Particle Theory Research Articles

Crowding-Regulated Binding of Divalent Biomolecules.

Macromolecular crowding affects biophysical processes as diverse as diffusion, gene expression, cell growth, and senescence. Yet, there is no comprehensive understanding of how crowding affects reactions, particularly multivalent binding. Herein, we use scaled particle theory and develop a molecular simulation method to investigate the binding of monovalent to divalent biomolecules. We find that crowding can increase or reduce cooperativity-the extent to which the binding of a second molecule is enhanced after binding a first molecule-by orders of magnitude, depending on the sizes of the involved molecular complexes. Cooperativity generally increases when a divalent molecule swells and then shrinks upon binding two ligands. Our calculations also reveal that, in some cases, crowding enables binding that does not occur otherwise. As an immunological example, we consider immunoglobulin G-antigen binding and show that crowding enhances its cooperativity in bulk but reduces it when an immunoglobulin G binds antigens on a surface.

From axion quality and naturalness problems to a high-quality ZN QCD relaxion

We highlight general issues associated with quality and naturalness problems in theories of light QCD-axions, axion-like particles, and relaxions. We show the presence of Planck-suppressed operators generically lead to scalar coupling of axions with the Standard model. We present a new class of $\mathbb Z_{N}$ QCD relaxion models that can address both the QCD relaxion CP problem as well as its quality problem. This new class of models also leads to interesting experimental signatures, which can be searched for at the precision frontier.

Cross-diffusion models in complex frameworks from microscopic to macroscopic

This paper deals with the micro–macro derivation of models from the underlying description provided by methods of the kinetic theory for active particles. We consider the so-called exotic models according to the definition proposed in [ N. Bellomo, N. Outada, J. Soler, Y. Tao and M. Winkler, Chemotaxis and cross diffusion models in complex environments: Modeling towards a multiscale vision, Math. Models Methods Appl. Sci. 32 (2022) 713–792]. The first part of the presentation focuses on a survey and a critical analysis of some phenomenological models known in the literature. We refer to a selection of case studies, in detail, the transport of virus models, social dynamics, and Keller–Segel in a fluid. The second part shows how a Hilbert-type approach can be developed to derive models at the macroscale from the underlying description provided by the kinetic theory of active particles. The third part deals with the derivation of macroscopic models corresponding to the selected case studies. Finally, a forward look into the future research perspectives is proposed.

Some Backscatter Modeling Issues Complicating the Sonar‐Based Monitoring of Suspended Sediments in Rivers

AbstractThe measurement of suspended sediments from acoustic backscatter was originally developed in marine science for monitoring near‐bottom suspensions usually composed of sand particles. In the last decade, there has been a growing interest in adapting these techniques to rivers. The so‐called “solid particle theory” developed in oceanography mainly applies to suspensions of non‐cohesive solid particles producing incoherent backscatter signal. So far, this theory has been used for interpreting river backscatter even if it relies on assumptions that are not obviously met in rivers. This study uses a set of measurements made on the Rhône River in France to discuss the typical issues which challenge the interpretation of sound backscattering for monitoring suspended sediments in rivers. Large discrepancies between model outputs and measurements for frequencies lower than 2.5 MHz suggest that other scattering processes including flocculation and air micro‐bubbles may have a large impact on acoustic backscatter and attenuation. Deviations of the backscatter echo distribution from Rayleigh statistics were observed, suggesting that the assumption of incoherent backscattering is not always met. This work calls for the development of a more complete theory for interpreting river backscatter.

Macromolecular crowding impact on anti-CRISPR AcrIIC3/NmeCas9 complex: Insights from scaled particle theory, molecular dynamics, and elastic networks models

The coupling of Cas9 and its inhibitor AcrIIC3, both from the bacterium Neisseria meningitidis (Nme), form a homodimer of the (NmeCas9/AcrIIC3)2 type. This coupling was studied to assess the impact of their interaction with the crowders in the following environments: (1) homogeneous crowded, (2) heterogeneous, and (3) microheterogeneous cytoplasmic. For this, statistical thermodynamic models based on the scaled particle theory (SPT) were used, considering the attractive and repulsive protein-crowders contributions and the stability of the formation of spherocylindrical homodimers and the effects of changes in the size of spherical dimers were estimated. Studies based on models of dynamics, elastic networks, and statistical potentials to the formation of complexes NmeCas9/AcrIIC3 using PEG as the crowding agent support the predictions from SPT. Macromolecular crowding stabilizes the formation of the dimers, being more significant when the attractive protein-crowder interactions are weaker and the crowders are smaller. The coupling is favored towards the formation of spherical and compact dimers due to crowding addition (excluded-volume effects) and the thermodynamic stability of the dimers is markedly dependent on the size of the crowders. These results support the experimental mechanistic proposal of inhibition of NmeCas9 mediated by AcrIIC3.

