Quarkyonic Matter and Neutron Stars

Momentum Shell in Quarkyonic Matter from Explicit Duality: A Dual Model for Cold, Dense QCD

We present a model of cold QCD matter that bridges nuclear and quark matter through the duality relation between quarks and baryons. The baryon number and energy densities are expressed as functionals of either the baryon momentum distribution, f_{B}, or the quark distribution, f_{Q}, which are subject to the constraints on fermions, 0≤f_{B,Q}≤1. The theory is ideal in the sense that the confinement of quarks into baryons is reflected in the duality relation between f_{Q} and f_{B}, while other possible interactions among quarks and baryons are all neglected. The variational problem with the duality constraints is formulated and we explicitly construct analytic solutions, finding two distinct regimes: a nuclear matter regime at low density and a quarkyonic regime at high density. In the quarkyonic regime, baryons underoccupy states at low momenta but form a momentum shell with f_{B}=1 on top of a quark Fermi sea. Such a theory describes a rapid transition from a soft nuclear equation of state to a stiff quarkyonic equation of state. At this transition, there is a rapid increase in the pressure.

An updated nuclear-physics and multi-messenger astrophysics framework for binary neutron star mergers

The multi-messenger detection of the gravitational-wave signal GW170817, the corresponding kilonova AT2017gfo and the short gamma-ray burst GRB170817A, as well as the observed afterglow has delivered a scientific breakthrough. For an accurate interpretation of all these different messengers, one requires robust theoretical models that describe the emitted gravitational-wave, the electromagnetic emission, and dense matter reliably. In addition, one needs efficient and accurate computational tools to ensure a correct cross-correlation between the models and the observational data. For this purpose, we have developed the Nuclear-physics and Multi-Messenger Astrophysics framework NMMA. The code allows incorporation of nuclear-physics constraints at low densities as well as X-ray and radio observations of isolated neutron stars. In previous works, the NMMA code has allowed us to constrain the equation of state of supranuclear dense matter, to measure the Hubble constant, and to compare dense-matter physics probed in neutron-star mergers and in heavy-ion collisions, and to classify electromagnetic observations and perform model selection. Here, we show an extension of the NMMA code as a first attempt of analyzing the gravitational-wave signal, the kilonova, and the gamma-ray burst afterglow simultaneously. Incorporating all available information, we estimate the radius of a 1.4M⊙ neutron star to be [Formula: see text] km.

Plausible presence of new state in neutron stars with masses above 0.98MTOV

We investigate the neutron star (NS) equation of state (EOS) by incorporating multi-messenger data of GW170817, PSR J0030 + 0451, PSR J0740 + 6620, and state-of-the-art theoretical progresses, including the information from chiral effective field theory (χEFT) and perturbative quantum chromodynamics (pQCD) calculation. Taking advantage of the various structures sampling by a single-layer feed-forward neural network model embedded in the Bayesian nonparametric inference, the structure of NS matter's sound speed c_s is explored in a model-agnostic way. It is found that a peak structure is common in the c_s² posterior, locating at 2.4-4.8ρ_sat (nuclear saturation density) and c_s² exceeds c²/3 at 90% credibility. The non-monotonic behavior suggests evidence of the state deviating from the hadronic matter inside the very massive NSs. Assuming the new/exotic state is featured as it is softer than typical hadronic models or even with hyperons, we find that a sizable (⩾10^-3M_⊙) exotic core, likely made of quark matter, is plausible for the NS with a gravitational mass above about 0.98M_TOV, where M_TOV represents the maximum gravitational mass of a non-rotating cold NS. The inferred M_TOV=2.18_-0.13^+0.27M_⊙ (90% credibility) is well consistent with the value of 2.17_-0.12^+0.15M_⊙ estimated independently with GW170817/GRB 170817A/AT2017gfo assuming a temporary supramassive NS remnant formed after the merger. PSR J0740 + 6620, the most massive NS detected so far, may host a sizable exotic core with a probability of ≈0.36.

Energy-Density Modeling of Strongly Interacting Matter: Atomic Nuclei and Dense Stars

We seek a simple but physically motivated model of strongly interacting matter applicable in atomic nuclei and the dense matter in the core of neutron stars. For densities below and somewhat above normal nuclear density, energy density functional (EDF) theory based on nucleonic degrees of freedom is the ideal candidate. We have explored that direction within the KIDS (Korea-IBS-Daegu-SKKU) framework, which we review in this contribution. The formalism for the KIDS-EoS and microscopic KIDS-EDF and optimization options for the EDF are described in a practical way to facilitate further applications. At densities higher than one nucleon per single-nucleon volume, i.e., roughly 0.4 fm−3, nucleonic degrees of freedom are no longer appropriate. The pseudo-conformal symmetry emergent in dense, topologically altered nuclear matter provides a simple expression for the energy per baryon in terms of the baryonic density. Besides resembling a simple EDF for dense matter, the expression has the appeal that it predicts a converged speed of sound at high densities. It can thus be implemented as a special case of the constant speed of sound (CSS) model. Here we consider a matching between representative nucleonic KIDS-EoSs and the CSS model, including the pseudo-conformal EoS, and apply the unified model to describe the mass–radius relation of neutron stars and examine the compatibility of CSS cores with heavy neutron stars. Although an abrupt transition to the pseudo-conformal regime at low densities does not favor heavy neutron stars, intermediate scenarios including a cusp in the speed of sound are not ruled out, while some appear more favorable to heavy stars than purely nucleonic matter.

