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Jump to: Review

9 pages, 1116 KiB

Open AccessArticle

Hurst Exponent and Event-by-Event Fluctuations in Relativistic Nucleus–Nucleus Collisions

by Anastasiya I. Fedosimova, Khusniddin K. Olimov, Igor A. Lebedev, Sayora A. Ibraimova, Ekaterina A. Bondar, Elena A. Dmitriyeva and Ernazar B. Mukanov

Particles 2024, 7(4), 918-926; https://doi.org/10.3390/particles7040055 - 15 Oct 2024

Viewed by 567

Abstract

A joint study of multi-particle pseudo-rapidity correlations and event-by-event fluctuations in the distributions of secondary particles and fragments of the target nucleus and the projectile nucleus was carried out in order to search for correlated clusters of secondary particles. An analysis of the [...] Read more.

A joint study of multi-particle pseudo-rapidity correlations and event-by-event fluctuations in the distributions of secondary particles and fragments of the target nucleus and the projectile nucleus was carried out in order to search for correlated clusters of secondary particles. An analysis of the collisions of the sulfur nucleus with photoemulsion nuclei at an energy of 200 A·GeV is presented based on experimental data obtained at the SPS at CERN. The analysis of multi-particle correlations was performed using the Hurst method. A detailed analysis of each individual event showed that in events of complete destruction of a projectile nucleus with a high multiplicity of secondary particles, long-distance multi-particle pseudo-rapidity correlations are observed. The distribution of average pseudo-rapidity in such events differs significantly from others, as it is much narrower, and its average value is noticeably shifted towards lower values <η>. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

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13 pages, 3624 KiB

Open AccessArticle

Unruh Entropy with Exponential Energy Distribution for a Spherically Symmetric Source

by Maksym Teslyk, Larissa Bravina and Evgeny Zabrodin

Particles 2024, 7(3), 634-646; https://doi.org/10.3390/particles7030036 - 22 Jul 2024

Viewed by 889

Abstract

Unruh effect entropy is estimated for a spherically symmetric source with an exponential energy distribution; angular degrees of freedom are suggested to be equally likely to contribute. Calculations are performed with an assumption about finite energy and multiplicity ranges. The result is represented [...] Read more.

Unruh effect entropy is estimated for a spherically symmetric source with an exponential energy distribution; angular degrees of freedom are suggested to be equally likely to contribute. Calculations are performed with an assumption about finite energy and multiplicity ranges. The result is represented in the units of Schwarzschild black hole entropy, with the analytical ratio being expressed analytically and generalized to homogeneous distribution over other degrees of freedom. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

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16 pages, 330 KiB

Open AccessArticle

Mass Spectrum of Noncharmed and Charmed Meson States in Extended Linear-Sigma Model

by Azar I. Ahmadov, Azzah A. Alshehri and Abdel Nasser Tawfik

Particles 2024, 7(3), 560-575; https://doi.org/10.3390/particles7030031 - 29 Jun 2024

Cited by 1 | Viewed by 830

Abstract

The mass spectrum of different meson particles is generated using an effective Lagrangian of the extended linear-sigma model (eLSM) for scalar and pseudoscalar meson fields and quark flavors, up, down, strange, and charm. Analytical formulas for the masses of scalar, pseudoscalar, vector, and [...] Read more.

The mass spectrum of different meson particles is generated using an effective Lagrangian of the extended linear-sigma model (eLSM) for scalar and pseudoscalar meson fields and quark flavors, up, down, strange, and charm. Analytical formulas for the masses of scalar, pseudoscalar, vector, and axialvector meson states are derived assuming global chiral symmetry. The various eLSM parameters are analytically deduced and numerically computed. This enables accurate estimations of the masses of sixteen noncharmed and thirteen charmed meson states at vanishing temperature. The comparison of these results to a recent compilation of the particle data group (PDG) allows us to draw the conclusion that the masses of sixteen noncharmed and thirteen charmed meson states calculated in the eLSM are in good agreement with the PDG. This shows that the eLSM, with its configurations and parameters, is an effective theoretical framework for determining the mass spectra of various noncharmed and charmed meson states, particularly at vanishing temperature. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

17 pages, 477 KiB

Open AccessArticle

Improved Equations of the Lagrange Top and Examples of Analytical Solutions

by Alexei A. Deriglazov

Particles 2024, 7(3), 543-559; https://doi.org/10.3390/particles7030030 - 24 Jun 2024

Cited by 2 | Viewed by 928

Abstract

Equations of a heavy rotating body with one fixed point can be deduced starting from a variational problem with holonomic constraints. When applying this formalism to the particular case of a Lagrange top, in the formulation with a diagonal inertia tensor the potential [...] Read more.

