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Particles, Volume 2, Issue 4 (December 2019) – 7 articles

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Open AccessArticle
Dynamical Freeze-Out Phenomena: The Case of K±, φ Transverse Momentum Spectra in Collisions of Au(1.23 A GeV) + Au
Particles 2019, 2(4), 511-528; https://doi.org/10.3390/particles2040032 - 15 Dec 2019
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Abstract
We argue for a continuous (dynamical) kinetic freeze-out of K ± , ϕ observed at midrapidity in collisions of Au(1.23 A GeV) + Au. The simulations, by means of a transport model of Boltzmann-Ühling-Uhlenbeck (BUU) type, point to time independent transverse momentum slope [...] Read more.
We argue for a continuous (dynamical) kinetic freeze-out of K ± , ϕ observed at midrapidity in collisions of Au(1.23 A GeV) + Au. The simulations, by means of a transport model of Boltzmann-Ühling-Uhlenbeck (BUU) type, point to time independent transverse momentum slope parameters after 20 fm/c. The complex interplay of expansion dynamics and strangeness production/exchange/absorption as well as elastic scatterings involved in the reaction network does not support the previous interpretation of a late freeze-out of K due to larger cross sections. Full article
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Open AccessArticle
Exploring Cosmic Matter in the Laboratory—The Compressed Baryonic Matter Experiment at FAIR
Particles 2019, 2(4), 499-510; https://doi.org/10.3390/particles2040031 - 11 Dec 2019
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Abstract
The Compressed Baryonic Matter (CBM) experiment is one of four scientific pillars of the future Facility for Antiproton and Ion Research (FAIR) in Darmstadt. In collisions between heavy nuclei at FAIR energies, it is expected that the matter in the reaction zone is [...] Read more.
The Compressed Baryonic Matter (CBM) experiment is one of four scientific pillars of the future Facility for Antiproton and Ion Research (FAIR) in Darmstadt. In collisions between heavy nuclei at FAIR energies, it is expected that the matter in the reaction zone is compressed to more than five times saturation density, corresponding to the density in the core of a massive neutron star. This offers the unique opportunity to study in the laboratory the high-density equation-of-state (EOS) of nuclear matter, and to search for new phases of Quantum Chromo Dynamics (QCD) matter at large baryon-chemical potentials. Promising experimental observables sensitive to the EOS and to possible phase transitions will be discussed, together with a brief description of the CBM experiment. Full article
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Open AccessArticle
Improving Center Vortex Detection by Usage of Center Regions as Guidance for the Direct Maximal Center Gauge
Particles 2019, 2(4), 491-498; https://doi.org/10.3390/particles2040030 - 11 Dec 2019
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Abstract
The center vortex model of quantum chromodynamic states that vortices, a closed color-magnetic flux, percolate the vacuum. Vortices are seen as the relevant excitations of the vacuum, causing confinement and dynamical chiral symmetry breaking. In an appropriate gauge, as direct maximal center gauge [...] Read more.
The center vortex model of quantum chromodynamic states that vortices, a closed color-magnetic flux, percolate the vacuum. Vortices are seen as the relevant excitations of the vacuum, causing confinement and dynamical chiral symmetry breaking. In an appropriate gauge, as direct maximal center gauge, vortices are detected by projecting onto the center degrees of freedom. Such gauges suffer from Gribov copy problems: different local maxima of the corresponding gauge functional can result in different predictions of the string tension. By using nontrivial center regions—that is, regions whose boundary evaluates to a nontrivial center element—a resolution of this issue seems possible. We use such nontrivial center regions to guide simulated annealing procedures, preventing an underestimation of the string tension in order to resolve the Gribov copy problem. Full article
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Open AccessArticle
Upgrading the Baryonic Matter at the Nuclotron Experiment at NICA for Studies of Dense Nuclear Matter
Particles 2019, 2(4), 481-490; https://doi.org/10.3390/particles2040029 - 15 Nov 2019
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Abstract
The Nuclotron at the Joint Institute for Nuclear Research in Dubna can deliver gold beams with kinetic energies between 2 and 4.5 A GeV. In heavy-ion collisions at these energies, it is expected that the nuclear fireball will be compressed by up to [...] Read more.
