Particle Density Fluctuations Research Articles

Particle Production and Density Fluctuations of Non-classical Inflaton in Coherent Squeezed Vacuum State of Flat FRW Universe

Particle Production and Density Fluctuations of Non-classical Inflaton in Coherent Squeezed Vacuum State of Flat FRW Universe

Scaling properties of particle density fluctuations at LHC energies

The availability of high-multiplicity events at LHC energies provides a unique opportunity to examine the nature of phase-transition (PT) in heavy-ion collisions. Within the framework of HYDJET++ model, a systematic study of particle density fluctuations in narrow phase-space bins in Pb-Pb collisions at and 5.02 TeV energies is performed using the method of scaled factorial moments (SFM). The findings reveal the presence of intermittency in the 2-dimensional (2D) phase space distribution of charged particles at the LHC energies. The anomalous fractal dimensions, d q , are observed to increase with the order of the moments q, suggesting the multifractal nature of the charged particle production. The parameter λ q is found to exhibit a monotonically decreasing trend with the order of the moments but with no clear minima in λ q values. Furthermore, the value of the critical exponent is estimated and compared with those predicted by other models reported at RHIC and LHC energies. The value of the critical exponent, ν, obtained in the present study, shows significant departure from the value ∼1.304, as expected for the Ginzberg-Landau (GL) type second-order PT but exhibits a good level of agreement with the results obtained by other models at LHC energies.

Study of Time Correlation and Memory Function for the Derivation of Spectral Density in Plasma

This paper is mainly focused on studying time correlation and memory functions in order to determine the spectral density in plasma. The research aims to understand and quantify the noise produced in the plasma as a result of random processes. Noise can arise from various sources, including fluctuations in particle density, temperature, and other plasma parameters. To ensure a fair comparison, the paper appears to keep the cross-sectional area of the plasma which is same for both the Q-Machine and Fusion Reactor. This means that any observed differences in spectral density can be attributed to the inherent characteristics of the two systems rather than differences in plasma volume. It is observed that Fusion Reactor produced more noise in comparison to the Q-Machine. This research seems to contribute to our understanding of plasma physics, specifically in the context of spectral density and noise production, by comparing two different plasma systems. The observed differences in noise levels between the Q-Machine and Fusion Reactor could have important implications for plasma-based technologies and fusion research.

Phase separation of passive particles in active liquids.

The transport properties of colloidal particles in active liquids have been studied extensively. It has led to a deeper understanding of the interactions between passive and active particles. However, the phase behavior of colloidal particles in active media has received little attention. Here, we present a combined experimental and numerical investigation of passive colloids dispersed in suspensions of active particles. Our study reveals dynamic clustering of colloids in active media due to an interplay of activity and attractive effective potential between the colloids. The strength of the effective potential is set by the size ratio of passive particles to the active ones. As the relative size of the passive particles increases, the effective potential becomes stronger and the average size of the clusters grows. The simulations reveal a macroscopic phase separation at sufficiently large size ratios. We will discuss the effect of density fluctuations of active particles on the nature of effective interactions between passive ones.

SPECIFIC ACOUSTIC-GRAVITY WAVE MODES IN ISOTHERMAL ATMOSPHERE

In the paper, we show that the spectrum of acoustic-gravity waves in an isothermal atmosphere includes four specific evanescent modes. These modes are the solutions of the system of hydrodynamic equations for small atmospheric disturbances under the assumption that one of the quantities (horizontal or vertical components of particle velocity, density fluctuations, or temperature) is equal to zero. Three of the four specific modes (the Lamb wave, the Brunt-Väisälä oscillation, and the f-mode) are well known, but they were previously obtained as independent solutions. The recent discovery by the authors of the evanescent γ-mode made it possible to show that all four specified modes form a certain family of special modes of the isothermal atmosphere. On the spectral diagram of the frequency and the wave vector, there are four dispersion curves of these special modes in which one of the perturbed quantities is equal to zero. These curves belong to the evanescent region of the acoustic-gravity wave spectrum. They intersect each other at five points. It is shown that the specific modes cannot interact at the intersection points. The polarization ratios between two perturbed quantities have a different sign on either side of a particular curve if one of the quantities on this curve is zero. These properties can be used as indicators of the specific modes in experimental studies of the evanescent spectrum of AGWs. By using polarization relations, the possibility of observing these modes in the Earth’s atmosphere and on the Sun is also analyzed.