Equations of a heavy rotating body with one fixed point can be deduced starting from a variational problem with holonomic constraints. When applying this formalism to the particular case of a Lagrange top, in the formulation with a diagonal inertia tensor the potential energy has a more complicated form as compared with that assumed in the literature on dynamics of a rigid body. This implies the corresponding improvements in equations of motion. Therefore, we revised this case, presenting several examples of analytical solutions to the improved equations. The case of precession without nutation has a surprisingly rich relationship between the rotation and precession rates, which is discussed in detail. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

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12 pages, 504 KiB

Open AccessArticle

Quantum Gravity Effective Action Provides Entropy of the Universe

by Ken-ji Hamada

Particles 2024, 7(2), 465-476; https://doi.org/10.3390/particles7020026 - 2 May 2024

Viewed by 1531

Abstract

The effective action in the renormalizable quantum theory of gravity provides entropy because the total Hamiltonian vanishes. Since it is a renormalization group invariant that is constant in the process of cosmic evolution, we can show conservation of entropy, which is an ansatz [...] Read more.

The effective action in the renormalizable quantum theory of gravity provides entropy because the total Hamiltonian vanishes. Since it is a renormalization group invariant that is constant in the process of cosmic evolution, we can show conservation of entropy, which is an ansatz in the standard cosmology. Here, we study renormalizable quantum gravity that exhibits conformal dominance at high energy beyond the Planck scale. The current entropy of the universe is derived by calculating the effective action under the scenario of quantum gravity inflation caused by its dynamics. We then argue that ghost modes must be unphysical but are necessary for the Hamiltonian to vanish and for entropy to exist in gravitational systems. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

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30 pages, 400 KiB

Open AccessArticle

Scales and Hierarchies: Planckian Signature in Standard Model

by Davide Fiscaletti and Ignazio Licata

Particles 2024, 7(2), 435-464; https://doi.org/10.3390/particles7020025 - 22 Apr 2024

Viewed by 1424

Abstract

A model of a physical vacuum defined by a Gross–Pitaevskij equation and characterized by dissipative features close to the Planck scale is proposed, which provides an emergent explanation of scales, hierarchies and Higgs mass generation of the Standard Model. A fundamental nonlocal and [...] Read more.

A model of a physical vacuum defined by a Gross–Pitaevskij equation and characterized by dissipative features close to the Planck scale is proposed, which provides an emergent explanation of scales, hierarchies and Higgs mass generation of the Standard Model. A fundamental nonlocal and nonlinear texture of the vacuum is introduced in terms of planckeons, sub-Planckian objects defined by a generalized Compton wavelength, which lead to find Planckian signatures of the Standard Model. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

12 pages, 264 KiB

Open AccessArticle

Can Black Holes or Other Relativistic Space Objects Be a Source of Dark Energy?

by Serge Parnovsky

Particles 2024, 7(2), 297-308; https://doi.org/10.3390/particles7020018 - 29 Mar 2024

Viewed by 1117

Abstract

We consider the hypothesis that the sources of dark energy (DE) could be black holes (BHs) or more exotic objects, such as naked singularities or gravastars. We propose a definition of the presence of DE in the Universe and a criterion for what [...] Read more.

