Particles

9 pages, 387 KiB

Open AccessArticle

Dynamical Pair Production at Sub-Barrier Energies for Light Nuclei

by Thomas Settlemyre, Hua Zheng and Aldo Bonasera

Particles 2022, 5(4), 580-588; https://doi.org/10.3390/particles5040041 - 17 Dec 2022

Cited by 2 | Viewed by 1510

In the collision of two heavy ions, the strong repulsion coming from the Coulomb field is enough to produce

e^{+} e^{-}

pair(s) from vacuum fluctuations. The energy is provided by the kinetic energy of the ions and the Coulomb interaction at [...] Read more.

In the collision of two heavy ions, the strong repulsion coming from the Coulomb field is enough to produce

e^{+} e^{-}

pair(s) from vacuum fluctuations. The energy is provided by the kinetic energy of the ions and the Coulomb interaction at the production point. If, for instance, the electron is located at the center of mass (C.M.) of the two ions moving along the z-axis, and the positron is at a distance x from the electron, the ions can be accelerated towards each other since the Coulomb barrier is lowered by the presence of the electron. This screening results in an increase in the kinetic energy of the colliding ions and may result in an increase in the fusion probability of light ions above the adiabatic limit. Full article

(This article belongs to the Collection High Energy Physics)

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19 pages, 1401 KiB

Open AccessArticle

Model Study of the Energy Dependence of Anisotropic Flow in Heavy-Ion Collisions at

\sqrt{s_{N N}}

= 2–4.5 GeV

by Petr Parfenov

Particles 2022, 5(4), 561-579; https://doi.org/10.3390/particles5040040 - 9 Dec 2022

Cited by 11 | Viewed by 1609

Abstract

The anisotropic flow is one of the important observables sensitive to the equation of state (EOS) and transport properties of the strongly interacting matter created in relativistic heavy-ion collisions. In this work, we report a detailed multi-differential study of the directed ( [...] Read more.

The anisotropic flow is one of the important observables sensitive to the equation of state (EOS) and transport properties of the strongly interacting matter created in relativistic heavy-ion collisions. In this work, we report a detailed multi-differential study of the directed (

v_{1}

), elliptic (

v_{2}

), triangular (

v_{3}

), and quadrangular (

v_{4}

) flow coefficients of protons in relativistic heavy-ion collisions at

\sqrt{s_{N N}}

= 2.2–4.5 GeV using several hadronic transport models. Recent published results for Au + Au collisions at

\sqrt{s_{N N}}

= 2.4 GeV from HADES experiment and at

\sqrt{s_{N N}}

= 3.0 GeV from the STAR experiment have been used for comparison. The study motivates further experimental investigations of the anisotropic collective flow of protons and neutrons in a high baryon density region. Full article

(This article belongs to the Special Issue Selected Papers from "Physics Performance Studies at FAIR and NICA")

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26 pages, 835 KiB

Open AccessArticle

Parametrizations of Collinear and k_T-Dependent Parton Densities in Proton

by Nizami A. Abdulov, Anatoly V. Kotikov and Artem Lipatov

Particles 2022, 5(4), 535-560; https://doi.org/10.3390/particles5040039 - 28 Nov 2022

Cited by 5 | Viewed by 2397

Abstract

A new type of parametrization for parton distribution functions in the proton, based on their

Q^{2}

-evolution at large and small x values, is constructed. In our analysis, the valence and nonsinglet parts obey the Gross–Llewellyn–Smith and Gottfried sum rules, respectively. For [...] Read more.

A new type of parametrization for parton distribution functions in the proton, based on their

Q^{2}

-evolution at large and small x values, is constructed. In our analysis, the valence and nonsinglet parts obey the Gross–Llewellyn–Smith and Gottfried sum rules, respectively. For the singlet quark and gluon densities, momentum conservation is taken into account. Then, using the Kimber–Martin–Ryskin prescription, we extend the consideration to Transverse Momentum Dependent (TMD, or unintegrated) gluon and quark distributions in the proton, which currently plays an important role in a the number of phenomenological applications. The analytical expressions for the latter, valid for both low and large x, are derived for the first time. Full article

(This article belongs to the Special Issue 2022 Feature Papers by Particles’ Editorial Board Members)

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21 pages, 7381 KiB

Open AccessArticle

Recovering the Conformal Limit of Color Superconducting Quark Matter within a Confining Density Functional Approach

by Oleksii Ivanytskyi and David B. Blaschke

Particles 2022, 5(4), 514-534; https://doi.org/10.3390/particles5040038 - 28 Nov 2022

Cited by 19 | Viewed by 1969

Abstract

We generalize a recently proposed confining relativistic density-functional approach to the case of density-dependent vector and diquark couplings. The particular behavior of these couplings is motivated by the non-perturbative gluon exchange in dense quark matter and provides the conformal limit at asymptotically high [...] Read more.

