Symmetries and Ultra Dense Matter of Compact Stars

A special issue of Symmetry (ISSN 2073-8994). This special issue belongs to the section "Physics".

Deadline for manuscript submissions: closed (31 March 2023) | Viewed by 14467

Special Issue Editor

Institut de Physique Theorique, Universite Paris-Saclay, CNRS, CEA, 91191 Gif-sur-Yvette, France
Interests: theoretical nuclear; hadron physics
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Ultra‐dense matter, expected to be revealed in massive compact stars in ongoing astrophysical measurements, in tandem with the terrestrial laboratories to come, is becoming an exciting area of research in physics. It encompasses the wide‐ranging areas of physics, gravity, nuclear, particle and condensed matters. With the advent of gravity‐wave detections, ongoing measurements from both space and terrestrial laboratories are poised to give precision results in various relations involving the equation of state (EoS) of the ultra‐dense matter necessary for mapping the structure of the core of the stars. How to reliably access the massive stars, the densest matter observable in the universe, presently remains a wide‐open issue. This is because even if one takes for granted that gravity is extremely well understood—which, of course, is still far from an unquestionably acceptable premise—the most crucial ingredient to describe the equation of state of matter, i.e., the strong interaction, is presently too poorly understood in the range of the matter density involved to enable one to “see” with confidence what actually takes place in ultra‐dense matter. QCD, the fundamental theory of strong interactions, is, to date, unable to access simultaneously both the low‐density regime—nuclear matter—and the non‐asymptotic higher‐density regime—the star core—in a consistent and unified way, neither are there any models in the literature that can be trusted. Presently available are mostly patchwork models, controlled within only a limited range of validity. This present state of matter therefore renders unanswerable the question: What is the fundamental physics involved in the densest “observable” object being stable against gravitational collapse?

In this series, we will have reviews and/or articles on new developments in the endeavor to address this issue by a few invited active researchers in the field coming from different strategies. One should of course ask to what extent the present knowledge on gravity can be taken as an established premise. We will then put focus on a variety of different approaches to the structure of the matter involved, exploiting both phenomenological and field theoretical tools, as well as the variety of efforts to construct—as model‐independently as feaible—the EoS directly from the wealth of observables that will become available from the low‐density to high‐density regimes involved with an emphasis on the future directions to be taken. The key issues will be on the symmetries considered—or assumed—to be consistent with QCD that are either intrinsic or emergent from strong correlations. There will be both bottom‐up and top‐down approaches. The former resorts to effective field theories anchored in chiral symmetry—with or without scale symmetry and hidden local symmetry—of QCD manifested at a low‐energy (density) scale and extrapolated to a high‐energy (density) scale—with or without “hadron‐quark continuity”—by implementing what are taken as the intrinsic QCD degrees of freedom. One of the top‐down approaches starts from where the perturbative QCD is applicable down to where the matching to a chiral EFT at low density becomes indispensable. The other top‐down approach is holographic dual QCD, brought from some non‐asymptotic scale, say, “orange scale,” down to the scale where a chiral EFT is forced upon. We will also present certain generic density‐functional approaches—in either relativistic or non‐relativistic mean‐field approximations—anchored on the Hohenberg–Kohn theorem applied to nuclear physics. These approaches purport to bypass the matching of the low–high scales.

Finally there will be an attempt to access the EoS directly from nuclear and astrophysical data without invoking nuclear models via, e.g., “deep learning” and other algorithms

Prof. Dr. Mannque Rho
Guest Editor

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Published Papers (11 papers)

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Editorial

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4 pages, 212 KiB  
Editorial
A Brief Overview of the Special Issue “Symmetry and Ultradense Matter in Compact Stars”
by Mannque Rho
Symmetry 2023, 15(12), 2109; https://doi.org/10.3390/sym15122109 - 24 Nov 2023
Viewed by 520
Abstract
The Standard Model, comprising electroweak (EW) and strong (QCD) interactions, has been established and tested with great accuracy [...] Full article
(This article belongs to the Special Issue Symmetries and Ultra Dense Matter of Compact Stars)