We consider the hypothesis that the sources of dark energy (DE) could be black holes (BHs) or more exotic objects, such as naked singularities or gravastars. We propose a definition of the presence of DE in the Universe and a criterion for what can be considered the source of this dark energy. It is based on the idea of the accelerated expansion of the Universe, which requires antigravity caused by large negative pressure. A recently proposed hypothesis, that the mass of BHs increases with time according to the same law as the volume of the part of the Universe containing it and the population of BHs can mimic DE, is examined. We demonstrate the reasons why it cannot be accepted, even if all the assumptions on which this hypothesis is based are considered true. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

8 pages, 241 KiB

Open AccessArticle

Cosmological Mass of the Photon Related to Stueckelberg and Higgs Mechanisms

by Lorenzo Gallerani Resca

Particles 2024, 7(2), 289-296; https://doi.org/10.3390/particles7020017 - 29 Mar 2024

Viewed by 1485

Abstract

I consider the electro-weak (EW) masses and interactions generated by photons using vacuum expectation values of Stueckelberg and Higgs fields. I provide a prescription to relate their parametric values to a cosmological range derived from the fundamental Heisenberg uncertainty principle and the Einstein–de [...] Read more.

I consider the electro-weak (EW) masses and interactions generated by photons using vacuum expectation values of Stueckelberg and Higgs fields. I provide a prescription to relate their parametric values to a cosmological range derived from the fundamental Heisenberg uncertainty principle and the Einstein–de Sitter cosmological constant and horizon. This yields qualitative connections between microscopic ranges acquired by

W^{\pm}

or

Z^{0}

gauge Bosons and the cosmological scale and minimal mass acquired by g-photons. I apply this procedure to an established Stueckelberg–Higgs mechanism, while I consider a similar procedure for a pair of Higgs fields that may spontaneously break all U(1) × SU(2) gauge invariances. My estimates of photon masses and their additional parity-breaking interactions with leptons and neutrinos may be detectable in suitable accelerator experiments. Their effects may also be observable astronomically through massive g-photon condensates that may contribute to dark matter and dark energy. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

14 pages, 338 KiB

Open AccessArticle

Z Boson Emission by a Neutrino in de Sitter Expanding Universe

by Mihaela-Andreea Băloi, Cosmin Crucean and Diana Dumitrele

Particles 2024, 7(1), 275-288; https://doi.org/10.3390/particles7010016 - 19 Mar 2024

Cited by 1 | Viewed by 1152

Abstract

The production of Z bosons in emission processes by neutrinos in the expanding de Sitter universe is studied by using perturbative methods. The total probability and transition rate for the spontaneous emission of a Z boson by a neutrino is computed analytically; then, [...] Read more.

The production of Z bosons in emission processes by neutrinos in the expanding de Sitter universe is studied by using perturbative methods. The total probability and transition rate for the spontaneous emission of a Z boson by a neutrino is computed analytically; then, we conduct a graphical analysis in terms of the expansion parameter. Our results prove that this process is possible only for large expansion conditions of the early universe. Finally, the density number of Z bosons is defined, and we obtain a quantitative estimation of this quantity in terms of the density number of neutrinos. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

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11 pages, 380 KiB

Open AccessArticle

Quantum Limit for the Emittance of Dirac Particles Carrying Orbital Angular Momentum

by Alessandro Curcio, Alessandro Cianchi and Massimo Ferrario

Particles 2024, 7(1), 264-274; https://doi.org/10.3390/particles7010015 - 17 Mar 2024

Viewed by 1318

Abstract

In this article, we highlight that the interaction potential confining Dirac particles in a box must be invariant under the charge conjugation to avoid the Klein paradox, in which an infinite number of negative-energy particles are excited. Furthermore, we derive the quantization rules [...] Read more.

In this article, we highlight that the interaction potential confining Dirac particles in a box must be invariant under the charge conjugation to avoid the Klein paradox, in which an infinite number of negative-energy particles are excited. Furthermore, we derive the quantization rules for a relativistic particle in a cylindrical box, which emulates the volume occupied by a beam of particles with a non-trivial aspect ratio. We apply our results to the evaluation of the quantum limit for emittance in particle accelerators. The developed theory allows the description of quantum beams carrying Orbital Angular Momentum (OAM). We demonstrate how the degeneracy pressure is such to increase the phase–space area of Dirac particles carrying OAM. The results dramatically differ from the classical evaluation of phase–space areas, that would foresee no increase in emittance for beams in a coherent state of OAM. We discuss the quantization of the phase–space cell’s area for single Dirac particles carrying OAM, and, finally, provide an interpretation of the beam entropy as the measure of how much the phase–space area occupied by the beam deviates from its quantum limit. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

