Particles doi: 10.3390/particles1010025

Authors: Ken-ichi Nanbu Yuki Saito Hirotoshi Saito Shigeru Kashiwagi Fujio Hinode Toshiya Muto Hiroyuki Hama

The temporal profile measurement for ultra-short electron bunches is one of the key issues for accelerator-based coherent light sources. A bunch length measurement system using Cherenkov radiation (CR) is under development at the Research Center for Electron Photon Science, Tohoku University. This system allows for the real-time diagnostics of electron bunches. The system comprises a thin silica aerogel as the Cherenkov radiator, a specially designed optical transport line, and a high-speed streak camera. The electron bunch length can be obtained by measuring the time spread of the CR from the electrons passing through the radiator medium using the streak camera. In this paper, we describe the novel bunch length measurement system using CR, discuss the expected time resolution of the system, and finally present the measurement results.

]]>Particles doi: 10.3390/particles1010024

Authors: Ludwik Turko

The fixed-target NA61/SHINE experiment (SPS CERN) looks for the critical point (CP) of strongly interacting matter and the properties of the onset of deconfinement. It is a scan of measurements of particle spectra and fluctuations in proton–proton, proton–nucleus, and nucleus–nucleus interactions as a function of collision energy and system size. This gives unique possibilities to researching critical properties of the dense hot hadronic matter created in the collision process. New measurements and their objectives, related to the third stage of the experiment after 2020, are presented and discussed here.

]]>Particles doi: 10.3390/particles1010023

Authors: Vladimir Morozov Vasyl’ Ignatyuk

The master equation for an open quantum system is derived in the weak-coupling approximation when the additional dynamical variable&mdash;the mean interaction energy&mdash;is included into the generic relevant statistical operator. This master equation is nonlocal in time and involves the &ldquo;quasi-temperature&rdquo;, which is a non- equilibrium state parameter conjugated thermodynamically to the mean interaction energy of the composite system. The evolution equation for the quasi-temperature is derived using the energy conservation law. Thus long-living dynamical correlations, which are associated with this conservation law and play an important role in transition to the Markovian regime and subsequent equilibration of the system, are properly taken into account.

]]>Particles doi: 10.3390/particles1010022

Authors: Shimin Jiang Weiwei Li Zhigang He Qika Jia

A new type of dielectric-loaded waveguide, the high-gradient dielectric-loaded waveguide (HG-DLW), where the Cherenkov radiation with a high gradient can be excited by relativistic electron, is proposed in this paper. Based on the simulation results, the process of the high-gradient Cherenkov radiation excited in the proposed structure is studied in details, and the amplitude of wakefields excited in proposed structure can be enhanced by over six times compared with that from ordinary dielectric-loaded waveguides.

]]>Particles doi: 10.3390/particles1010021

Authors: Ruixuan Huang Weiwei Li Zhouyu Zhao Heting Li Jigang Wang Tian Ma Qiuping Huang Zhigang He Qika Jia Lin Wang Yalin Lu

Terahertz (THz) radiation has attracted much attention in new scientific and industrial applications. There has been significant recent progress in generating THz with accelerators. To investigate the collective behavior of electron dynamics, we have proposed a new high throughput material characterization system, which supplies a multiple light source. The system includes a pre-bunched THz free electron laser (FEL), which is a high-power narrow-band THz source with a wide tuning range of frequency. The physical design with the main components of the facility is introduced, and the simulation results are illustrated. Radiation of 0.5&ndash;3.0 THz is obtained by the fundamental wave of the pre-bunched beam, and radiation covering 3.0&ndash;5.0 THz is realized by second harmonic generation. As the simulation shows, intense THz radiation could be achieved in a frequency from 0.5&ndash;5.0 THz, with a peak power of several megawatts (MWs) and a bandwidth of a few percent.