We generalize a recently proposed confining relativistic density-functional approach to the case of density-dependent vector and diquark couplings. The particular behavior of these couplings is motivated by the non-perturbative gluon exchange in dense quark matter and provides the conformal limit at asymptotically high densities. We demonstrate that this feature of the quark matter EoS is consistent with a significant stiffness in the density range typical for the interiors of neutron stars. In order to model these astrophysical objects, we construct a family of hybrid quark-hadron EoSs of cold stellar matter. We also confront our approach with the observational constraints on the mass–radius relation of neutron stars and their tidal deformabilities and argue in favor of a quark matter onset at masses below

1.0 M_{⊙}

. Full article

(This article belongs to the Special Issue Selected Papers from “The Modern Physics of Compact Stars and Relativistic Gravity 2021”)

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21 pages, 1563 KiB

Open AccessReview

Formation, Possible Detection and Consequences of Highly Magnetized Compact Stars

by Banibrata Mukhopadhyay and Mukul Bhattacharya

Particles 2022, 5(4), 493-513; https://doi.org/10.3390/particles5040037 - 17 Nov 2022

Cited by 5 | Viewed by 1804

Abstract

Over the past several years, there has been enormous interest in massive neutron stars and white dwarfs due to either their direct or indirect evidence. The recent detection of gravitational wave event GW190814 has confirmed the existence of compact stars with masses as [...] Read more.

Over the past several years, there has been enormous interest in massive neutron stars and white dwarfs due to either their direct or indirect evidence. The recent detection of gravitational wave event GW190814 has confirmed the existence of compact stars with masses as high as ∼2.5–2.67

M_{⊙}

within the so-called mass gap, indicating the existence of highly massive neutron stars. One of the primary goals to invoke massive compact objects was to explain the recent detections of over a dozen Type Ia supernovae, whose peculiarity lies with their unusual light curve, in particular the high luminosity and low ejecta velocity. In a series of recent papers, our group has proposed that highly magnetised white dwarfs with super-Chandrasekhar masses can be promising candidates for the progenitors of these peculiar supernovae. The mass-radius relations of these magnetised stars are significantly different from those of their non-magnetised counterparts, which leads to a revised super-Chandrasekhar mass-limit. These compact stars have wider ranging implications, including those for soft gamma-ray repeaters, anomalous X-ray pulsars, white dwarf pulsars and gravitational radiation. Here we review the development of the subject over the last decade or so, describing the overall state of the art of the subject as it stands now. We mainly touch upon the possible formation channels of these intriguing stars as well as the effectiveness of direct detection methods. These magnetised stars can have many interesting consequences, including reconsideration of them as possible standard candles. Full article

(This article belongs to the Special Issue Selected Papers from “The Modern Physics of Compact Stars and Relativistic Gravity 2021”)

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5 pages, 581 KiB

Open AccessArticle

The Number of Elementary Fermions and the Electromagnetic Coupling

by Liberato De Caro

Particles 2022, 5(4), 488-492; https://doi.org/10.3390/particles5040036 - 10 Nov 2022

Viewed by 1445

Abstract

Electric charges and masses of elementary fermions of the Standard Model and fundamental physical constants (speed of light in vacuum, Planck constant, gravitational constant, vacuum permittivity, electron charge) are related through a simple equation. This new relation links 10 of the free parameters [...] Read more.