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15 pages, 6416 KiB

Open AccessArticle

Surface Vibrations of Bubble-like Superheavy Nuclei

by Şerban Mişicu

Particles 2024, 7(1), 214-228; https://doi.org/10.3390/particles7010012 - 5 Mar 2024

Viewed by 1295

Abstract

The shape vibrations of a superheavy nucleus with a complete (bubble) or a partially (semi-bubble) depleted density in its central region and sharp-edge inner and outer surfaces are investigated in the frame of the Liquid-Drop Model. The quadrupole oscillations of the two existing [...] Read more.

The shape vibrations of a superheavy nucleus with a complete (bubble) or a partially (semi-bubble) depleted density in its central region and sharp-edge inner and outer surfaces are investigated in the frame of the Liquid-Drop Model. The quadrupole oscillations of the two existing surfaces are coupled in both velocity and coordinate and, upon decoupling, a low-energy and a high-energy component are predicted. The electric transition probabilities are estimated for the decay of the low-lying mode first

2^{+}

state to the ground state for the entire range of the radius and density of the depleted core. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

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18 pages, 1314 KiB

Open AccessArticle

A New Look at b → s Observables in 331 Models

by Francesco Loparco

Particles 2024, 7(1), 161-178; https://doi.org/10.3390/particles7010009 - 27 Feb 2024

Cited by 3 | Viewed by 1403

Abstract

Flavour changing neutral current (FCNC) processes are described by loop diagrams in the Standard Model (SM), while in 331 models, based on the gauge group

SU {(3)}_{C} \times SU {(3)}_{L} \times U {(1)}_{X}

, [...] Read more.

Flavour changing neutral current (FCNC) processes are described by loop diagrams in the Standard Model (SM), while in 331 models, based on the gauge group

SU {(3)}_{C} \times SU {(3)}_{L} \times U {(1)}_{X}

, they are dominated by tree-level exchanges of a new heavy neutral gauge boson

Z^{'}

. By exploiting this feature, observables related to FCNC decays of K,

B_{d}

and

B_{s}

mesons can be considered in several variants of 331 models. The variants are distinguished by the value of a parameter

β

that plays a key role in this framework. Imposing constraints on the

Δ F = 2

observables, we select possible ranges for the mass of the

Z^{'}

boson in correspondence to the values

β = \pm k / \sqrt{3}

, with

k = 1, 2

. The results are used to determine the impact of 331 models on

b \to s

processes and on the correlations among them, in the light of new experimental data recently released. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

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23 pages, 3800 KiB

Open AccessArticle

Numerical Modeling of Erosion in Hall Effect Thrusters

by Matteo Passet, Mario Panelli and Francesco Battista

Particles 2024, 7(1), 121-143; https://doi.org/10.3390/particles7010007 - 30 Jan 2024

Viewed by 1992

Abstract

The erosion of the accelerating chamber walls is one of the main factors limiting the operational life of Hall effect thrusters (HETs), and it is mainly related to the sputtering of ceramic walls due to the impacting energetic ion particles. The erosion phenomenon [...] Read more.

The erosion of the accelerating chamber walls is one of the main factors limiting the operational life of Hall effect thrusters (HETs), and it is mainly related to the sputtering of ceramic walls due to the impacting energetic ion particles. The erosion phenomenon is investigated by means of a numerical model that couples the plasma model HYPICFLU2, used for evaluating the local distributions of ion energies and incidence angles, and a sputtering model specific for the xenon–Borosil pair, which is the most used in HETs application. The sputtering yield model is based on the measurements by Ranjan et al. that are improved with a linear factor to include wall temperature effect, recently studied by Parida et al. The experimental eroded profiles of SPT100 walls are selected as benchmark. The results show that there is a decrease in erosion speed with time, in accordance with experimental measurements, but the model underestimates, by about 50–60%, the erosion at the channel exit, which suggests a stronger dependence of sputter yield on surface temperature. Thus, the need for new experimental measurements of sputtering in the range of impact energy, angle, and wall temperature, respectively, of 10–250 eV, 0–85°, 30–600 °C, arises. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