Symmetries and Asymmetries in Branching Processes

As is known in stochastic particle theory, the same random process can be described by two different master equations for the evolution of the probability density, namely, by a forward or a backward master equation. These are the generalised analogues of the direct and adjoint equations of traditional transport theory. At the level of the first moment, these two equations show considerable resemblance to each other, but they become increasingly different with increasing moment order. The purpose of this paper is to demonstrate this increasing asymmetry and to discuss the underlying reasons. It is argued that since the reason of the different forms of the forward and the backward equations lies in the lack of invariance of the process to time reversal, the reason for the increasing asymmetry between the two forms for higher-order moments or processes with several variables (particle types) can be related to the increasing level of the violation of the invariance to time reversal, as is illustrated with some examples.

Study on Segregation and Heat Transfer of Polydisperse Particles in a Three-Dimensional Rotating Drum by Using the DEM

The segregation and heat transfer of polydisperse particles in the rotating drum have an important impact on many industrial fields, and the theory is not completely clear. Based on the verified discrete element method model, the segregation and heat transfer of polydisperse particles in the rotating drum indirectly heated by the side wall are studied. The results show that the polydisperse particles form the segregation structure of the radial core and axial band in the rolling regime rotating drum. The comparison with monodisperse particles indicates that the heat transfer of polydisperse particles is influenced by the segregation structure. Small changes in rotation speed and filling level in the rolling regime do not alter the particle segregation and heat transfer structure. The addition of lifters is an effective means to reduce particle segregation and improve the heat transfer effect. This work provides a reference for revealing the segregation and heat transfer theory of polydisperse particles in the rotating drum.

Revisiting MMPBSA by Adoption of MC-Based Surface Area/Volume, ANI-ML Potentials, and Two-Valued Interior Dielectric Constant.

Here, we report the accuracy improvements of molecular mechanics Poisson-Boltzmann surface area (MMPBSA) calculations by adoption of ANI-ML potentials in replacement of MM terms, the use of solvent-accessible surface area (SASA) and volume (SAV) values from the Monte Carlo sampling of the probe, and introducing two different interior dielectric constants for electrostatic interactions of protein-ligand (P-L) and polar solvation term in the MMPBSA calculations. Our results show that the Pearson correlation coefficients of MMPBSA-calculated values with respect to experimental binding free energies can be drastically improved from 0.48 to 0.90 by adoption of ANI-ML potentials in replacement of MM energy terms in the equation, referred to as ANI-PBSA. Moreover, we show that the SASA/SAV-combined equation in the scaled particle theory (SPT) can be a better choice to model nonpolar solvation term, reaching nearly the same accuracy by ANI-PBSA calculations. Finally, we introduce two different values of interior dielectric constants, which could be an alternative strategy between the single and variable constant definitions.

Magnonic Klein and acausal tunneling enabled by breaking the anti parity-time symmetry in antiferromagnets

Klein tunneling associated with particle-antiparticle pair productions across a potential barrier is a key prediction of quantum-field theory for relativistic particles. Yet, a direct experimental realization is hampered by the particles large rest mass resulting in high potential barrier. Here, for non-Hermitian antiferromagnets, at the verge of the anti-parity-time symmetry transition, chiral magnons are demonstrated to offer a bosonic platform to access Klein tunneling at meV energies in experimentally feasible settings. Our analytical and numerical simulations evidence that magnetic damping renders a low energy mechanism for the breakdown of the magnonic vacuum and for creating particle-antiparticle pairs in strong magnetic fields. Adopting Feynman’s picture for antiparticles, the tunneling time of an incident magnon wave packet across a supercritical barrier is found to be negative. The uncovered aspects point to the potential of chiral magnons for addressing fundamental physics in a conceptually simple setup with the potential for use in chirality-dependent magnonic computing.

Scaling laws for single-file diffusion of adhesive particles.

Single-file diffusion refers to the Brownian motion in narrow channels where particles cannot pass each other. In such processes, the diffusion of a tagged particle is typically normal at short times and becomes subdiffusive at long times. For hard-sphere interparticle interaction, the time-dependent mean squared displacement of a tracer is well understood. Here we develop a scaling theory for adhesive particles. It provides a full description of the time-dependent diffusive behavior with a scaling function that depends on an effective strength of adhesive interaction. Particle clustering induced by the adhesive interaction slows down the diffusion at short times, while it enhances subdiffusion at long times. The enhancement effect can be quantified in measurements irrespective of how tagged particles are injected into the system. Combined effects of pore structure and particle adhesiveness should speed up translocation of molecules through narrow pores.

