Medical Physics
APHABAMAS: An analytical phantom-based scheme for assessing the accuracy of high-resolution 3D MRI motion-artifact simulations
Tianqi Wu and Hui Zhang
Purpose: Motion compromises the utility of high-resolution 3D MRI, an established tool in quantitative neuroimaging research. Deep learning-based methods have shown promise for mitigating motion-induced artifacts, but their development typically requires simulated motion-corrupted data. Several open-source tools exist for this task, each implementing different
algorithms. However, no scheme currently exists for evaluating the accuracy of these simulations, making it difficult for users to choose the most suitable tool. This study aims to develop such a scheme. Methods: The essential ingredient of the desired scheme is a ground-truth reference that does not suffer from sampling-induced error. To meet this requirement, the proposed scheme, APHABAMAS, leverages a digital phantom whose image- and
Fourier-domain representations can be expressed analytically under arbitrary rigid-body transformations. APHABAMAS is used to quantify sampling-induced errors of three existing
algorithms under synthetic and tracking-data-derived motion trajectories at different motion severity levels. Results: All three
algorithms produce distinct but visually plausible artifacts, demonstrating the need for a ground-truth reference. APHABAMAS shows that the
algorithm utilizing the uniform-to-non-uniform (Type-2) NUFFT provides the most consistent agreement with the ground-truth reference across motion types and severity levels. The non-uniform-to-uniform (Type-1) NUFFT-based
algorithm, which does not mimic the MR acquisition process, deteriorates substantially with increasing motion severity. Conclusions: APHABAMAS provides a rigorous tool for assessing the accuracy of high-resolution 3D MRI motion-artifact simulations. It enables accuracy-based assessment of three existing
algorithms, thereby facilitating informed selection of the most suitable one for synthesizing motion-corrupted data.
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Fluid Dynamics
A Multispecies ESBGK Model for Gas Mixtures with Variable Hard Sphere Transport: Theory and Verification
Marcel Pfeiffer, Franziska Tuttas, Julien Mathiaud et al. (+1 more)
A multi-species Bhatnagar-Gross-Krook (BGK) model for gas mixtures is presented that achieves the correct species-wise relaxation of velocities, temperatures, and pressure tensors according to the Boltzmann collision integral, as well as the correct mixture Prandtl number, while retaining a single relaxation term per species. The model extends the ellipsoidal statistical BGK (ESBGK) model by introducing relative relaxation targets for each species, derived from the Variable Hard Sphere (VHS) production rates of the Grad 13 approximation. Three approaches for the species relaxation frequency are proposed and analyzed: a Grad 13-based per-species frequency, a mixture-averaged frequency, and an empirical harmonic mean of the two. The model is implemented in the particle-based code PICLas and verified against Direct Simulation Monte Carlo (DSMC) results for a range of test cases, including 0D reservoir relaxation, mass
diffusion, supersonic Couette flow, and hypersonic flow around a 70{\deg} blunted cone for binary and ternary gas mixtures. Across all test cases, the proposed model reproduces the correct Prandtl number, species temperature, velocity relaxation rates and pressure tensor relaxation, with the empirical relaxation frequency consistently yielding the best agreement with DSMC.
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All authors Marcel Pfeiffer, Franziska Tuttas, Julien Mathiaud, Luc Mieussens
Fluid Dynamics
Flow and Heat Transfer Characteristics of Forced Convection Past an Isoflux Circular Cylinder in Galinstan for Reynolds Numbers up to 600
Dipjyoti Nath
A numerical investigation of steady forced convection heat transfer from an isoflux circular cylinder immersed in the
liquid metal Galinstan is presented. The governing streamfunction, vorticity, and energy equations are solved using a fourth-order compact finite difference scheme in cylindrical coordinates (FOCS–CC) coupled with a stable pseudo-time iteration (PTI) technique. The influence of the Reynolds number ($1 \leq Re \leq 600$) on the flow and heat transfer characteristics is systematically investigated for Galinstan with a Prandtl number of $Pr=0.025$. The performance and accuracy of the proposed scheme are first established through grid independence studies and validation against previously published numerical results for the average Nusselt number and total drag coefficient over a range of Reynolds numbers. Excellent agreement with the available literature confirms the reliability and robustness of the present formulation. The effects of Reynolds number ($1\leq Re\leq600$) on the flow and thermal fields are examined through streamline patterns, isotherm distributions, and local Nusselt number variations. The results reveal that increasing the Reynolds number promotes flow separation, enlarges the wake region, intensifies downstream thermal transport, and significantly enhances convective heat transfer from the cylinder surface. Furthermore, a new empirical correlation for the average Nusselt number is proposed for Galinstan fluid over the Reynolds number range $1\leq Re\leq600$, exhibiting excellent agreement with the numerical data with a coefficient of determination of $R^2=0.99939$.