]]>Particles doi: 10.3390/particles1010020

Authors: Kaichun Zhang Ke Chen Qian Xu Wangju Xu Neng Xiong Xiaoxing Chen Diwei Liu

The analytical expressions of the beam&ndash;wave coupling coefficients and the beam loading conductance for a 2&pi; mode in a multi-gap cavity is proposed as a circuit of the extended interaction klystron (EIK), are derived by space-charge wave theory. The mechanism of the beam&ndash;wave synchronization and the coupling in the multi-gap cavity at 225 GHz are studied in detail by calculating the coupling coefficient and the normalized beam loading conductance as a function of gap number, gap dimension, and beam voltage as well as the perveance. The stability of the circuit is analyzed by considering the quality factor of the electron beam. It is found that the stability of the operating 2&pi; mode is more sensitive to the beam voltage and gap number. Based on the theory and analysis, a 5-gap coupled cavity is proposed as a section of EIK&rsquo;s circuit. A low voltage EIK with a 4-cavity circuit at 225 GHz is designed and is simulated by a particle-in-cell (PIC) code. The EIK can achieve a maximum output power of ~36 W with more than 30 dB gain at 225 GHz.

]]>Particles doi: 10.3390/particles1010019

Authors: Linbo Liang Weihao Liu Qika Jia Yalin Lu

We propose and design a dual dielectric grating structure for generating electron bunches with THz repetition frequency. Here we apply transversally polarized sub-terahertz waves to transversally illuminate a dielectric dual-grating, within which the sinusoidally varied deflection fields are induced and exerted on the electron bunches passing through the structure. The velocities of electrons within the bunches are modulated in the transversal direction, which then leads to the generation of a train of micro-bunches. The spectra of these micro-bunches have a series of profound high harmonics in the terahertz region. The simulation result of the electron beam’s bunching behavior is discussed in this paper.

]]>Particles doi: 10.3390/particles1010018

Authors: Siriwan Krainara Shuya Chatani Heishun Zen Toshiteru Kii Hideaki Ohgaki

A THz coherent undulator radiation (THz-CUR) source has been developed at the Institute of Advanced Energy, Kyoto University. A photocathode Radio-Frequency (RF) gun and a bunch compressor chicane are used for generating short-bunch electron beams. When the electron beam energy is low, the space-charge effect strongly degrades the beam quality, such as the bunch length and the energy spread at the high bunch charge condition at around 160 pC, and results in the reduction of the highest frequency and the maximum radiated power of the THz-CUR. To mitigate the space charge effect, we have investigated the dependence of the electron beam quality on the laser distribution in transverse and longitudinal directions by using a numerical simulation code, General Particle Tracer GPT. The manipulation of the laser distribution has potential for improving the performance of the THz-CUR source. The electron bunch was effectively compressed with the chicane magnet when the laser transverse distribution was the truncated Gaussian profile, illuminating a cathode. Moreover, the compressed electron bunch was shortened by enlarging the laser pulse width. Consequently, an enhancement of the radiated power of the THz-CUR has been indicated.

]]>Particles doi: 10.3390/particles1010017

Authors: Alessandro Pesci

The effects on Raychaudhuri&rsquo;s equation of an intrinsically-discrete or particle nature of spacetime are investigated. This is done through the consideration of null congruences emerging from, or converging to, a generic point of spacetime, i.e., in geometric circumstances somehow prototypical of singularity issues. We do this from an effective point of view, that is through a (continuous) description of spacetime modified to embody the existence of an intrinsic discreteness on the small scale, this adding to previous results for non-null congruences. Various expressions for the effective rate of change of expansion are derived. They in particular provide finite values for the limiting effective expansion and its rate of variation when approaching the focal point. Further, this results in a non-vanishing of the limiting cross-sectional area itself of the congruence.

]]>Particles doi: 10.3390/particles1010016

Authors: Arus Harutyunyan Armen Sedrakian

We provide a discussion of the bulk viscosity of two-flavor quark plasma, described by the Nambu&ndash;Jona-Lasinio model, within the framework of Kubo-Zubarev formalism. This discussion, which is complementary to our earlier study, contains a new, detailed derivation of the bulk viscosity in the case of multiple conserved charges. We also provide some numerical details of the computation of the bulk viscosity close to the Mott transition line, where the dissipation is dominated by decays of mesons into quarks and their inverse processes. We close with a summary of our current understanding of this quantity, which stresses the importance of loop resummation for obtaining the qualitatively correct result near the Mott line.