Completing the fifth PN precision frontier via the EFT of spinning gravitating objects

We put forward a broader picture of the effective theory of a spinning particle within the EFT of spinning gravitating objects, through which we derive and establish the new precision frontier at the fifth PN (5PN) order. This frontier includes higher-spin sectors, quadratic and quartic in the spin, which both display novel physical features, due to the extension of the effective theory beyond linear order in the curvature. The quadratic-in-spin sectors give rise to a new tidal effect, and the quartic-in-spin sectors exhibit a new multipolar deformation. We then generalize the concept of tidal operators and of spin-induced multipolar operators, and make conjectures on the numerical values of their Wilson coefficients, and on the effective point-particle action of Kerr black holes. We confirm the generalized actions for generic compact binaries of the NLO quartic-in-spin sectors which were derived via the extension of the EFT of gravitating spinning objects. We first present the corresponding interaction potentials and general Hamiltonians, which consist of 12 distinct sectors, with a new one due to the new multipolar deformation. These Hamiltonians give the full physical information on the binary system, which mostly gets lost in higher-spin sectors, when going to the aligned-spins configuration. Moreover these general Hamiltonians uniquely allow us to find the complete Poincaré algebra at the 5PN order with spins, including the third subleading quadratic-in-spin sectors. We derive consequent observables for GW applications. Finally, to make contact with the scattering problem, we also derive the extrapolated scattering angles for aligned spins. Our completion of the Poincaré algebra provides the strongest validation of our most comprehensive new results, and thus that the 5PN order has now been established as the new precision frontier.

Modeling horizon absorption in spinning binary black holes using effective worldline theory

The mass and spin of black holes (BHs) in binary systems may change due to the infall of gravitational-wave (GW) energy down the horizons. For spinning BHs, this effect enters at 2.5 post-Newtonian (PN) order relative to the leading-order energy flux at infinity. There is currently a discrepancy in the literature in the expressions of these horizon fluxes in the test-body limit at 4PN order (relative 1.5PN order). Here, we model the horizon absorption as tidal heating in an effective worldline theory of a spinning particle equipped with tidally-induced quadrupole and octupole moments. We match the tidal response to analytic solutions of the Teukolsky equation in a scattering scenario, and obtain general formulae for the evolution of mass and spin. We then specialize to the case of aligned-spin--quasi-circular binaries, obtaining the corresponding contributions to the GW phasing through 4PN order. Importantly, we find that the number of GW cycles due to horizon fluxes with masses observed by LIGO-Virgo-KAGRA detectors is about 2-3 orders of magnitude smaller than the other contributions to the phasing at the same PN order. Furthermore, in the test-body limit, we find full agreement with results obtained earlier from BH perturbation theory, with a small mass in an equatorial circular orbit treated as a source perturbing the Kerr metric. Thus, we weigh in on one side of the previous discrepancy.

Dual-Wavelength Smoke Detector Measuring Both Light Scattering and Extinction to Reduce False Alarms

Existing fire smoke detectors use the intensity of scattering light or the light extinction coefficient as the indicator of fire smoke to trigger fire alarms. However, false fire alarms could be triggered by dust and water fog. Achieving reliable early fire detection with minimal false alarms is a challenge. Based on the Mie scattering theory of spherical particles, it is derived that the ratio of scattering intensity and the ratio of optical extinction of two incident lights with different wavelengths only depends on the intrinsic properties of the aerosol (the average particle size and refractive index). This paper then presents an improved dual-wavelength smoke detection by measuring scattering light and extinction simultaneously to reduce false alarms. Simulations and verification with test fires of European Standard EN 54 were performed, demonstrating that fires can be distinguished from nuisance sources without complicated calculations. These results indicate that the improved detection system can be applied for smoke monitoring and fire protection.

Eulerian multifluid simulations of proppant transport with different sizes

Proppant transport is critical in hydraulic fractures and enhanced geothermal systems. Proppant transport is essentially a dense granular flow in narrow slots, and the Euler–Euler methods are commonly used to study the principle of proppant transport at the field scale. However, the simulated results cannot reproduce the laboratory observations well because some closure equations are not suitable for describing the quasi-static state of proppants after settlement, and only monodisperse granular flow is considered in simulations, which neglects the interaction between large and small particles. To improve the applicability of the numerical simulation of proppant transport in hydraulic fracturing treatment, binary-size proppant transport numerical simulations using the Eulerian multifluid method (EMM) are performed in this study. First, the motion characteristics of the suspended and settled proppants were analyzed using the kinetic theory of granular flow (KTGF) and the frictional theory of viscous particles. Thereafter, the solid–liquid momentum exchange considering the wall retardation effect and the solid–solid momentum exchange considering the endurable contact among the particles are discussed. Finally, the numerical results are qualitatively and quantitatively verified using proppant transport experiments and particle image velocimetry tests. The combination of traditional KTGF models and frictional models exhibits better performance than the modified KTGF models when considering the inertia flow regime in the proppant transport simulation, and the contribution of viscous-particle cohesion to friction must be considered. Notably, the simulated results are close to the experimental results for the development process of sand banks and the velocity distribution of particles. This verified method is efficient in computing and it will provide new insights into the pumping procedure design for hydraulic fracturing.

