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Symmetry
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Advances on Chiral Symmetry and Its Restoration
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Advances on Chiral Symmetry and Its Restoration

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

Deadline for manuscript submissions: closed (28 March 2022) | Viewed by 9911

Special Issue Editors

Prof. Dr. Angel Gómez Nicola

E-Mail Website
Guest Editor
Departamento de Física Teórica, Universidad Complutense de Madrid, Madrid, Spain
Interests: chiral symmetry restoration; QCD
Dr. Jacobo Ruiz de Elvira

E-Mail Website
Guest Editor
Institute for Theoretical Physics, University of Bern, Bern, Switzerland
Interests: low-energy hadron physics; effective field theories; dispersion relations
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues, 

Chiral Symmetry is the main building block for our current understanding of low energy hadron physics. For many years, the pattern of chiral symmetry breaking of strong interactions has served as a guideline for the construction of effective theories describing hadron phenomenology, with great success as far as the comparison with experimental data and lattice simulations are concerned. The physics behind this guiding principle covers a wide range of energy scales from hadron spectroscopy to heavy-ion collisions, where the great variety of techniques based on chiral symmetry have allowed us to reach an era of precision in this field.

In particular, the restoration of chiral symmetry under external conditions, like those of temperature and chemical potentials achieved in relativistic heavy-ion collisions, has become a key ingredient to understand properly the phase diagram of quantum chromodynamics. Substantial advances in this direction have come from lattice simulations of thermodynamics and fluctuations, theoretical developments, and experiments at various facilities, such as RHIC at BNL, LHC at CERN, or GSI-FAIR at Darmstadt, which have significantly improved our knowledge of the phase diagram. Open problems such as the existence and properties of a critical point or the nature of the transition still challenge theoretical approaches and offer interesting lines of research for the near future.

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Symmetry is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2400 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Published Papers (4 papers)

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Research

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18 pages, 1578 KiB  
Article
Gliding Down the QCD Transition Line, from Nf = 2 till the Onset of Conformality
by Andrey Yu Kotov, Maria Paola Lombardo and Anton Trunin
Symmetry 2021, 13(10), 1833; https://doi.org/10.3390/sym13101833 - 1 Oct 2021
Cited by 6 | Viewed by 1369
Abstract
We review the hot QCD transition with varying number of flavours, from two till the onset of the conformal window. We discuss the universality class for Nf=2, along the critical line for two massless light flavours, and a third [...] Read more.
We review the hot QCD transition with varying number of flavours, from two till the onset of the conformal window. We discuss the universality class for Nf=2, along the critical line for two massless light flavours, and a third flavour whose mass serves as an interpolator between Nf=2 and Nf=3. We identify a possible scaling window for the 3D O(4) universality class transition, and its crossover to a mean field behaviour. We follow the transition from Nf=3 to larger Nf, when it remains of first order, with an increasing coupling strength; we summarise its known properties, including possible cosmological applications as a model for a strong electroweak transition. The first order transition, and its accompanying second order endpoint, finally morphs into the essential singularity at the onset of the conformal window, following the singular behaviour predicted by the functional renormalisation group. Full article
(This article belongs to the Special Issue Advances on Chiral Symmetry and Its Restoration)
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Review

