu--Jona-Lasinio (mPNJL) model, which gives consistent results with lattice QCD data, is used. Most importantly, we introduce an isoscalar-vector interaction in the description of quark matter, and we study its influence on the hadron-quark phase transition in the interior of massive neutron stars. With the constraints of neutron star observations, our calculation shows that the isoscalar-vector interaction between quarks is indispensable if massive hybrids star exist in the universe, and its strength determines the onset density of quark matter, as well as the mass-radius relations of hybrid stars. Furthermore, as a connection with heavy-ion-collision experiments we give some discussions about the strength of isoscalar-vector interaction and its effect on the signals of hadron-quark phase transition in heavy-ion collisions, in the energy range of the NICA at JINR-Dubna and FAIR at GSI-Darmstadt facilities.

Phase diagrams in the Hadron-PNJL model

The two-Equation of State (Two-EoS) model is used to describe the hadron-quark phase transition in dense-hot matter formed in heavy-ion collisions. The non-linear Walecka model is used to describe the hadronic phase. For the quark phase, the Nambu--Jona-Lasinio model coupled to Polyakov-Loop fields (PNJL) is used to include both the chiral and (de)confinement dynamics. The phase diagrams are derived from the Gibbs conditions and compared with the results obtained in the Hadron-NJL model without confinement. As in the Hadron-NJL case a first order transition is observed, but with a Critical-End-Point at much higher temperature, consequence of the confinement mechanism that reduces the degrees of freedom of the quark matter in proximity of the phase transition. Particular attention is devoted to the phase transition in isospin asymmetric matter. Interesting isospin effects are found at high baryon density and reduced temperatures, in fact common also to other quark models, like MIT-Bag and NJL model. Some possible observation signals are suggested to probe in Heavy-Ion Collision (HIC) experiments at intermediate energies.

Phase transitions of hadronic to quark matter at finite T and mu_B

The phase transition of hadronic to quark matter and the boundaries of the mixed hadron-quark coexistence phase are studied within the two Equation of State (EoS) model. The relativistic effective mean field approach with constant and density dependent meson-nucleon couplings is used to describe hadronic matter, and the MIT Bag model is adopted to describe quark matter. The boundaries of the mixed phase for different Bag constants are obtained solving the Gibbs equations. We notice that the dependence on the Bag parameter of the critical temperatures (at zero chemical potential) can be well reproduced by a fermion ultrarelativistic quark gas model, without contribution from the hadron part. At variance the critical chemical potentials (at zero temperature) are very sensitive to the EoS of the hadron sector. Hence the study of the hadronic EoS is much more relevant for the determination of the transition to the quark-gluon-plasma at finite baryon density and low-T. Moreover in the low temperature and finite chemical potential region no solutions of the Gibbs conditions are existing for small Bag constant values, B < (135 MeV)^4. Isospin effects in asymmetric matter appear relevant in the high chemical potential regions at lower temperatures, of interest for the inner core properties of neutron stars and for heavy ion collisions at intermediate energies.