]]>Particles doi: 10.3390/particles1010015

Authors: Levon Sh. Grigorian Hrant F. Khachatryan Aram A. Saharian

We investigate static spherically-symmetric configurations of gravitating masses in the bimetric scalar-tensor theory of gravitation. In the gravitational sector, the theory contains the metric tensor, a scalar field and a background metric as an absolute variable of the theory. The analysis is presented for the simplest version of the theory with a constant coupling function and a zero cosmological function. We show that, depending on the value of the theory parameter, the masses for superdense compact configurations can be essentially larger compared to the configurations in general relativity.

]]>Particles doi: 10.3390/particles1010014

Authors: Dmitry Ofengeim Dmitry Zyuzin

We reanalyse the X-ray spectrum of the PSR B0833–45 (the Vela pulsar) using the data of the Chandra space observatory. In contrast to previous works, we consider a wide range of possible masses and radii of the pulsar. The derived surface temperature of the star Ts∞=0.66−0.01+0.04MK (1σ level over the entire mass and radius range of our study) is consistent with earlier results. However, the preferable values of Vela’s mass and radius given by the spectral analysis are different from those used previously; they are consistent with modern equation of state models of neutron star matter. In addition, we evaluate the Vela’s surface temperature as a function of assumed values of its mass and radius. This allows us to analyse the neutrino cooling rates consistent with the evaluated surface temperatures and explore the additional restrictions that could be set on the Vela’s mass and radius using different versions of the neutron star cooling theory.

]]>Particles doi: 10.3390/particles1010013

Authors: Aram Saharian Vardan Manukyan Nvard Saharyan

We investigate the influence of a generalized cosmic string in (D+1)-dimensional spacetime on the local characteristics of the electromagnetic vacuum. Two special cases are considered with flat and locally de Sitter background geometries. The topological contributions in the vacuum expectation values (VEVs) of the squared electric and magnetic fields are explicitly separated. Depending on the number of spatial dimensions and on the planar angle deficit induced by the cosmic string, these contributions can be either negative or positive. In the case of the flat bulk, the VEV of the energy–momentum tensor is evaluated as well. For the locally de Sitter bulk, the influence of the background gravitational field essentially changes the behavior of the vacuum densities at distances from the string larger than the curvature radius of the spacetime.

]]>Particles doi: 10.3390/particles1010012

Authors: Nobuyuki Nishimori Ryoji Nagai Masaru Sawamura Ryoichi Hajima

We have developed a photocathode dc gun for a compact Smith-Purcell free-electron laser in the terahertz wavelength region. The gun system consists of an alkali antimonide photocathode preparation chamber, a dc gun with a 250 kV-50 mA Cockcroft-Walton high-voltage power supply, and a downstream beamline with a water-cooled beam dump to accommodate a beam power of 5 kW. We fabricated a Cs3Sb photocathode with quantum efficiency of 5.8% at a wavelength of 532 nm and generated a 150 keV beam with current of up to 4.3 mA with a 500 mW laser. A vacuum chamber for the Smith-Purcell free-electron laser has been installed in the downstream beamline. We describe the present status of our work.

]]>Particles doi: 10.3390/particles1010011

Authors: Arus Harutyunyan Armen Sedrakian Dirk Rischke

We present a new derivation of second-order relativistic dissipative fluid dynamics for quantum systems using Zubarev’s formalism for the non-equilibrium statistical operator. In particular, we discuss the shear-stress tensor to second order in gradients and argue that the relaxation terms for the dissipative quantities arise from memory effects contained in the statistical operator. We also identify new transport coefficients which describe the relaxation of dissipative processes to second order and express them in terms of equilibrium correlation functions, thus establishing Kubo-type formulae for the second-order transport coefficients.

]]>Particles doi: 10.3390/particles1010010

Authors: Matt Visser

Some 67 years ago (1951), Wolfgang Pauli noted that the net zero-point energy density could be set to zero by a carefully fine-tuned cancellation between bosons and fermions. In the current article, I will argue in a slightly different direction: the zero-point energy density is only one component of the zero-point stress energy tensor, and it is this tensor quantity that is in many ways the more fundamental object of interest. I shall demonstrate that Lorentz invariance of the zero-point stress energy tensor implies finiteness of the zero-point stress energy tensor, and vice versa. Under certain circumstances (in particular, but not limited to, the finite quantum field theories (QFTs)), Pauli’s cancellation mechanism will survive the introduction of particle interactions. I shall then relate the discussion to beyond standard model (BSM) physics, to the cosmological constant, and to Sakharov-style induced gravity.