Bubble nucleation in a cold spin 1 gas

Cold atomic gases offer the prospect of simulating the physics of the very early Universe in the laboratory. In the condensate phase, the gas is described by a field theory with key features of high energy particle theory. This paper describes a three level system which undergoes a first order phase transition through the nucleation of bubbles. The theoretical investigation shows bubbles nucleating in two dimensions at non-zero temperature. There is good agreement between the bubble nucleation rates calculated from a stochastic projected Gross–Pitaevskii equation and from a non-perturbative instanton method. When an optical box trap is included in the simulations, the bubbles nucleate preferentially near the walls of the trap.

Mathematical modeling of glioma invasion and therapy approaches via kinetic theory of active particles

A multiscale model for glioma spread in brain tissue under the influence of vascularization and vascular endothelial growth factors is proposed. It accounts for the interplay between the different components of the neoplasm and the healthy tissue and it investigates and compares various therapy approaches. Precisely, these involve radiotherapy and chemotherapy in a concurrent or adjuvant manner together with anti-angiogenic therapy affecting the vascular component of the system. We assess tumor growth and spread on the basis of diffusion tensor imaging (DTI) data, which allows us to reconstruct a realistic brain geometry and tissue structure, and we apply our model to real glioma patient data. In this latter case, a space-dependent radiotherapy description is considered using data about the corresponding isodose curves.

Higgs boson origin from a gauge symmetric theory of massive composite particles and massless W± and Z0 bosons at the TeV scale

The ultraviolet completion is the Standard Model (SM) gauge-symmetric four-fermion couplings at the high-energy cutoff. Composite particles appear in the gauge symmetric phase in contrast with SM particles in the spontaneous symmetry-breaking phase. The critical point between the two phases is a weak first-order transition. It relates to an ultraviolet fixed point for an SM gauge symmetric theory of composite particles in the strong coupling regime. The low-energy SM realizes at an infrared fixed point in the weak coupling regime. Composite bosons dissolve into SM particles at the phase transition, and in the top-quark channel, they become a composite SM Higgs boson and three Goldstone bosons. Extrapolation of SM renormalization-group solutions to high energies implies that the gauge-symmetric theory of composite particles has a characteristic scale of about 5.1 TeV. We discuss the phenomenological implications of composite SM Higgs boson in the gauge symmetry-breaking phase, and massive composite bosons coupling to massless W± and Z0 gauge bosons in the gauge symmetric phase.

Possible Particles that Build Up for the Existence of Dark Matter

Even in the view of modern physics, dark matter is still one of the most mysterious topics. Although physicists have not yet been able to confirm the exact particle that makes up for dark matter, or even if there are any, some self-consistent theories are established. This paper explores two most convincing explanations about the composition of the dark matter, the axions and weakly interacting massive particles (WIMPS) theory. The content and development of the two theories regarding the composition of the dark matter are mainly discussed.

Kinetic modeling of a leader–follower system in crowd evacuation with collective learning

A kinetic modeling of crowd evacuation with leaders and followers is considered in this paper, in which the followers may not know the full information about the walking environment and the evacuation strategy, but they follow the leaders and learn the walking strategy to get out of the walking venue. Based on the kinetic theory of active particles, the learning dynamics are considered by introducing an activity variable [Formula: see text], which represents the learning level of the followers and measures how much knowledge a follower has learned about the walking strategy, the walking environment, or the geometry of the walking venue. Several fundamental factors are considered in this leader–follower learning system of crowd evacuation, including: (1) the rational motion of all pedestrians, i.e. the trend to the exit or to a preferred direction, the ability to avoid collisions with walls or obstacles, and the tendency to search for less crowded direction with minimal density gradient, (2) the irrational motion of followers to follow other pedestrians induced by panic, (3) the learning dynamics of the followers who learn the walking strategy during interaction with others, and, (4) the transition from a follower to a leader when one’s activity reaches the highest level of learning. A numerical comparison of a metro platform evacuation with and without learning shows a reasonably good predictive ability of the model that the learning effect plays a significant role in the evacuation dynamics.

Editorial: Active particle models and methods in science and society

This editorial paper reviews the articles published in a special issue devoted to the application of active particle methods to the study of the collective dynamics of large systems of interacting entities in science and society. The applications presented in this special issue focus on the study of financial markets, cell dynamics in the context of cancer modeling, crowd vehicle and crowd dynamics, and classical problems in the kinetic theory of active particles and swarm theory. A critical analysis is proposed to look forward at new perspectives, with emphasis on the interaction of multiple social dynamics.