Research

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26 pages, 16642 KiB  
Article
A Deep Learning Approach to Extracting Nuclear Matter Properties from Neutron Star Observations
by Plamen G. Krastev
Symmetry 2023, 15(5), 1123; https://doi.org/10.3390/sym15051123 - 20 May 2023
Cited by 10 | Viewed by 1314
Abstract
Understanding the equation of state of dense neutron-rich matter remains a major challenge in modern physics and astrophysics. Neutron star observations from electromagnetic and gravitational wave spectra provide critical insights into the behavior of dense neutron-rich matter. The next generation of telescopes and [...] Read more.
Understanding the equation of state of dense neutron-rich matter remains a major challenge in modern physics and astrophysics. Neutron star observations from electromagnetic and gravitational wave spectra provide critical insights into the behavior of dense neutron-rich matter. The next generation of telescopes and gravitational wave detectors will offer even more detailed neutron-star observations. Employing deep learning techniques to map neutron star mass and radius observations to the equation of state allows for its accurate and reliable determination. This work demonstrates the feasibility of using deep learning to extract the equation of state directly from observations of neutron stars, and to also obtain related nuclear matter properties such as the slope, curvature, and skewness of nuclear symmetry energy at saturation density. Most importantly, it shows that this deep learning approach is able to reconstruct realistic equations of state and deduce realistic nuclear matter properties. This highlights the potential of artificial neural networks in providing a reliable and efficient means to extract crucial information about the equation of state and related properties of dense neutron-rich matter in the era of multi-messenger astrophysics. Full article
(This article belongs to the Special Issue Symmetries and Ultra Dense Matter of Compact Stars)
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12 pages, 6811 KiB  
Article
Building an Equation of State Density Ladder
by Marc Salinas and Jorge Piekarewicz
Symmetry 2023, 15(5), 994; https://doi.org/10.3390/sym15050994 - 27 Apr 2023
Cited by 4 | Viewed by 860
Abstract
The confluence of major theoretical, experimental, and observational advances are providing a unique perspective on the equation of state of dense neutron-rich matter—particularly its symmetry energy—and its imprint on the mass-radius relation for neutron stars. In this contribution, we organize these developments in [...] Read more.
The confluence of major theoretical, experimental, and observational advances are providing a unique perspective on the equation of state of dense neutron-rich matter—particularly its symmetry energy—and its imprint on the mass-radius relation for neutron stars. In this contribution, we organize these developments in an equation of the state density ladder. Of particular relevance to this discussion are the impact of the various rungs on the equation of state and the identification of possible discrepancies among the various methods. A preliminary analysis identifies possible tension between laboratory measurements and gravitational-wave detections that could indicate the emergence of a phase transition in the stellar core. Full article
(This article belongs to the Special Issue Symmetries and Ultra Dense Matter of Compact Stars)
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12 pages, 296 KiB  
Article
Chiral Symmetry Breaking and the Masses of Hadrons: A Review
by Su Houng Lee
Symmetry 2023, 15(4), 799; https://doi.org/10.3390/sym15040799 - 24 Mar 2023
Cited by 3 | Viewed by 839
Abstract
The masses of hadrons in the vacuum, where the chiral symmetry is restored, and in the medium are generally different even when the changes in the order parameters of chiral symmetry are the same. Here, we first discuss the relation between the hadron [...] Read more.
The masses of hadrons in the vacuum, where the chiral symmetry is restored, and in the medium are generally different even when the changes in the order parameters of chiral symmetry are the same. Here, we first discuss the relation between the hadron masses and the chiral symmetry breaking in approaches based on operator product expansion (OPE). We then discuss what additional changes occur to the hadron masses when going from the chiral symmetry restored vacuum to nuclear medium and/or finite temperature. The work will highlight how we can identify the effects of chiral symmetry restoration from experimental observations. Full article
(This article belongs to the Special Issue Symmetries and Ultra Dense Matter of Compact Stars)
26 pages, 1417 KiB  
Article
Topology and Emergent Symmetries in Dense Compact Star Matter
by Yong-Liang Ma and Wen-Cong Yang
Symmetry 2023, 15(3), 776; https://doi.org/10.3390/sym15030776 - 22 Mar 2023
Cited by 5 | Viewed by 1396
Abstract
It has been found that the topology effect and the possible emergent hidden scale and hidden local flavor symmetries at high density reveal a novel structure of compact star matter. When Nf2, baryons can be described by skyrmions when [...] Read more.