The Nuclotron at the Joint Institute for Nuclear Research in Dubna can deliver gold beams with kinetic energies between 2 and 4.5 A GeV. In heavy-ion collisions at these energies, it is expected that the nuclear fireball will be compressed by up to approximately four times the saturation density. This offers the opportunity to study the high-density equation-of-state (EOS) of nuclear matter in the laboratory, which is needed for our understanding of the structure of neutron stars and the dynamics of neutron star mergers. The Baryonic Matter at the Nuclotron ([email protected]) experiment will be upgraded to perform multi-differential measurements of hadrons including (multi-) strange hyperons, which are promising probes of the high-density EOS, and of new phases of quantum chromodynamic (QCD) matter. The layout of the upgraded [email protected] experiment and the results of feasibility studies are presented. Full article
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Open AccessArticle
Dissipation Triggers Dynamical Two-Stream Instability
Particles 2019, 2(4), 457-480; https://doi.org/10.3390/particles2040028 - 31 Oct 2019
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Abstract
Two coupled, interpenetrating fluids suffer instabilities beyond certain critical counterflows. For ideal fluids, an energetic instability occurs at the point where a sound mode inverts its direction due to the counterflow, while dynamical instabilities only occur at larger relative velocities. Here, we discuss [...] Read more.
Two coupled, interpenetrating fluids suffer instabilities beyond certain critical counterflows. For ideal fluids, an energetic instability occurs at the point where a sound mode inverts its direction due to the counterflow, while dynamical instabilities only occur at larger relative velocities. Here, we discuss two relativistic fluids, one of which is dissipative. Using linearized hydrodynamics, we show that, in this case, the energetic instability turns dynamical, i.e., there is an exponentially growing mode, and this exponential growth only occurs in the presence of dissipation. This result is general and does not rely on an underlying microscopic theory. It can be applied to various two-fluid systems, for instance, in the interior of neutron stars. We also point out that, under certain circumstances, the two-fluid system exhibits a mode analogous to the r-mode in neutron stars that can become unstable for arbitrarily small values of the counterflow. Full article
(This article belongs to the Special Issue Dense QCD and neutron stars)
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Open AccessArticle
Strange Stars in the Vector Interaction Enhanced Bag Model
Particles 2019, 2(4), 447-456; https://doi.org/10.3390/particles2040027 - 18 Oct 2019
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Abstract
The vector interaction enhanced Bag model (vBag) for dense quark matter extends the commonly used thermodynamic Bag model (tdBag) by incorporating effects of dynamical chiral symmetry breaking (D χ SB) and vector repulsion. Motivated by the suggestion that the stability of strange matter [...] Read more.
The vector interaction enhanced Bag model (vBag) for dense quark matter extends the commonly used thermodynamic Bag model (tdBag) by incorporating effects of dynamical chiral symmetry breaking (D χ SB) and vector repulsion. Motivated by the suggestion that the stability of strange matter is in tension with chiral symmetry breaking (D χ SB) we examine the parameter space for its stability in the vBag model in this work. Assuming the chiral transition occurs at sufficiently low density, we determine the stability region of strange matter as a function of the effective Bag constant and the vector coupling. As an astrophysical application, we construct contours of maximum mass M max and radius at maximum mass R max in this region of parameter space. We also study the stability of strange stars in the vBag model with maximum mass in the 2 M range by computing the spectrum of radial oscillations, and comparing to results from the tdBag model, find some notable differences. Full article
(This article belongs to the Special Issue Dense QCD and neutron stars)
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Open AccessEditorial
Editorial for the Special Issue “The Modern Physics of Compact Stars and Relativistic Gravity 2017”
Particles 2019, 2(4), 444-446; https://doi.org/10.3390/particles2040026 - 23 Sep 2019
Viewed by 290
Abstract
This Special Issue arose from the presentations of the authors at the international conference “The Modern Physics of Compact Stars and Relativistic Gravity 2017” https://indico [...] Full article
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