]]>Particles doi: 10.3390/particles1010009

Authors: Robert Ajvazyan John Annand Dimiter Balabanski Nersik Grigoryan Vanik Kakoyan Patrik Khachatryan Vachik Khachatryan Kenneth Livingston Rachel Montgomery Henrik Vardanyan Branislav Vlahovic Simon Zhamkochyan Amur Margaryan

An active target is being developed to be used in low-energy nuclear astrophysics experiments. It is a position- and time-sensitive detector system based on the low-pressure Multi Wire Proportional Chamber (MWPC) technique. Methylal ((OCH3)2CH2), at a pressure of a few Torr, serves as the working gas for MWPC operation, and in addition, the oxygen atoms of the methylal molecules serve as an experimental target. The main advantage of this new target detector system is that it has high sensitivity to the low-energy, highly-ionizing particles produced after photodisintegration of 16O and insensitivity to γ-rays and minimum ionizing particles. This allows users to detect only the products of the nuclear reaction of interest. The threshold energies for detection of α particles and 12C nuclei are about 50 keV and 100 keV, respectively. The main disadvantage of this detector is the small target thickness, which is around a few tens of μg/cm2. However, reasonable luminosity can be achieved by using a multimodule detector system and an intense, Laser Compton Backscattered (LCB) γ-ray beam. This paper summarizes the architecture of the active target and reports test results of the prototype detector. The tests investigated the timing and position resolutions of 30 × 30 mm2 low-pressure MWPC units using an α-particle source. The possibility of measuring the 16O(γ, α)12C cross-section in the 8–10 MeV energy region by using a LCB γ-ray beam is also discussed. A measurement of the 16O(γ, α)12C cross-section will enable the reaction rate of 12C(α, γ)16O to be determined with significantly improved precision compared to previous experiments.

]]>Particles doi: 10.3390/particles1010008

Authors: Monika Sinha

Magnetars have been observationally determined to have surface magnetic fields of order of 10 14 – 10 15 G, and the implied internal field strength may be even larger. We discuss the effect of strong field on the dense matter expected to be inside neutron stars. We describe the microphysics, phenomenology, and astrophysical implications of strong field induced unpairing effect that may occur in magnetars, if the local magnetic field in the core of a magnetar exceeds a critical value. The density dependence of the pairing of proton condensate implies that the critical value required for the unpairing effect to occur is maximal at the crust–core interface and decreases towards the center of the star. As a consequence, magnetar cores with homogeneous constant fields will be partially superconducting for “medium-field” magnetars, whereas “strong-field” magnetars will be void of superconductivity. We also discuss its effect on some observational phenomena which depend on the nature and composition of matter inside neutron stars.

]]>Particles doi: 10.3390/particles1010007

Authors: Ernazar Abdikamalov César Huete Ayan Nussupbekov Shapagat Berdibek

Convective instabilities in the advanced stages of nuclear shell burning can play an important role in neutrino-driven supernova explosions. In our previous work, we studied the interaction of vorticity and entropy waves with the supernova shock using a linear perturbations theory. In this paper, we extend our work by studying the effect of acoustic waves. As the acoustic waves cross the shock, the perturbed shock induces a field of entropy and vorticity waves in the post-shock flow. We find that, even when the upstream flow is assumed to be dominated by sonic perturbations, the shock-generated vorticity waves contain most of the turbulent kinetic energy in the post-shock region, while the entropy waves produced behind the shock are responsible for most of the density perturbations. The entropy perturbations are expected to become buoyant as a response to the gravity force and then generate additional turbulence in the post-shock region. This leads to a modest reduction of the critical neutrino luminosity necessary for producing an explosion, which we estimate to be less than ~5%.