Periodicities and Plasma Density Structure of Jupiter's Dawnside Magnetosphere

AbstractAbility to quantify variations in magnetic field topology and density within Jupiter's magnetosphere is an important step in understanding the overall structure and dynamics. The Juno spacecraft has provided a rich data set in the dawnside magnetosphere. The recent Grid Agnostic MHD for Extended Research Applications (GAMERA) global simulation study by Zhang et al. (2021, https://doi.org/10.1126/sciadv.abd1204) showed a highly structured plasmadisc with closed magnetic field lines mapped between the outer dawn‐tail flank and the high‐latitude polar region. To test these model predictions, we examined Juno's magnetic field data and electron/energetic particle data to categorize portions of orbits 1–15 into one of three regions based on plasma confinement: the flux pileup region, the intermediate region, and the plasmadisc region. For each region we examined periodicities from magnetic field fluctuations and particle density fluctuations on the 1–10 hr time scale. Periodicities on this time scale could relate to internal (e.g., plasmadisc structure) or external processes (e.g., Kelvin‐Helmholtz vortices). Similar analysis was performed on the GAMERA simulation with the data split into two regions, an outer (150 > R > 60) region and an inner (R < 60) region. Finally, using published density moments from Huscher et al. (2021, https://doi.org/10.1029/2021JA029446), we compared the relative density variations of the Juno moments and the GAMERA simulation to further understand the overall structure and dynamics of the plasmadisc. The agreement between data and simulation supports the existence of such a highly structured plasmadisc.

Conditions for the propulsion of a colloid surrounded by a mesoscale phase separation.

We study a two-dimensional model of an active isotropic colloid whose propulsion is linked to the interactions between solute particles of the bath. The colloid catalyzes a chemical reaction in its vicinity, that yields a local phase separation of solute particles. The density fluctuations of solute particles result in the enhanced diffusion of the colloid. Using numerical simulations, we thoroughly investigate the conditions under which activity occurs, and we establish a state diagram for the activity of the colloid as a function of the parameters of the model. We use the generated data to unravel a key observable that controls the existence and the intensity of activity: The filling fraction of the reaction area. Remarkably, we finally show that propulsion also occurs in three-dimensional geometries, which confirms the interest of this mechanism for experimental applications.

Kinetic theory of particle-in-cell simulation plasma and the ensemble averaging technique

We derive the kinetic theory of fluctuations in physically and numerically stable particle-in-cell (PIC) simulations of electrostatic plasmas. The starting point is the single-time correlation at the start of the simulation between the statistical fluctuations of the weighted densities of macroparticle centers in the plasma particle phase-space. The fluctuations are associated with different initial conditions, typically due to the random initial conditions (in velocity space) of the macroparticles/simulation plasma, assigned according to their initial distribution of probability. The single-time correlations at all time steps and in each spatial grid cell are then determined from the Laplace–Fourier transforms of the discretized Klimontovich-like equation for the macroparticles and Maxwell’s equations for the fields, as computed by modern PIC codes. We recover the expressions for the electrostatic field and the plasma particle density fluctuation autocorrelation spectra as well as the kinetic equations describing the average evolution of PIC-simulated plasma particles, first derived by Langdon (1970b Proc. 4th Conf. Numerical Simulation of Plasmas) using a test macroparticle approach perturbing a discretized Vlasovian plasma and then averaging the obtained physical quantity over the initial macroparticle velocity distribution. We generalize and extend these results to the modern algorithms in PIC codes using arbitrary macroparticle weights. Analytical estimates of statistical fluctuation amplitudes are derived as a function of the plasma simulation parameters, using the central limit theorem in the limit of a large number of macroparticles per cell. The theory is then used to analyze the ensemble averaging technique of PIC simulations where statistical averages are performed over ensembles of PIC simulations, modeling the same plasma physics problem but using different statistical realizations of the initial distribution functions of the macroparticles. This method is illustrated by linear Landau damping uncovering (from noise, which is usually considered numerical) the physical fluctuations driven by a single small amplitude electrostatic wave perturbing a PIC simulation plasma in equilibrium.