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25 pages, 1962 KiB

Open AccessArticle

Searching for Dark Matter Axions via Atomic Excitations

by J. D. Vergados, S. Cohen, F. T. Avignone and R. Creswick

Particles 2024, 7(1), 96-120; https://doi.org/10.3390/particles7010006 - 27 Jan 2024

Cited by 1 | Viewed by 1618

Abstract

Axions can be considered as good dark matter candidates. The detection of such light particles can be achieved by observing axion-induced atomic excitations. The target is in a magnetic field so that the m-degeneracy is removed, and the energy levels can be [...] Read more.

Axions can be considered as good dark matter candidates. The detection of such light particles can be achieved by observing axion-induced atomic excitations. The target is in a magnetic field so that the m-degeneracy is removed, and the energy levels can be suitably adjusted. Using an axion-electron coupling indicated by the limit obtained by the Borexino experiment, which is quite stringent, reasonable axion absorption rates have been obtained for various atomic targets The obtained results depend, of course, on the atom considered through the parameters

ϵ

(the spin−orbit splitting) as well as

δ

( the energy splitting due to the magnetic moment interaction). This assumption allows axion masses in the tens of

μ

eV if the transition occurs between members of the same multiplet, i.e.,

| J_{1}, M_{1} = - J_{1} ⟩ \to | J_{1}, M_{1} = - J + 1 ⟩, J_{1} \neq 0

, and axion masses in the range 1 meV–1 eV for transitions of the spin−orbit splitting type

| J_{1}, M = - J_{1} ⟩ \to | J_{2}, M_{2} = - J_{1} + q ⟩, q = - 1, 0, 1

, i.e., three types of transition. The axion mass that can be detected is very close to the excitation energy involved, which can vary by adjusting the magnetic field. Furthermore, since the axion is absorbed by the atom, the calculated cross-section exhibits the behavior of a resonance, which can be exploited by experiments to minimize any background events. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

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15 pages, 6051 KiB

Open AccessArticle

Stationary Schrödinger Equation and Darwin Term from Maximal Entropy Random Walk

by Manfried Faber

Particles 2024, 7(1), 25-39; https://doi.org/10.3390/particles7010002 - 26 Dec 2023

Cited by 1 | Viewed by 1597

Abstract

We describe particles in a potential by a special diffusion process, the maximal entropy random walk (MERW) on a lattice. Since MERW originates in a variational problem, it shares the linear algebra of Hilbert spaces with quantum mechanics. The Born rule appears from [...] Read more.

We describe particles in a potential by a special diffusion process, the maximal entropy random walk (MERW) on a lattice. Since MERW originates in a variational problem, it shares the linear algebra of Hilbert spaces with quantum mechanics. The Born rule appears from measurements between equilibrium states in the past and the same equilibrium states in the future. Introducing potentials by the observation that time, in a gravitational field running in different heights with a different speed, MERW respects the rule that all trajectories of the same duration are counted with equal probability. In this way, MERW allows us to derive the Schrödinger equation for a particle in a potential and the Darwin term of the nonrelativistic expansion of the Dirac equation. Finally, we discuss why quantum mechanics cannot be simply a result of MERW, but, due to the many analogies, MERW may pave the way for further understanding. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

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20 pages, 3853 KiB

Open AccessArticle

Order, Chaos and Born’s Distribution of Bohmian Particles

by Athanasios C. Tzemos and George Contopoulos

Particles 2023, 6(4), 923-942; https://doi.org/10.3390/particles6040060 - 1 Nov 2023

Cited by 1 | Viewed by 1286

Abstract

We study order, chaos and ergodicity in the Bohmian trajectories of a 2D quantum harmonic oscillator. We first present all the possible types (chaotic, ordered) of Bohmian trajectories in wavefunctions made of superpositions of two and three energy eigenstates of the oscillator. There [...] Read more.

