Special Issue "Interplay of QCD, Cosmology and Astroparticle Physics"

A special issue of Universe (ISSN 2218-1997).

Deadline for manuscript submissions: closed (31 March 2019).

Special Issue Editor

Prof. Dr. Carlos Pajares
E-Mail Website
Guest Editor
Univeristy of Santiago de Compostela, Spain

Special Issue Information

Dear Colleagues,

Quantum Chromodynamics (QCD) encodes a large richness of physical phenomena, and is intensively studied theoretically and experimentally. However, in spite of its success, some of its aspects are not yet fully understood; there remain open questions that demand answers. Most of these questions have important implications in cosmology and astroparticle physics.

The QCD Lagrangian contains ingredients that can clarify key questions concerning cosmology. The term, which breaks conformal symmetry, even in the massless case, is related to the axion field and its search concerns the nature of dark matter and also could contribute to the cosmological constant. Other possibilities of dark matter have been speculated, such as the existence of exotic hadrons made of color-octet complexes. The mentioned term, which is not CP invariant, plays an important role in the equation of the state of the deconfinement transition from hadronic matter to quark gluon matter, such as what happened in the first moments of the Universe after the Big Bang. This term is proportional to the trace anomaly, which measures the departure from a free quark-gluon gas of the obtained strongly-coupled quark gluon matter, and is also related to vacuum structure.

The experiments of the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) have created strongly-coupled quark gluon matter in nucleus–nucleus collisions. Most of the observed collective effects have also been seen in pp collisions. In this case, it is not clear how hydrodynamic models can be applied. There is not a unified picture of the transverse momentum distribution of pp data, as well as its azimuthal distribution. The interplay between soft and hard collisions can show interesting relationships between parton entanglement and thermalization. On the other hand, the forward LHC detectors provide important information on elastic and diffractive scattering, which play important roles in determining the hadronic cascade produced in ultrarelativistic cosmic rays. Usual hadronic models, previously-matched to LHC data, are not able to describe some of the cosmic ray data at higher energies, such as the excess of muons and the energy dependence of the distribution of the length of maximum depth. Phenomena like gluon saturation, color reconnection, string interactions, percolation, and string junction working at LHC energies could have implications in the hadronic cascade.

The QCD conformal breaking term, the axion field and the relation to dark matter and the cosmological constant, the strong CP problem, the dependence on the temperature of the trace anomaly, the equation of state close to the deconfinement phase transition, the collective effects produced in colliding small systems and its thermalization, the transverse momentum distributions, including azimuthal distributions and the interplay between soft and hard interactions, the elastic and diffractive scatterings and in general forward physics at LHC and ultrahigh cosmic ray energies, models of hadronic cascade are the subject of special interest at an interplay of QCD with two related fields: Cosmology and Astroparticle Physics.

Prof. Dr. Carlos Pajares
Guest Editor

Manuscript Submission Information

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Keywords

  • conformal symmetry breaking
  • axion field
  • dark matter
  • cosmological constant
  • trace anomaly
  • equation of state
  • deconfinement phase transition
  • collectivity in small systems
  • thermalization
  • interplay between soft and hard interactions
  • gluon saturation
  • string interactions
  • forward LHC Physics
  • elastic and diffraction scattering
  • proton profile
  • ultrahigh cosmic ray energy
  • hadronic cascade models

Published Papers (3 papers)

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Research

Open AccessArticle
Taming the Energy Rise of the Total Proton-Proton Cross-Section
Universe 2019, 5(5), 106; https://doi.org/10.3390/universe5050106 - 07 May 2019
Cited by 1 | Viewed by 1069
Abstract
Steep rise of parton densities in the limit of small parton momentum fraction x poses a challenge for describing the observed energy-dependence of the total and inelastic proton-proton cross sections σ p p tot / inel : considering a realistic parton spatial distribution, one obtains a too-strong increase of σ p p tot / inel in the limit of very high energies. We discuss various mechanisms which allow one to tame such a rise, paying special attention to the role of parton-parton correlations. In addition, we investigate a potential impact on model predictions for σ p p tot, related to dynamical higher twist corrections to parton-production processes. Full article
(This article belongs to the Special Issue Interplay of QCD, Cosmology and Astroparticle Physics)
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Open AccessArticle
Hot Dense Matter: Deconfinement and Clustering of Color Sources in Nuclear Collisions
Universe 2018, 4(9), 96; https://doi.org/10.3390/universe4090096 - 18 Sep 2018
Cited by 1 | Viewed by 972
Abstract
Within the first few microseconds from after the Big Bang, the hot dense matter was in the form of the Quark Gluon Plasm (QGP) consisting of free quarks and gluons. By colliding heavy nuclei at RHIC and LHC at a velocity close to [...] Read more.
Within the first few microseconds from after the Big Bang, the hot dense matter was in the form of the Quark Gluon Plasm (QGP) consisting of free quarks and gluons. By colliding heavy nuclei at RHIC and LHC at a velocity close to the speed of light, we were able to create the primordial matter and observe the matter after expansion and cooling. In this report we present the thermodynamics and transport coefficients obtained in the framework of clustering of color sources in both hadron-hadron and nucleus-nucleus collisions at RHIC and LHC energies. Multiparticle production at high energies can be described in terms of color strings stretched between the projectile and target. At high string density single strings overlap and form color sources. This addition belongs to the non-perturbative domain of Quantum Chromo Dynamics (QGP) and manifests its most fundamental features. The Schwinger QED 2 mechanism produces color neutral q q ¯ pairs when color source strings break. Subsequent hardonization produces the observed hadrons. With growing energy and atomic number of the colliding nuclei the density of strings grows and more color sources form clusters in the transverse plane. At a certain critical density a macroscopic cluster appears, which marks the percolation phase transition. This is the Color String Percolation Model (CSPM). The critical density is identified as the deconfinement transition and happens at the hadronization temperature. The stochastic thermalization in p p and A-A is a consequence of the quantum tunneling through the event horizon introduced by the confining color fields, the Hawking-Unruh effect. The percolation approach within CSPM is successfully used to describe the crossover phase transition in the soft collision region. The same phenomenology when applied to both hadron-hadron and nucleus-nucleus collisions emphasizes the importance of color string density, creating a macroscopic cluster which identifies the connectivity required for a finite droplet of the QGP. Full article
(This article belongs to the Special Issue Interplay of QCD, Cosmology and Astroparticle Physics)
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Open AccessArticle
Several Effects Unexplained by QCD
Universe 2018, 4(5), 65; https://doi.org/10.3390/universe4050065 - 16 May 2018
Cited by 9 | Viewed by 1119
Abstract
Several new experimental discoveries in high energy proton interactions, yet unexplained by QCD, are discussed in the paper. The increase of the cross sections with increasing energy from ISR to LHC, the correlation between it and the behavior of the slope of the [...] Read more.
Several new experimental discoveries in high energy proton interactions, yet unexplained by QCD, are discussed in the paper. The increase of the cross sections with increasing energy from ISR to LHC, the correlation between it and the behavior of the slope of the elastic diffraction cone, the unexpected increase of the survival probability of protons in the same energy range, the new structure of the elastic differential cross section at rather large transferred momenta (small distances) and the peculiar ridge effect in high multiplicity inelastic processes are still waiting for QCD interpretation and deeper insight in vacuum. Full article
(This article belongs to the Special Issue Interplay of QCD, Cosmology and Astroparticle Physics)
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