Density and Velocity Fluctuations of Alpha Particles in Magnetic Switchbacks

Magnetic switchbacks, or sudden reversals in the magnetic field’s radial direction, are one of the more striking observations of the Parker Solar Probe (PSP) in its mission thus far. While their precise production mechanisms are still unknown, the two main theories are via interchange reconnection events and in situ generation. In this work, density and abundance variations of alpha particles are studied inside and outside individual switchbacks. We find no consistent compositional differences in the alpha particle abundance ratio, n α p , inside versus outside switchbacks, nor do we observe any signature when separating the switchbacks according to V α p /V pw , the ratio of the alpha–proton differential speed to the wave phase speed (the speed at which the switchback is traveling). We argue that these measurements cannot be used to rule in favor of one production mechanism over the other, due to the distance between PSP and the postulated interchange reconnection events. In addition, we examine the 3D velocity fluctuations of protons and alpha particles within individual switchbacks. While switchbacks are always associated with increases in proton velocity, alpha velocities may be enhanced, unchanged, or decrease. This is due to the interplay between V pw and V α p , with the Alfvénic motion of the alpha particles vanishing as the difference ∣V pw – V α p ∣ decreases. We show how the Alfvénic motion of both the alphas and the protons through switchbacks can be understood as an approximately rigid arm rotation about the location of the wave frame, and illustrate that the wave frame can therefore be estimated using particle measurements alone, via sphere fitting.

Numerical turbulence simulations of intermittent fluctuations in the scrape-off layer of magnetized plasmas

Intermittent fluctuations in the boundary of magnetically confined plasmas are investigated by numerical turbulence simulations of a reduced fluid model describing the evolution of the plasma density and electric drift vorticity in the two-dimensional plane perpendicular to the magnetic field. Two different cases are considered: one describing resistive drift waves in the edge region and another including only the interchange instability due to unfavorable magnetic field curvature in the scrape-off layer. Analysis of long data time series obtained by single-point recordings is compared to predictions of a stochastic model describing the plasma fluctuations as a superposition of uncorrelated pulses. For both cases investigated, the radial particle density profile in the scrape-off layer is exponential with a radially constant scale length. The probability density function for the particle density fluctuations in the far scrape-off layer has an exponential tail. Radial motion of blob-like structures leads to large-amplitude bursts with an exponential distribution of peak amplitudes and the waiting times between them. The average burst shape is well described by a two-sided exponential function. The frequency power spectral density of the particle density is simply that of the average burst shape and is the same for all radial positions in the scrape-off layer. The fluctuation statistics obtained from the numerical simulations are in excellent agreement with recent experimental measurements on magnetically confined plasmas. The statistical framework defines a new validation metric for boundary turbulence simulations.

Parallel temperature interfaces in the Katz–Lebowitz–Spohn driven lattice gas

We explore a variant of the Katz–Lebowitz–Spohn (KLS) driven lattice gas in two dimensions, where the lattice is split into two regions that are coupled to heat baths with distinct temperatures. The geometry was arranged such that the temperature boundaries are oriented parallel to the external particle drive and resulting net current. We have explored the changes in the dynamical behavior that are induced by our choice of the hopping rates across the temperature boundaries. If these hopping rates at the interfaces satisfy particle-hole symmetry, the current difference across them generates a vector flow diagram akin to a vortex sheet. We have studied the finite-size scaling of the particle density fluctuations in both temperature regions, and observed that it is controlled by the respective temperature values. Specifically, if the colder subsystem is maintained at the KLS critical temperature, while the hotter subsystem’s temperature is set much higher, the interface current greatly suppresses particle exchange between the two regions. As a result of the ensuing effective subsystem decoupling, strong fluctuations persist in the critical region, whence the particle density fluctuations scale with the KLS critical exponents. However, if both temperatures are set well above the critical temperature, the particle density fluctuations scale according to the totally asymmetric exclusion process. In addition, we have measured the entropy production rate in both subsystems; it displays intriguing algebraic decay in the critical region, while it saturates quickly at a small but non-zero level in the hotter region. We have also considered another possible choice of the hopping rates across the temperature interfaces that explicitly breaks particle-hole symmetry. In that case the boundary rates induce a net particle flux across the interfaces that displays power-law behavior, until ultimately the particle exclusion constraints generate a clogging transition to an inert state.

Investigation of centrality dependence of dynamical fluctuations in narrow pseudo-rapidity interval on event-by-event basis

A detailed study of centrality dependence of event-by-event fluctuations of maximum particle density of the produced particles in narrow pseudo-rapidity interval in terms of the scaled variance [Formula: see text] has been carried out for [Formula: see text]O-emulsion interactions at 4.5[Formula: see text]AGeV/[Formula: see text]. Depending on the values of the total charges or sum of the charges of noninteracting projectile fragments, event samples were classified into four centrality classes. Presence of event-by-event fluctuations of maximum particle density is reflected in the multiparticle production process for different centrality classes. The event-by-event fluctuations are found to decrease with the increase of pseudo-rapidity interval. The event-by-event fluctuations are found to decrease with decreasing centrality of collisions. A comparison with the analyzed results of the total disintegration events has also been carried out. Experimental analysis results have been compared with those obtained from the analysis of Monte Carlo simulated (MC-RAND) events in order to extract the dynamical fluctuations.