We study order, chaos and ergodicity in the Bohmian trajectories of a 2D quantum harmonic oscillator. We first present all the possible types (chaotic, ordered) of Bohmian trajectories in wavefunctions made of superpositions of two and three energy eigenstates of the oscillator. There is no chaos in the case of two terms and in some cases of three terms. Then, we show the different geometries of nodal points in bipartite Bohmian systems of entangled qubits. Finally, we study multinodal wavefunctions and find that a large number of nodal points does not always imply the dominance of chaos. We show that, in some cases, the Born distribution is dominated by ordered trajectories, something that has a significant impact on the accessibility of Born’s rule

P = {| Ψ |}^{2}

by initial distributions of Bohmian particles with

P_{0} \neq {| Ψ_{0} |}^{2}

. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

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Review

Jump to: Research

30 pages, 414 KiB

Open AccessReview

CPT Symmetry Searches in the Neutral Meson System

by Ágnes Roberts

Particles 2024, 7(3), 717-746; https://doi.org/10.3390/particles7030042 - 14 Aug 2024

Viewed by 810

Abstract

A review of the landscape of CPT symmetry tests is presented, centered around the Standard-Model Extension and focusing on tests in the neutral meson system. A discussion of the relevant theories summarizes original ideas. It is followed by a short transition into phenomenology. [...] Read more.

A review of the landscape of CPT symmetry tests is presented, centered around the Standard-Model Extension and focusing on tests in the neutral meson system. A discussion of the relevant theories summarizes original ideas. It is followed by a short transition into phenomenology. A more detailed parameterization is presented. Various experiments are used to deliver an overview of testing CPT from every angle that the theory suggested and that the neutral meson (NM) system could accommodate. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

41 pages, 6388 KiB

Open AccessReview

Forward & Far-Forward Heavy Hadrons with Jethad: A High-Energy Viewpoint

by Francesco Giovanni Celiberto

Particles 2024, 7(3), 502-542; https://doi.org/10.3390/particles7030029 - 24 Jun 2024

Cited by 3 | Viewed by 855

Abstract

Inspired by recent findings that semi-inclusive detections of heavy hadrons exhibit fair stabilization patterns in high-energy resummed distributions against (missing) higher-order corrections, we review and extend our studies on the hadroproduction of light and heavy hadrons tagged in forward and far-forward rapidity ranges. [...] Read more.

Inspired by recent findings that semi-inclusive detections of heavy hadrons exhibit fair stabilization patterns in high-energy resummed distributions against (missing) higher-order corrections, we review and extend our studies on the hadroproduction of light and heavy hadrons tagged in forward and far-forward rapidity ranges. We analyze the

NLL / {NLO}^{+}

behavior of rapidity rates and angular multiplicities via the Jethad method, where the resummation of next-to-leading energy logarithms and beyond is consistently embodied in the collinear picture. We explore kinematic regions that are within LHC typical acceptances, as well as novel sectors accessible thanks the combined tagging of a far-forward light or heavy hadron at future Forward Physics Facilities and a of central particle at LHC experiments via a precise timing-coincidence setup. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

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18 pages, 1376 KiB

Open AccessReview

Feature Selection Techniques for CR Isotope Identification with the AMS-02 Experiment in Space

by Marta Borchiellini, Leandro Mano, Fernando Barão and Manuela Vecchi

Particles 2024, 7(2), 417-434; https://doi.org/10.3390/particles7020024 - 20 Apr 2024

Viewed by 1168

Abstract

Isotopic composition measurements of singly charged cosmic rays (CR) provide essential insights into CR transport in the Galaxy. The Alpha Magnetic Spectrometer (AMS-02) can identify singly charged isotopes up to about 10 GeV/n. However, their identification presents challenges due to the small abundance [...] Read more.