It has been found that the topology effect and the possible emergent hidden scale and hidden local flavor symmetries at high density reveal a novel structure of compact star matter. When Nf2, baryons can be described by skyrmions when the number of color Nc is regarded as a large parameter and there is a robust topology change—the transition from skyrmion to half-skyrmion—in the skyrmion matter approach to dense nuclear matter. The hidden scale and local flavor symmetries, which are sources introducing the scalar meson and vector mesons, are significant elements for understanding the nuclear force in nonlinear chiral effective theories. We review in this paper how the robust conclusions from the topology approach to dense matter and emergent hidden scale and hidden local flavor symmetries figure in generalized nuclear effective field theory (GnEFT), which is applicable to nuclear matter from low density to compact star density. The topology change encoded in the parameters of the effective field theory is interpreted as the hadron-quark continuity in the sense of the Cheshire Cat Principle. A novel feature predicted in this theory that has not been found before is the precocious appearance of the conformal sound velocity in the cores of massive stars, although the trace of the energy-momentum tensor of the system is not zero. That is, there is a pseudoconformal structure in the compact star matter and, in contrast to the usual picture, the matter is made of colorless quasiparticles of fractional baryon charges. A possible resolution of the longstanding gA quench problem in nuclei transition and the compatibility of the predictions of the GnEFT with the global properties of neutron star and the data from gravitational wave detections are also discussed. Full article
(This article belongs to the Special Issue Symmetries and Ultra Dense Matter of Compact Stars)
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20 pages, 706 KiB  
Article
Energy-Density Modeling of Strongly Interacting Matter: Atomic Nuclei and Dense Stars
by Panagiota Papakonstantinou and Chang Ho Hyun
Symmetry 2023, 15(3), 683; https://doi.org/10.3390/sym15030683 - 08 Mar 2023
Cited by 5 | Viewed by 1200
Abstract
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 [...] Read more.
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 fm3, 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. Full article
(This article belongs to the Special Issue Symmetries and Ultra Dense Matter of Compact Stars)
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20 pages, 756 KiB  
Article
Popcorn Transitions and Approach to Conformality in Homogeneous Holographic Nuclear Matter
by Jesús Cruz Rojas, Tuna Demircik and Matti Järvinen
Symmetry 2023, 15(2), 331; https://doi.org/10.3390/sym15020331 - 25 Jan 2023
Cited by 9 | Viewed by 924
Abstract
We study cold and dense nuclear matter by using the gauge/gravity duality. To this end, we use the Witten–Sakai–Sugimoto model and the V-QCD models with an approach where the nuclear matter is taken to be spatially homogeneous. We focus on the “popcorn” transitions, [...] Read more.
We study cold and dense nuclear matter by using the gauge/gravity duality. To this end, we use the Witten–Sakai–Sugimoto model and the V-QCD models with an approach where the nuclear matter is taken to be spatially homogeneous. We focus on the “popcorn” transitions, which are phase transitions in the nuclear matter phases induced by changes in the layer structure of the configuration on the gravity side. We demonstrate that the equation of state for the homogeneous nuclear matter becomes approximately conformal at high densities, and compare our results to other approaches. Full article
(This article belongs to the Special Issue Symmetries and Ultra Dense Matter of Compact Stars)
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15 pages, 641 KiB  
Article
From Skyrmions to One Flavored Baryons and Beyond
by Avner Karasik
Symmetry 2022, 14(11), 2347; https://doi.org/10.3390/sym14112347 - 08 Nov 2022
Cited by 1 | Viewed by 718
Abstract
While the identification of skyrmions as the low energy description of baryons in Nf2 QCD is known for decades, a parallel construction for the case of Nf=1 is more mysterious. In the case of one fermionic flavor, [...] Read more.
While the identification of skyrmions as the low energy description of baryons in Nf2 QCD is known for decades, a parallel construction for the case of Nf=1 is more mysterious. In the case of one fermionic flavor, there is no chiral symmetry breaking, no non-linear sigma model, and the conventional construction of skyrmions fails to work. In this article, I will review developments from the last couple of years trying to identify baryons as certain singular configurations in the large Nc limit of Nf=1 QCD. We will give various arguments supporting this identification, and discuss some of its applications. Unlike skyrmions, the new baryons are not contained completely inside the low energy effective theory. They give rise to a singular ring on which the chiral condensate must vanish, with new degrees of freedom living on this ring. These configurations may serve as a bridge between the UV and the IR, and hopefully shed some light on the connection between different phases of QCD. Full article
(This article belongs to the Special Issue Symmetries and Ultra Dense Matter of Compact Stars)
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13 pages, 508 KiB  
Article
Pseudo-Conformal Sound Speed in the Core of Compact Stars
by Mannque Rho
Symmetry 2022, 14(10), 2154; https://doi.org/10.3390/sym14102154 - 14 Oct 2022
Cited by 9 | Viewed by 988
Abstract
By implementing the putative “hadron-quark continuity” conjectured in QCD in terms of skyrmion-half-skyrmion topological change in an effective field theory for dense matter, we argue that (quasi-)baryons could “masquerade” deconfined quarks in the interior of compact stars. We interpret this phenomenon as a [...] Read more.
By implementing the putative “hadron-quark continuity” conjectured in QCD in terms of skyrmion-half-skyrmion topological change in an effective field theory for dense matter, we argue that (quasi-)baryons could “masquerade” deconfined quarks in the interior of compact stars. We interpret this phenomenon as a consequence of possible interplay between hidden scale symmetry and hidden local symmetry at high density. A surprising spin-off of the emerging symmetry that we call “pseudo-conformality” is that the long-standing puzzle of the quenched gA1 in nuclei can be given a simple resolution by the way the hidden symmetries impact nuclear dynamics at low density. Full article
(This article belongs to the Special Issue Symmetries and Ultra Dense Matter of Compact Stars)
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Review