Event-by-event fluctuations of maximum particle density with respect to the width of the pseudo-rapidity interval at a few A GeV/c

A detailed study of event-by-event fluctuations of maximum particle density of the produced particles with respect to the width of the pseudo-rapidity interval in terms of the scaled variance ω has been carried out for 16O-AgBr, 28Si-AgBr and 32S-AgBr interactions at 4.5 A GeV/c. For all the interactions the values of scaled variance are found to be greater than one indicating the presence of strong event-by-event fluctuations of maximum particle density values in the multiparticle production process. The event-by-event fluctuations are found to decrease with the increase of the width of the pseudo-rapidity interval. Experimental analysis has been compared with the results obtained from the analysis of events simulated by generating random numbers (MC-RAND) and the Ultra-Relativistic Quantum Molecular Dynamics (UrQMD) model. MC-RAND events show significantly less event-by-event fluctuations in comparison to the experimental data. The UrQMD model also failed to reproduce the experimental results. Comparison of our analysis with the outcome of other analysed emulsion data has also been presented.

Effect of Calcination Temperature on Magnetic Properties of MnZn Ferrites for High Frequency Applications

With the development of switching power supplies, miniaturization and high efficiency become hot research issues, and decreasing high-frequency losses is an effective method to achieve it. In this article, the effect of different calcination temperature on the power losses of MnZn ferrites at high frequency (500 kHz) over a broad temperature range is reported. The MnZn ferrites samples were prepared by ceramic process and the effect of calcination temperature was analyzed. The raw materials were calcined at 775 ℃, 800 ℃, 825 ℃, 850 ℃, 875℃, 900 ℃, 925 ℃ and 950 ℃, and the regular fluctuations of particle size (as-calcined), density (as-sintered) and magnetic properties are presented in this work. It is shown that the samples calcined at 850 ℃ exhibit optimal microstructure and magnetic properties. The newly developed MnZn ferrites are characterized by sintered density of 4.61 g/cm³, initial permeability of 1223 (10 kHz/0.1 mT/25 ℃), saturation magnetic flux density of 488 mT (10 kHz/1200 A/m/25 ℃) and power losses of 68 mW/cm³ (500 kHz/50 mT/100 ℃).

Cosmic microwave background dipole asymmetry could be explained by axion monodromy cosmic strings

Observations by the Wilkinson Microwave Anisotropy Probe and the Planck mission suggest a hemispherical power amplitude asymmetry in the cosmic microwave background, with a correlation length on the order of the size of the observable Universe. We find that this anomaly can be naturally explained by an axion-like particle (ALP) cosmic string formed near our visible Universe. The field variation associated to this cosmic string creates particle density fluctuations after inflation, which consequently decay into radiation before the Big Bang Nucleosynthesis (BBN) era and resulted in the observed power asymmetry. We find in this scenario that the hemispherical power amplitude asymmetry is strongly scale dependent: $A(k)\propto {\rm exp}(-kl)/k$. Admittedly, typical inflation models predict a relic number density of topological defects of order one per observable Universe and so in our model the cosmic string must be tuned to have an impact factor of order $1/H_0$. Interestingly, the constraints based on purely cosmological considerations also give rise to a Peccei-Quinn scale $F_a$ of order $10^3$ larger then the Hubble scale of inflation $H_I$. Assuming $H_I\sim 10^{13}$GeV, we then have an ALP with $F_a\sim 10^{16}$GeV, which coincides with the presumed scale of grand unification. As we require ALP decays occur before the BBN era, which implies a relatively heavy mass or strong self-coupling, and considering that the associated potential should break the shift symmetry softly in order to protect the system from radiative corrections, we also conclude that the required ALP potential should be monodromic in nature.

Effect of turbulent fluctuations on neutral particles transport with the TOKAM3X-EIRENE turbulence code

The effect of turbulent fluctuations on the transport of neutral particles (atoms, molecules) in tokamak plasmas is investigated with the 3D global turbulence code TOKAM3X-EIRENE in limiter geometry. The statistical properties of turbulent fields relevant to this work are discussed, including the recycling flux. The neutral particle transport is recalculated on the mean field plasma, and compared to the mean neutral particle density/flows obtained from the turbulent simulation, so as to assess the effects of the fluctuations, in particular on the ionization balance. The latter effects are remarkably modest in the simulation presented here, but are expected to become more and more pronounced as the high recycling regime is approached, in particular because the plasma temperature becomes low enough so that ionization is strongly non-linear. However, the turbulent fluctuations in the SOL do have a substantial effect on the neutral densities on the low field side of the limiter, including in the confined plasma. These effects are traced back to non-linearities in the plasma flux at the wall, and the fluctuations in the latter are identified as an important contributor both to neutral particle density fluctuations and to deviations from mean neutral particle density/flows in the turbulent simulation with respect to the same quantities recalculated on the mean plasma fields.