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25 pages, 2976 KiB  
Review
Lattice Constraints on the QCD Chiral Phase Transition at Finite Temperature and Baryon Density
by Owe Philipsen
Symmetry 2021, 13(11), 2079; https://doi.org/10.3390/sym13112079 - 3 Nov 2021
Cited by 17 | Viewed by 3801
Abstract
The thermal restoration of chiral symmetry in QCD is known to proceed by an analytic crossover, which is widely expected to turn into a phase transition with a critical endpoint as the baryon density is increased. In the absence of a genuine solution [...] Read more.
The thermal restoration of chiral symmetry in QCD is known to proceed by an analytic crossover, which is widely expected to turn into a phase transition with a critical endpoint as the baryon density is increased. In the absence of a genuine solution to the sign problem of lattice QCD, simulations at zero and imaginary baryon chemical potential in a parameter space enlarged by a variable number of quark flavours and quark masses constitute a viable way to constrain the location of a possible non-analytic phase transition and its critical endpoint. In this article I review recent progress towards an understanding of the nature of the transition in the massless limit, and its critical temperature at zero density. Combined with increasingly detailed studies of the physical crossover region, current data bound a possible critical point to μB ≳ 3T. Full article
(This article belongs to the Special Issue Advances on Chiral Symmetry and Its Restoration)
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31 pages, 2392 KiB  
Review
Degeneracy Patterns of Chiral Companions at Finite Temperature
by Juan M. Torres-Rincon
Symmetry 2021, 13(8), 1400; https://doi.org/10.3390/sym13081400 - 1 Aug 2021
Cited by 2 | Viewed by 1722
Abstract
Chiral symmetry represents a fundamental concept lying at the core of particle and nuclear physics. Its spontaneous breaking in vacuum can be exploited to distinguish chiral hadronic partners, whose masses differ. In fact, the features of this breaking serve as guiding principles for [...] Read more.
Chiral symmetry represents a fundamental concept lying at the core of particle and nuclear physics. Its spontaneous breaking in vacuum can be exploited to distinguish chiral hadronic partners, whose masses differ. In fact, the features of this breaking serve as guiding principles for the construction of effective approaches of QCD at low energies, e.g., the chiral perturbation theory, the linear sigma model, the (Polyakov)–Nambu–Jona-Lasinio model, etc. At high temperatures/densities chiral symmetry can be restored bringing the chiral partners to be nearly degenerated in mass. At vanishing baryochemical potential, such restoration follows a smooth transition, and the chiral companions reach this degeneration above the transition temperature. In this work I review how different realizations of chiral partner degeneracy arise in different effective theories/models of QCD. I distinguish the cases where the chiral states are either fundamental degrees of freedom or (dynamically-generated) composed states. In particular, I discuss the intriguing case in which chiral symmetry restoration involves more than two chiral partners, recently addressed in the literature. Full article
(This article belongs to the Special Issue Advances on Chiral Symmetry and Its Restoration)
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27 pages, 405 KiB  
Review
Axial UA(1) Anomaly: A New Mechanism to Generate Massless Bosons
by Vicente Azcoiti
Symmetry 2021, 13(2), 209; https://doi.org/10.3390/sym13020209 - 28 Jan 2021
Cited by 3 | Viewed by 1639
Abstract
Prior to the establishment of QCD as the correct theory describing hadronic physics, it was realized that the essential ingredients of the hadronic world at low energies are chiral symmetry and its spontaneous breaking. Spontaneous symmetry breaking is a non-perturbative phenomenon, and, thanks [...] Read more.
Prior to the establishment of QCD as the correct theory describing hadronic physics, it was realized that the essential ingredients of the hadronic world at low energies are chiral symmetry and its spontaneous breaking. Spontaneous symmetry breaking is a non-perturbative phenomenon, and, thanks to massive QCD simulations on the lattice, we have at present a good understanding of the vacuum realization of the non-abelian chiral symmetry as a function of the physical temperature. As far as the UA(1) anomaly is concerned, and especially in the high temperature phase, the current situation is however far from satisfactory. The first part of this article is devoted to reviewing the present status of lattice calculations, in the high temperature phase of QCD, of quantities directly related to the UA(1) axial anomaly. In the second part, some recently suggested interesting physical implications of the UA(1) anomaly in systems where the non-abelian axial symmetry is fulfilled in the vacuum are analyzed. More precisely it is argued that, if the UA(1) symmetry remains effectively broken, the topological properties of the theory can be the basis of a mechanism, other than Goldstone’s theorem, to generate a rich spectrum of massless bosons at the chiral limit. Full article
(This article belongs to the Special Issue Advances on Chiral Symmetry and Its Restoration)
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