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48 pages, 6164 KiB  
Review
Skyrme Crystals, Nuclear Matter and Compact Stars
by Christoph Adam, Alberto García Martín-Caro, Miguel Huidobro and Andrzej Wereszczynski
Symmetry 2023, 15(4), 899; https://doi.org/10.3390/sym15040899 - 12 Apr 2023
Cited by 6 | Viewed by 1269
Abstract
A general review of the crystalline solutions of the generalized Skyrmemodel and their application to the study of cold nuclear matter at finite density and the Equation of State (EOS) of neutron stars is presented. For the relevant range of densities, the ground [...] Read more.
A general review of the crystalline solutions of the generalized Skyrmemodel and their application to the study of cold nuclear matter at finite density and the Equation of State (EOS) of neutron stars is presented. For the relevant range of densities, the ground state of the Skyrmemodel on the three torus is shown to correspond to configurations with different symmetries, with a sequence of phase transitions between such configurations. The effects of nonzero finite isospin asymmetry are taken into account by the canonical quantization of isospin collective coordinates, and some thermodynamical and nuclear observables (such as the symmetry energy) are computed as a function of the density. We also explore the extension of the model to accommodate strange degrees of freedom, and find a first-order transition for the condensation of kaons in the Skyrme crystal background in a thermodynamically consistent, non-perturbative way. Finally, an approximate EOS of dense matter is constructed by fitting the free parameters of the model to some nuclear observables close to saturation density, which are particularly relevant for the description of nuclear matter. The resulting neutron star mass–radius curves already reasonably satisfy current astrophysical constraints. Full article
(This article belongs to the Special Issue Symmetries and Ultra Dense Matter of Compact Stars)
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30 pages, 3480 KiB  
Review
Chiral Restoration of Nucleons in Neutron Star Matter: Studies Based on a Parity Doublet Model
by Takuya Minamikawa, Bikai Gao, Toru Kojo and Masayasu Harada
Symmetry 2023, 15(3), 745; https://doi.org/10.3390/sym15030745 - 17 Mar 2023
Cited by 7 | Viewed by 3125
Abstract
We review the chiral variant and invariant components of nucleon masses and the consequence of their existence on the chiral restoration in extreme conditions, particularly in neutron star matter. We consider a model of linear realization of chiral symmetry with the nucleon parity [...] Read more.
We review the chiral variant and invariant components of nucleon masses and the consequence of their existence on the chiral restoration in extreme conditions, particularly in neutron star matter. We consider a model of linear realization of chiral symmetry with the nucleon parity doublet structure that permits the chiral invariant mass, m0, for positive and negative parity nucleons. The nuclear matter is constructed with the parity doublet nucleon model coupled to scalar fields σ, vector fields (ω,ρ), and mesons with strangeness through the U(1)A anomaly. In models with a large m0, the nucleon mass is insensitive to the medium, and the nuclear saturation properties can be reproduced without demanding strong couplings of the nucleons to the scalar fields σ and vector fields ω. We confront the resulting nuclear equations of state with nuclear constraints and neutron star observations and delineate the chiral invariant mass and effective interactions. To further examine the nuclear equations of state beyond the saturation density, we supplement quark models to set the boundary conditions from the high-density side. The quark models are constrained by the two-solar-mass conditions, and such constraints are transferred to nuclear models through the causality and thermodynamic stability conditions. We also calculate various condensates and the matter composition from nuclear to quark matter in a unified matter by constructing a generating functional that interpolates the nuclear and quark matter with external fields. Two types of chiral restoration are discussed: one due to the positive scalar charges of nucleons and the other triggered by the evolution of the Dirac sea. We found that the U(1)A anomaly softens equations of state from low to high density. Full article
(This article belongs to the Special Issue Symmetries and Ultra Dense Matter of Compact Stars)
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