Particle-laden two-dimensional elastic turbulence

The aggregation properties of heavy inertial particles in the elastic turbulence regime of an Oldroyd-B fluid with periodic Kolmogorov mean flow are investigated by means of extensive numerical simulations in two dimensions. Both the small- and large-scale features of the resulting inhomogeneous particle distribution are examined, focusing on their connection with the properties of the advecting viscoelastic flow. We find that particles preferentially accumulate on thin highly elastic propagating structures and that this effect is the largest for intermediate values of particle inertia. We provide a quantitative characterization of this phenomenon that allows to relate it to the accumulation of particles in filamentary highly strained flow regions producing clusters of correlation dimension close to 1. At larger scales, particles are found to undergo turbophoretic-like segregation. Indeed, our results indicate a close relationship between the profiles of particle density and fluid velocity fluctuations. The large-scale inhomogeneity of the particle distribution is interpreted in the framework of a model derived in the limit of small, but finite, particle inertia. The qualitative characteristics of different observables are, to a good extent, independent of the flow elasticity. When increased, the latter is found, however, to slightly reduce the globally averaged degree of turbophoretic unmixing.

Hydrodynamic fluctuations in quasi-two dimensional diffusion

We study diffusion of colloids on a fluid–fluid interface using particle simulations and fluctuating hydrodynamics. Diffusion on a two-dimensional interface with three-dimensional hydrodynamics is known to be anomalous, with the collective diffusion coefficient diverging like the inverse of the wavenumber. This unusual collective effect arises because of the compressibility of the fluid flow in the plane of the interface, and leads to a nonlinear nonlocal convolution term in the diffusion equation for the ensemble-averaged concentration. We extend the previous hydrodynamic theory to account for a species/color labeling of the particles, as necessary to model experiments based on fluorescent techniques. We study the magnitude and dynamics of density and color density fluctuations using a novel Brownian dynamics algorithm, as well as fluctuating hydrodynamics theory and simulation. We find that hydrodynamic coupling between a single tagged particle and collective density fluctuations leads to a reduction of the long-time self-diffusion coefficient, even for an ideal gas of non-interacting particles. This unexpected finding demonstrates that density functional theories that do not account for thermal fluctuations are incomplete even for ideal systems. Using linearized fluctuating hydrodynamics theory, we show that for diffusion on a fluid–fluid interface, nonequilibrium fluctuations of the total density are small compared to the equilibrium fluctuations, but fluctuations of color density are giant and exhibit a spectrum that decays as the inverse cubed power of the wavenumber. We confirm these predictions through Brownian dynamics simulations of diffusive mixing with two indistinguishable species. We also examine nonequilibrium fluctuations in systems with two-dimensional hydrodynamics, such as thin smectic films in vacuum. We find that nonequilibrium fluctuations are colossal and comparable in magnitude to the mean, and can be accurately modeled using numerical solvers for the nonlinear equations of fluctuating hydrodynamics.

Does a Growing Static Length Scale Control the Glass Transition?

Several theories of the glass transition propose that the structural relaxation time τ_{α} is controlled by a growing static length scale ξ that is determined by the free energy landscape but not by the local dynamic rules governing its exploration. We argue, based on recent simulations using particle-radius-swap dynamics, that only a modest factor in the increase in τ_{α} on approach to the glass transition may stem from the growth of a static length, with a vastly larger contribution attributable, instead, to a slowdown of local dynamics. This reinforces arguments that we base on the observed strong coupling of particle diffusion and density fluctuations in real glasses.

Effects of shear flows on the evolution of fluctuations in interchange turbulence

We report a non-perturbative study of the effect of different types of shear flows on the evolution of vorticity and particle density fluctuations in interchange turbulence. For the same shear strength, the transport of density is less reduced by streamers than by zonal flows, with zonal flows leading to oscillation death. In the inviscid limit, vorticity (density) grows (decays) as a power law due to the streamer or zonal flow and exponentially due to the combined effect of the zonal flow and streamer with the same sign of shear. The zonal flow and streamer with the opposite sign of shear lead to oscillation at multiple frequencies.