Electric charges and masses of elementary fermions of the Standard Model and fundamental physical constants (speed of light in vacuum, Planck constant, gravitational constant, vacuum permittivity, electron charge) are related through a simple equation. This new relation links 10 of the free parameters of the Standard Model—the masses of the three charged leptons and six quarks, and the electromagnetic coupling—in a compact formula, leaving strong constraints for allowing further elementary charged fermions beyond the Standard Model’s physics. The formula is not derived by theoretical calculations, but it is based on the empirically measured values of the electric charges and proper masses of the known elementary fermions. Full article

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37 pages, 5374 KiB

Open AccessArticle

Analysis of Overlapping Resonances with Unitary Breit–Wigner and K-Matrix Approaches

by Victor Henner and Tatyana Belozerova

Particles 2022, 5(4), 451-487; https://doi.org/10.3390/particles5040035 - 27 Oct 2022

Viewed by 1841

Abstract

We compare two methods for obtaining the parameters of overlapping resonances. The convenience of the Breit–Wigner (BW) approach is based on the fact that it operates with the masses and widths of the states. For several resonances with the same quantum numbers, a [...] Read more.

We compare two methods for obtaining the parameters of overlapping resonances. The convenience of the Breit–Wigner (BW) approach is based on the fact that it operates with the masses and widths of the states. For several resonances with the same quantum numbers, a sum of BW functions violates the unitarity of the S-matrix. However, unitarity can be maintained by introducing interference phases to a BW implementation of scattering matrix formalism. A background can be added to the BW amplitudes in the standard way by using background phases. The K-matrix method is often used to analyze data related to several resonances with the same quantum numbers. It guarantees the unitarity of the S-matrix, but its parameters can be considered as resonance masses and widths only for well-spaced states. It also does not allow the separation of the resonant and background contributions in scattering amplitudes, which is critically important for determining parameters of wide resonances. To demonstrate the features of these methods, we consider several examples using simulated data. Full article

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9 pages, 381 KiB

Open AccessArticle

Electromagnetic Response in an Expanding Quark–Gluon Plasma

by Igor A. Shovkovy

Particles 2022, 5(4), 442-450; https://doi.org/10.3390/particles5040034 - 22 Oct 2022

Cited by 9 | Viewed by 1640

Abstract

The validity of conventional Ohm’s law is tested in the context of a rapidly evolving quark–gluon plasma produced in heavy-ion collisions. Here, we discuss the electromagnetic response using an analytical solution in kinetic theory. As conjectured previously, after switching on an electric field [...] Read more.

The validity of conventional Ohm’s law is tested in the context of a rapidly evolving quark–gluon plasma produced in heavy-ion collisions. Here, we discuss the electromagnetic response using an analytical solution in kinetic theory. As conjectured previously, after switching on an electric field in a nonexpanding plasma, the time-dependent current is given by

J (t) = (1 - e^{- t / τ_{0}}) σ_{0} E

, where

τ_{0}

is the transport relaxation time and

σ_{0}

is the steady-state electrical conductivity. Such an incomplete electromagnetic response reduces the efficiency of the magnetic flux trapping in the quark–gluon plasma, and may prevent the observation of the chiral magnetic effect. Here, we extend the study to the case of a rapidly expanding plasma. We find that the decreasing temperature and the increasing transport relaxation time have opposite effects on the electromagnetic response. While the former suppresses the time-dependent conductivity, the latter enhances it. Full article

(This article belongs to the Special Issue 2022 Feature Papers by Particles’ Editorial Board Members)

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16 pages, 322 KiB

Open AccessArticle

Quantum States for a Minimum-Length Spacetime

by Alessandro Pesci

Particles 2022, 5(4), 426-441; https://doi.org/10.3390/particles5040033 - 23 Sep 2022

Cited by 2 | Viewed by 1588

Abstract

Starting from some results regarding the form of the Ricci scalar at a point P in a (particle-like) spacetime endowed with a minimum distance, we investigate how they might be accommodated, specifically for the case of null separations, in a as-simple-as-possible quantum structure [...] Read more.

Starting from some results regarding the form of the Ricci scalar at a point P in a (particle-like) spacetime endowed with a minimum distance, we investigate how they might be accommodated, specifically for the case of null separations, in a as-simple-as-possible quantum structure for spacetime at P, and we try to accomplish this in terms of potentially operationally defined concepts. In so doing, we provide a possible explicit form for the operator expressing the Ricci scalar as a quantum observable, and give quantum-informational support, thus regardless of or before field equations, to associating with a patch of horizon an entropy proportional to its area. Full article

(This article belongs to the Special Issue Particles: Feature Papers)

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Particles, Volume 5, Issue 4 (December 2022) – 9 articles

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