Massive hybrid stars within the extended three-flavor quark-meson diquark model

Abstract

We discuss the properties of the extended three-flavor quark-meson diquark (EQMD) model as a renormalizable low-energy effective model for QCD. The effective degrees of freedom are quarks, scalar- and pseudoscalar mesons, diquarks, vector- and axial-vector mesons. We calculate the equation of state (EoS) in the mean-field approximation at $T=0$ imposing charge neutrality for electric and color charges. We match the EoS with a low-density nuclear equation of state. We discuss how the choice of parameters in the model affects the EoS and thereby the mass-radius for hybrid stars. We show that it is possible to construct hybrid stars whose masses and radii are in agreement with recent astrophysical observations and perturbative QCD (pQCD). The addition of vector and axial vector mesons to the quark-meson diquark is essential, since it makes the EoS sufficiently stiff for intermediate densities. Our results suggest that stars with a mass larger than $M\sim2M_{\odot}$ have a quark core with a central density $n_B\geq 3.9n_{\rm sat}$, where $n_{\rm sat}\approx0.165$fm$^{-3}$ is the saturation density. The speed of sound has a double-peak structure and relaxes to the conformal limit from above for large baryon chemical potentials $\mu_B$. This structure is caused by the decrease in the mass of the $s$ quark as $\mu_B$ increases.