Gravitational Wave Signal for Quark Matter with Realistic Phase Transition

The cores of neutron stars (NSs) near the maximum mass can realize a transitional change to quark matter (QM). Gravitational waves from binary NS mergers are expected to convey information about the equation of state (EOS) sensitive to the QM transition. Here, we present the first results of gravitational wave simulation with the realistic EOS that is consistent with ab initio approaches of χEFT and pQCD and is assumed to go through smooth crossover. We compare them to results obtained with another EOS with a first-order hadron-quark phase transition. Our results suggest that early collapse to a black hole in the post-merger stage after NS merger robustly signifies softening of the EOS associated with the QM onset in the crossover scenario. The nature of the hadron-quark phase transition can be further constrained by the condition that electromagnetic counterparts should be energized by the material left outside the remnant black hole.

Trace Anomaly as Signature of Conformality in Neutron Stars

We discuss an interpretation that a peak in the sound velocity in neutron star matter, as suggested by the observational data, signifies strongly coupled conformal matter. The normalized trace anomaly is a dimensionless measure of conformality leading to the derivative and the nonderivative contributions to the sound velocity. We find that the peak in the sound velocity is attributed to the derivative contribution from the trace anomaly that steeply approaches the conformal limit. Smooth continuity to the behavior of high-density QCD implies that the matter part of the trace anomaly may be positive definite. We discuss a possible implication of the positivity condition of the trace anomaly on the M-R relation of the neutron stars.

Merger and Postmerger of Binary Neutron Stars with a Quark-Hadron Crossover Equation of State

Fully general-relativistic binary-neutron-star (BNS) merger simulations with quark-hadron crossover (QHC) equations of state (EOS) are studied for the first time. In contrast to EOS with purely hadronic matter or with a first-order quark-hadron phase transition (1PT), in the transition region QHC EOS show a peak in sound speed and thus a stiffening. We study the effects of such stiffening in the merger and postmerger gravitational (GW) signals. Through simulations in the binary-mass range 2.5<M/M_{⊙}<2.75, characteristic differences due to different EOS appear in the frequency of the main peak of the postmerger GW spectrum (f_{2}), extracted through Bayesian inference. In particular, we found that (i) for lower-mass binaries, since the maximum baryon number density (n_{max}) after the merger stays below 3-4 times the nuclear-matter density (n_{0}), the characteristic stiffening of the QHC models in that density range results in a lower f_{2} than that computed for the underlying hadronic EOS and thus also than that for EOS with a 1PT; (ii) for higher-mass binaries, where n_{max} may exceed 4-5n_{0} depending on the EOS model, whether f_{2} in QHC models is higher or lower than that in the underlying hadronic model depends on the height of the sound-speed peak. Comparing the values of f_{2} for different EOS and BNS masses gives important clues on how to discriminate different types of quark dynamics in the high-density end of EOS and is relevant to future kilohertz GW observations with third-generation GW detectors.

The Macro-Physics of the Quark-Nova: Astrophysical Implications

A quark-nova is a hypothetical stellar evolution branch where a neutron star converts explosively into a quark star. Here, we discuss the intimate coupling between the micro-physics and macro-physics of the quark-nova and provide a prescription for how to couple the Burn-UD code to the stellar evolution code in order to simulate neutron-star-to-quark-star burning at stellar scales and estimate the resulting energy release and ejecta. Once formed, the thermal evolution of the proto-quark star follows. We found much higher peak neutrino luminosities (>1055 erg/s) and a higher energy neutrino (i.e., harder) spectrum than previous stellar evolution studies of proto-neutron stars. We derived the neutrino counts that observatories such as Super-Kamiokande-III and Halo-II should expect and suggest how these can differentiate between a supernova and a quark-nova. Due to the high peak neutrino luminosities, neutrino pair annihilation can deposit as much as 1052 ergs in kinetic energy in the matter overlaying the neutrinosphere, yielding relativistic quark-nova ejecta. We show how the quark-nova could help us understand many still enigmatic high-energy astrophysical transients, such as super-luminous supernovae, gamma-ray bursts and fast radio bursts.

