From hadrons to quarks in neutron stars: a review

Abstract

In recent years our understanding of neutron stars has advanced remarkably, thanks to research converging from many directions. The importance of understanding neutron star behavior and structure has been underlined by the recent direct detection of gravitational radiation from merging neutron stars. The clean identification of several heavy neutron stars, of order two solar masses, challenges our current understanding of how dense matter can be sufficiently stiff to support such a mass against gravitational collapse. Programs underway to determine simultaneously the mass and radius of neutron stars will continue to constrain and inform theories of neutron star interiors. At the same time, an emerging understanding in quantum chromodynamics (QCD) of how nuclear matter can evolve into deconfined quark matter at high baryon densities is leading to advances in understanding the equation of state of the matter under the extreme conditions in neutron star interiors. We review here 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.

Quantum chromodynamics (QCD)-like phase diagram with Efimov trimers and Cooper pairs in resonantly interacting SU(3) Fermi gases

“…As another example, strongly interacting twocomponent Fermi gases can be used as quantum simulators of dilute neutron matter [12][13][14]. Indeed, the observed thermodynamic quantities of homogeneous Fermi gases near the unitarity limit [15][16][17][18] quantitatively reproduce the equation of state of neutron matter obtained by numerical simulations [19] which is crucial for understanding the interior of a neutron star [20,21].…”

Polyakov loop fluctuations in the presence of external fields

“…It would also be interesting to investigate how the susceptibility observables behave at large baryon densities [55] and react to other external fields, e.g., a magnetic field [56,57]. Since the curvatures dictate how reluctant the system is to deviate from the equilibrium position in the presence of external disturbances, we expect these susceptibilities would be crucial to successfully describing the system.…”

Constraining the Dense Matter Equation of State with Joint Analysis of NICER and LIGO/Virgo Measurements

“…Astronomical multimessenger observations yield statistical measurements of neutron star (NS) properties such as gravitational mass, radius, and tidal deformability, providing a way to study matter under extreme conditions. Theoretical predictions for the phases of matter in NS cores span a wide range, from neutron-rich nucleonic matter to hy-peron or deconfined quark formation or the emergence of a Bose-Einstein condensate or a color superconducting phase (see Hebeler et al 2015;Lattimer & Prakash 2016;Oertel et al 2017;Baym et al 2018, for recent reviews on this topic). In practice, our uncertainty about dense matter is usually expressed in terms of a space of viable equation of state (EOS) models (see, e.g., Abbott et al 2018;Raaijmakers et al 2019, and references therein).…”