Isotopic composition measurements of singly charged cosmic rays (CR) provide essential insights into CR transport in the Galaxy. The Alpha Magnetic Spectrometer (AMS-02) can identify singly charged isotopes up to about 10 GeV/n. However, their identification presents challenges due to the small abundance of CR deuterons compared to the proton background. In particular, a high accuracy for the velocity measured by a ring-imaging Cherenkov detector (RICH) is needed to achieve a good isotopic mass separation over a wide range of energies. The velocity measurement with the RICH is particularly challenging for

Z = 1

isotopes due to the low number of photons produced in the Cherenkov rings. This faint signal is easily disrupted by noisy hits leading to a misreconstruction of the particles’ ring. Hence, an efficient background reduction process is needed to ensure the quality of the reconstructed Cherenkov rings and provide a correct measurement of the particles’ velocity. Machine learning methods, particularly boosted decision trees, are well suited for this task, but their performance relies on the choice of the features needed for their training phase. While physics-driven feature selection methods based on the knowledge of the detector are often used, machine learning algorithms for automated feature selection can provide a helpful alternative that optimises the classification method’s performance. We compare five algorithms for selecting the feature samples for RICH background reduction, achieving the best results with the Random Forest method. We also test its performance against the physics-driven selection method, obtaining better results. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

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55 pages, 652 KiB

Open AccessReview

Hadronic Light-by-Light Corrections to the Muon Anomalous Magnetic Moment

by Daniel Melo, Edilson Reyes and Raffaele Fazio

Particles 2024, 7(2), 327-381; https://doi.org/10.3390/particles7020020 - 10 Apr 2024

Cited by 1 | Viewed by 1387

Abstract

We review the hadronic light-by-light (HLbL) contribution to the muon anomalous magnetic moment. Upcoming measurements will reduce the experimental uncertainty of this observable by a factor of four; therefore, the theoretical precision must improve accordingly to fully harness such an experimental breakthrough. With [...] Read more.

We review the hadronic light-by-light (HLbL) contribution to the muon anomalous magnetic moment. Upcoming measurements will reduce the experimental uncertainty of this observable by a factor of four; therefore, the theoretical precision must improve accordingly to fully harness such an experimental breakthrough. With regards to the HLbL contribution, this implies a study of the high-energy intermediate states that are neglected in dispersive estimates. We focus on the maximally symmetric high-energy regime and in-quark loop approximation of perturbation theory, following the method of the OPE with background fields proposed by Bijnens et al. in 2019 and 2020. We confirm their results regarding the contributions to the muon

g - 2

. For this, we use an alternative computational method based on a reduction in the full quark loop amplitude, instead of projecting on a supposedly complete system of tensor structures motivated by first principles. Concerning scalar coefficients, mass corrections have been obtained by hypergeometric representations of Mellin–Barnes integrals. By our technique, the completeness of such kinematic singularity/zero-free tensor decomposition of the HLbL amplitude is explicitly checked. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

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18 pages, 348 KiB

Open AccessReview

Review on Minimally Extended Varying Speed of Light Model

by Seokcheon Lee

Particles 2024, 7(2), 309-326; https://doi.org/10.3390/particles7020019 - 9 Apr 2024

Cited by 4 | Viewed by 1890

Abstract

It is known that dimensional constants, such as ℏ, c, G, e, and k, are merely human constructs whose values and units vary depending on the chosen system of measurement. Therefore, the time variations in dimensional constants lack [...] Read more.

It is known that dimensional constants, such as ℏ, c, G, e, and k, are merely human constructs whose values and units vary depending on the chosen system of measurement. Therefore, the time variations in dimensional constants lack operational significance due to their dependence on these dimensional constants. They are well structured and represent a valid discussion. However, this fact only becomes a meaningful debate within the context of a static or present Universe. As theoretically and observationally well established, the current Universe is undergoing accelerated expansion, wherein dimensional quantities, like the wavelength of light, also experience redshift phenomena elongating over cosmic time. In other words, in an expanding Universe, dimensional quantities of physical parameters vary with cosmic time. From this perspective, there exists the possibility that dimensional constants, such as the speed of light, could vary with the expansion of the Universe. In this review paper, we contemplate under what circumstances the speed of light may change or remain constant over cosmic time and discuss the potential for distinguishing these cases observationally. Full article

(This article belongs to the Special Issue Feature Papers for Particles 2023)

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