]]>Particles doi: 10.3390/particles1010006

Authors: Mikael Maior de Sousa Rubens Freire Ribeiro Eugênio Bezerra de Mello

In this paper, we calculate the vacuum expectation value of the azimuthal fermionic current, associated with a massive fermionic quantum field in the spacetime of an idealized cosmic string, considering the presence of a magnetic tube of radius a, coaxial to the string. In this analysis three distinct configurations of magnetic field are considered: (i) a magnetic field concentrated on a surface of the tube; (ii) a magnetic field presenting a 1 / r radial dependence; and (iii) an homogeneous magnetic field. In order to develop this analysis , we construct the complete set of normalized solution of the Dirac equation in the region outside the tube. By using the mode-sum formula, we show that the azimuthal induced current is formed by two contributions: the first being the current induced by a line of magnetic flux running along the string, and the second, named core-induced current, is induced by the non-vanishing extension of the magnetic tube. The first contribution depends only on the fractional part of the ration of the magnetic flux inside the tube by the quantum one; as to the second contribution, it depends on the total magnetic flux. We specifically analyze the core-induced current in several limits of the parameters and distance to the tube.

]]>Particles doi: 10.3390/particles1010005

Authors: Kirill Bronnikov

We review nonsingular static, spherically symmetric solutions of general relativity with minimally coupled scalar fields. Considered are wormholes and regular black holes (BHs) without a center, including black universes (BHs with expanding cosmology beyond the horizon). Such configurations require a “ghost” field with negative kinetic energy K. Ghosts can be invisible under usual conditions if K &lt; 0 only in strong-field region (“trapped ghost”), or they rapidly decay at large radii. Before discussing particular examples, some general results are presented, such as the necessity of anisotropic matter for asymptotically flat or AdS wormholes, no-hair and global structure theorems for BHs with scalar fields. The stability properties of scalar wormholes and regular BHs under spherical perturbations are discussed. It is stressed that the effective potential V eff for perturbations has universal shapes near generic wormhole throats (a positive pole regularizable by a Darboux transformation) and near transition surfaces from canonical to ghost scalar field behavior (a negative pole at which the perturbation finiteness requirement plays a stabilizing role). Positive poles of V eff emerging at “long throats” (with the radius r ≈ r 0 + const · x 2 n , n &gt; 1 , x = 0 is the throat) may be regularized by repeated Darboux transformations for some values of n.

]]>Particles doi: 10.3390/particles1010004

Authors: Sergey Pavluchenko

We report the results of a study on the dynamical compactification of spatially flat cosmological models in Einstein–Gauss–Bonnet gravity. The analysis was performed in the arbitrary dimension in order to be more general. We consider both vacuum and Λ -term cases. Our results suggest that for vacuum case, realistic compactification into the Kasner (power law) regime occurs with any number of dimensions (D), while the compactification into the exponential solution occurs only for D ⩾ 2 . For the Λ -term case only compactification into the exponential solution exists, and it only occurs for D ⩾ 2 as well. Our results, combined with the bounds on Gauss–Bonnet coupling and the Λ -term ( α , Λ , respectively) from other considerations, allow for the tightening of the existing constraints and forbid α &lt; 0 .

]]>Particles doi: 10.3390/particles1010003

Authors: Arpine Piloyan Sergey Pavluchenko Luca Amendola

In this paper we perform a reconstruction of the scalar field potential responsible for cosmic acceleration using SNe Ia data. After describing the method, we test it with real SNe Ia data—Union2.1 and JLA SNe datasets. We demonstrate that with the current data precision level, the full reconstruction is not possible. We discuss the problems which arise during the reconstruction process and the ways to overcome them.

]]>Particles doi: 10.3390/particles1010002

Authors: Stefan Typel

Relativistic mean-field models are successfully used for the description of finite nuclei and nuclear matter. Approaches with density-dependent meson-nucleon couplings assume specific functional forms and a dependence on vector densities in most cases. In this work, parametrizations with a larger sample of functions and dependencies on vector and scalar densities are investigated. They are obtained from fitting properties of finite nuclei. The quality of the description of nuclei and the obtained equations of state of symmetric nuclear matter and neutron matter below saturation are very similar. However, characteristic nuclear matter parameters, the equations of state and the symmetry energy at suprasaturation densities show some correlations with the choice of the density dependence and functional form of the couplings. Conditions are identified that can lead to problems for some of the parametrizations.

]]>Particles doi: 10.3390/particles1010001

Authors: Roy Lacey

Particles, a new online, open access journal, has been launched to facilitate the publication of original research papers, review articles, and communications of new results and progress in a broad set of subfields which span nuclear and particle physics, experimental/theoretical high-energy physics and astrophysics.[...]

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