Quark-hybrid matter in the cores of massive neutron stars

Abstract

Using a nonlocal extension of the SU(3) Nambu-Jona Lasinio model, which reproduces several of the key features of Quantum Chromodynamics, we show that mixed phases of deconfined quarks and confined hadrons (quark-hybrid matter) may exist in the cores of neutron stars as massive as around 2.1 M⊙. The radii of these objects are found to be in the canonical range of ∼ 12 − 13 km. According to our study, the transition to pure quark matter does not occur in stable neutron stars, but is shifted to neutron stars which are unstable against radial oscillations. The implications of our study for the recently discovered, massive neutron star PSR J1614-2230, whose gravitational mass is 1.97 ± 0.04 M⊙, are that this neutron star may contain an extended region of quark-hybrid matter at it center, but no pure quark matter. Introduction -Quantum Chromodynamics (QCD) has the properties of asymptotic freedom and confinement. The former implies that in the high-momentum transfer regime, the quarks behave essentially as free particles, i.e., the interaction between two quarks due to gluon exchange is very weak. This regime can therefore be treated using perturbation theory, where the quark-gluon coupling constant serves as an expansion parameter. For this momentum range, the dispersion processes can be calculated very accurately. By contrast, at low-momentum transfers ( 1 GeV) QCD becomes highly nonlinear, which prevents the use of perturbative methods. One of the renowned effective models that serves as a suitable approximation to QCD in the low-energy regime is the quark version of the Nambu JonaLasinio (NJL) model [1][2][3]. In this model chiral symmetry constraints are taken into account via effective interactions between quarks, through local four-point vertex interactions. The drawbacks of using local interactions are that the model must be regularized to avoid divergences in the loop integrals, and that the model is non-confining. The absence of confinement is essentially related to the fact that the dynamically generated constituent quark masses are momentum independent. Since the 1990's, there have been investigations proposing nonlocal interactions to solve these problems [4]. One interesting suggestion arises from the relationship between the NJL model and the model of one-gluon exchange where an effective gluon propagator is used to generate effective interactions between quarks. This provides a natural way to introduce a nonlocality in the quark-quark interaction, which can be characterized by a model-dependent form factor, g(p) [5].In this paper we analyze the global structure and composition of massive neutron stars in the framework of an extended version of the nonlocal SU(3) NJL model. Of particular interest is the question as to whether or not massive neutron stars, such as the recently discovered pulsar PSR J1614-2230 whose gravitational mass was found to be 1.97 ± 0.04 M ⊙ [6], may contain stellar cores made of deconfined quark matter [7][8][9]16]. Tendentially, one could argue that quark deconfinement may not...

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“…We have found that hybrid star configurations reaching or even exceeding the 2M ⊙ mass constraint have cores comprised of a mixed phase of quarks and hadrons. This conclusion is similar to the one drawn when promoting the local charge neutrality condition to a global one via Gibbs construction [20].…”

Dense baryonic matter: Constraints from recent neutron star observations

“…[79] using a model that combines a relativistic mean field (RMF) equation of state for the hadronic sector with a non-local PNJL model for quark matter. While the non-local effective interaction between quarks does not make much of a difference compared to the local couplings used in the present work, it should be noted that RMF-based equations of state usually fail to satisfy at least one of the EoS criteria, namely the requirement of consistency with the most advanced many-body calculations of neutron matter [12,16].…”

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

“…The hadron-quark phase transition is one of the most concerned topics, and so far it is still controversial whether quarks can appear in cold neutron star [34][35][36][37][38]. It is also an important topic in heavy-ion collisions, and related experiments at medium and high densities will be performed in the near future on the updated facilities of NICA at JINR-Dubna and FAIR at GSI-Darmstadt.…”