From hadrons to quarks in neutron stars: a review (1707.04966v3)

Abstract: We review the equation of state of matter in neutron stars from the solid crust through the liquid nuclear matter interior to the quark regime at higher densities. We focus in detail on the question of how quark matter appears in neutron stars, and how it affects the equation of state. After discussing the crust and liquid nuclear matter in the core we briefly review aspects of microscopic quark physics relevant to neutron stars, and quark models of dense matter based on the Nambu--Jona-Lasinio framework, in which gluonic processes are replaced by effective quark interactions. We turn then to describing equations of state useful for interpretation of both electromagnetic and gravitational observations, reviewing the emerging picture of hadron-quark continuity in which hadronic matter turns relatively smoothly, with at most only a weak first order transition, into quark matter with increasing density. We review construction of unified equations of state that interpolate between the reasonably well understood nuclear matter regime at low densities and the quark matter regime at higher densities. The utility of such interpolations is driven by the present inability to calculate the dense matter equation of state in QCD from first principles. As we review, the parameters of effective quark models -- which have direct relevance to the more general structure of the QCD phase diagram of dense and hot matter -- are constrained by neutron star mass and radii measurements, in particular favoring large repulsive density-density and attractive diquark pairing interactions. We describe the structure of neutron stars constructed from the unified equations of states with crossover. Lastly we present the current equations of state -- called "QHC18" for quark-hadron crossover -- in a parametrized form practical for neutron star modeling.

• The paper presents integrated theoretical frameworks using the NJL model to connect low-density hadronic and high-density quark regimes in neutron stars.
• It demonstrates that incorporating repulsive vector interactions and diquark pairing produces EOS models capable of supporting 2-solar-mass stars.
• Observational constraints from NICER and gravitational wave detections are used to refine mass-radius relations and guide future research.

A Review of "From Hadrons to Quarks in Neutron Stars"

The paper by Gordon Baym et al., titled "From Hadrons to Quarks in Neutron Stars: A Review," provides a comprehensive examination of the transition of matter within neutron stars from hadronic to quark states. This review focuses on various theoretical frameworks and observational data that contribute to our understanding of the equation of state (EOS) within neutron stars, which critically influences their mass and radius.

Key Concepts and Frameworks

The paper addresses the fundamental challenge of connecting the low-density nuclear matter equations of state, prevalent in neutron star crusts and outer cores, with the high-density regimes expected in the inner core where quark matter might dominate. The hadronic equation of state is well-characterized at densities up to approximately twice the saturation density, $n_0$, using nucleon-nucleon interactions derived from scattering data and the properties of light nuclei. However, the regime beyond $2n_0$ necessitates modeling approaches due to unknown many-body forces and the potential emergence of non-nucleonic degrees of freedom, such as hyperons and mesons.

To model the quark matter phase, the authors utilize the Nambu-Jona-Lasinio (NJL) model, which effectively captures the chiral properties of QCD at high densities while circumventing the computational challenges of lattice QCD at finite baryon density. The NJL model encapsulates the dynamics of quark-antiquark and diquark pairing, which are essential components of the chiral phase transition and color superconductivity, respectively.

Strong Numerical Results and Model Implications

One of the critical numerical challenges addressed in the paper is constructing unified equations of state that are compatible with observations of neutron stars, particularly those with masses around 2 solar masses. The authors highlight that achieving these masses requires EOS with significant stiffness, often realized by including repulsive vector interactions and diquark pairing in the quark matter models.

This review presents several parameter sets within the NJL model that yield equations of state capable of supporting observed neutron star properties, including mass-radius relations. The consideration of hadron-quark continuity, rather than a distinct first-order phase transition, offers a more flexible framework that can accommodate the observational and theoretical constraints without resorting to extreme assumptions about matter at supra-nuclear densities.

Observational Constraints and Future Directions

Observations of neutron stars in binary systems through electromagnetic signals and gravitational waves, such as those from the NICER mission and events like GW170817, provide essential empirical data for constraining the EOS. These observations are cross-examined with theoretical predictions to refine our understanding of neutron star interiors.

The authors also speculate on the future developments in this field, emphasizing the need to account for finite temperature effects in neutron star mergers and the role of potential exotic phases such as quarkyonic matter. The nuances of detailed structural transitions in neutron star interiors remain an area rich for exploration, with implications for the EOS continuity approach.

In conclusion, the paper contributes significantly to the discourse on neutron star matter by bridging traditional nuclear physics and modern QCD-based models, while also emphasizing the value of observational data in refining theoretical models. This integrative approach provides a robust platform for future advancements in understanding the dense matter equation of state within neutron stars.