Mapping Topology of Skyrmions and Fractional Quantum Hall Droplets to Nuclear EFT for Ultra-Dense Baryonic Matter

We describe the mapping at high density of topological structure of baryonic matter to a nuclear effective field theory that implements hidden symmetries emergent from strong nuclear correlations. The theory constructed is found to be consistent with no conflicts with the presently available observations in both normal nuclear matter and compact-star matter. The hidden symmetries involved are “local flavor symmetry” of the vector mesons identified to be (Seiberg-)dual to the gluons of QCD and hidden “quantum scale symmetry” with an IR fixed point with a “genuine dilaton (GD)” characterized by non-vanishing pion and dilaton decay constants. Both the skyrmion topology for Nf≥2 baryons and the fractional quantum Hall (FQH) droplet topology for Nf=1 baryons are unified in the “homogeneous/hidden” Wess–Zumino term in the hidden local symmetry (HLS) Lagrangian. The possible indispensable role of the FQH droplets in going beyond the density regime of compact stars approaching scale-chiral restoration is explored by moving toward the limit where both the dilaton and the pion go massless.

Effects of a Finite Volume in the Phase Structure of QCD

Working in the SU(2) flavor version of the NJL model, we study the effect of taking a finite system volume on a strongly interacting system of quarks, and, in particular, the location of the chiral phase transition and the CEP. We consider two shapes for the volume, spherical and cubic regions with different sizes and different boundary conditions. To analyze the QCD phase diagram, we use a novel criterion to study the crossover zone. A comparison between the results obtained from the two different shapes and several boundary conditions is carried out. We use the method of Multiple Reflection Expansion to determine the density of states and three kinds of boundary conditions over the cubic shape. These boundary conditions are: periodic, anti-periodic and stationary boundary conditions on the quark fields.

Finding Structure in the Speed of Sound of Supranuclear Matter from Binary Love Relations

Analyses that connect observations of neutron stars with nuclear-matter properties can rely on equation-of-state insensitive relations. We show that the slope of the binary Love relations (between the tidal deformabilities of binary neutron stars) encodes the baryon density at which the speed of sound rapidly changes. Twin stars lead to relations that present a signature "hill," "drop," and "swoosh" due to the second (mass-radius) stable branch, requiring a new description of the binary Love relations. Together, these features can reveal new properties and phases of nuclear matter.

Conformality and percolation threshold in neutron stars

Speed of sound is given attention in multi-messenger astronomy as it encodes information of the dense matter equation of state. Recently the trace anomaly was proposed as a more informative quantity. In this work, we statistically determine the speed of sound and trace anomaly and show that they are driven to their conformal values at the centers of maximally massive neutron stars. We show that the local peak in the speed of sound can be associated deconfinement along with percolation conditions in QCD matter.

Equation of state and strangeness in neutron stars - role of hyperon-nuclear three-body forces -

A brief survey is presented of our present understanding of the equation-of-state of cold, dense matter and the speed of sound in the interior of neutron stars, based on the constraints inferred from observational data. The second part focuses on strangeness in baryonic matter and the role of hyperonnuclear two- and three-body forces, with reference to the "hyperon puzzle" in neutron stars and possible scenarios for its solution.

Different Faces of Confinement

In this review, we provide a short outlook of some of the current most popular pictures and promising approaches to non-perturbative physics and confinement in gauge theories. A qualitative and by no means exhaustive discussion presented here covers such key topics as the phases of QCD matter, the order parameters for confinement, the central vortex and monopole pictures of the QCD vacuum structure, fundamental properties of the string tension, confinement realisations in gauge-Higgs and Yang–Mills theories, magnetic order/disorder phase transition, among others.

Speed of Sound and Baryon Cumulants in Heavy-Ion Collisions

We present a method that may allow an estimate of the value of the speed of sound as well as its logarithmic derivative with respect to the baryon number density in matter created in heavy-ion collisions. To this end, we use well-known observables: cumulants of the baryon number distribution. In analyses aimed at uncovering the phase diagram of strongly interacting matter, cumulants gather considerable attention as their qualitative behavior along the explored range of collision energies is expected to aid in detecting the QCD critical point. We show that the cumulants may also reveal the behavior of the speed of sound in the temperature and baryon chemical potential plane. We demonstrate the applicability of such estimates within two models of nuclear matter and explore what might be understood from known experimental data.

Neutron Star Equation of State in Light of GW190814

The observation of gravitational waves from an asymmetric binary opens the possibility for heavy neutron stars, but these pose challenges to models of the neutron star equation of state. We construct heavy neutron stars by introducing nontrivial structure in the speed of sound sourced by deconfined QCD matter, which cannot be well recovered by spectral representations. Their moment of inertia, Love number, and quadrupole moment are very small, so a tenfold increase in sensitivity may be needed to test this possibility with gravitational waves, which is feasible with third generation detectors.