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Optics
Photonic Theta Cavity: Engineering Bound States in the Continuum in Topological Resonators Beyond the Limitations of Near Field Coupling
Eric Seabron, Robert E. Coleman, Chuanyu Lian et al. (+3 more)
The Theta Cavity is a unique topological resonator architecture which utilizes interferometric coupling to overcome fundamental design limitations associated with near-field evanescent coupling which currently dominates the design space for integrated photonics. The defining device physics is established by the mirror symmetric cross junctions which preserves efficient power transfer between a waveguide and ring resonator creating a strongly correlated phase relationship between the multiple paths. The unique mode selection physics allows for interference driven suppression of radiative pathways enabling strong cavity confinement and the emergence of Bound States in the Continuum (BICs). Analytical models reveal non-Hermitian optical band structure displaying non-trivial topological transitions between BIC and quasi-BIC modes that are robust to attenuation, temperature variations, and typical fabrication non-idealities, which is critical for overcoming intrinsic limitations associated with silicon based integrated photonics. The Theta Cavity architecture also circumvents limitations that arise from proximity requirements of the physical gap used in near-field coupled waveguides which enables a flexible design space for new devices. In this study, we demonstrate phase mediated long-range strong coupling of multiple ring resonators in the Nested Theta Cavity architecture showcasing band structure hybridization resulting in the formation of anti-crossing bandgaps, Dirac crossings, and Fano resonances. The photonic Theta Cavity architecture provides a scalable, topologically robust platform for engineering modes across a multidimensional parameter space with high resilience to perturbation and attenuation, enabling a new approach for the designing of integrated cavity devices.
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All authors Eric Seabron, Robert E. Coleman, Chuanyu Lian, Malcolm Bogroff, Carlos Rios, Antonio Levy
Optics · Cross-listed from quant-ph
Heterodyne position detection of an optomechanical system
Daniel Tandeitnik, Gabriel Dias and Thiago Guerreiro
We report a heterodyne detection scheme for position readout of an optomechanical system, in particular an optically levitated particle, implemented via digital In-phase and Quadrature demodulation on a field-programmable gate array. Compared to the standard homodyne approach, the proposed method offers three key advantages: it remains robust in the presence of strong parasitic back-reflected fields that would otherwise prevent stable phase locking; it produces a signal linearly proportional to the particle displacement, eliminating phase-wrapping distortion; and its calibration factor is intrinsically immune to drifts in the optical power of the local oscillator or scattered field. We experimentally demonstrate and quantify all three advantages through simultaneous homodyne and heterodyne measurements on the same trapped particle. The proposed method can be used in any optomechanical system based on phase readout.
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Optics
Dark Optical Trapping of Resonant Transition-Metal Dichalcogenide Particles
Patrick Illetschek, Gleb Fedorovich, Albert Seredin et al. (+6 more)
Mitigating
recoil events and minimizing optically induced heating are central challenges in the precise control and cooling of macroscopic particles. To overcome this, we propose trapping resonant dielectric particles for applications in ultra-high
vacuum (UHV) levitodynamics. Contrary to other approaches, where suppressing the parasitic resonant scattering was achieved in a standing wave geometry, here we propose a single beam geometry in a dark trap regime. As a promising material platform, we focus on a class of transition-metal dichalcogenide (TMD) particles with high polarizability, characterized by refractive indices in the range $3.7$-$4.8$ and densities up to $9.3~\mathrm{g\,cm^{-3}}$. Using full Mie theory, we identify a range of TMD particle radii that support stable axial and radial
magnetic quadrupole trapping in a bottle-beam configuration. We predict that for WS$_2$ particles with a mass of $0.5 \times 10^{12}\,\mathrm{amu}$, one can expect suppression of the scattering rate relative to the mechanical frequency down to $\Gamma/\Omega \simeq 0.02$. This corresponds to a coherence time extended by approximately three orders of magnitude compared with silica particles of the same mass trapped in conventional bright optical traps at UHV. Combined with significantly reduced internal heating, remaining well below the melting point of the material, dark trapping of resonant TMD macroscopic particles emerges as a promising platform for exploring
quantum physics with large masses.
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All authors Patrick Illetschek, Gleb Fedorovich, Albert Seredin, Gleb Tselikov, Valentin S. Volkov, Nikolai Kiesel, Markus Aspelmeyer, Mihail Petrov, Anton V. Zasedatelev
History and Philosophy of Physics
Rasmus Jaksland
In AdS/CFT, one often finds claims along the lines that “spacetime emerges from
entanglement." This paper argues that behind these general statements hide three distinct emergence claims about, respectively, metric, gravitational dynamics, and topological connectivity. Thus, despite being advertised with the same terminology, these results are not about the same spatiotemporal aspects. They can therefore not just be grouped as evidence for one unanimous conclusion, though they do point in similar directions. The paper also investigates whether the three emergence claims satisfy two of the necessary conditions for emergence: Determination and novelty. The paper argues that none of the emergence claims satisfies the determination condition: More than
entanglement is needed to furnish the emergence basis. Besides
entanglement, the emergence basis for the bulk metric must include the induced metric on the boundary. Thus, this claim might not satisfy the novelty condition for emergence that the emergent should be novel as compared to the emergence basis. Likewise, the emergence basis for topological connectivity seems to include connectivity whereby this emergence claim is also questionable. The paper concludes that only gravitational dynamics is novel compared to the fully furnished emergence basis.
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History and Philosophy of Physics
The New Tractatus Program: Quantum Mechanics, Anti-Totalitarian Ontology, and Consciousness
Mikołaj Sienicki and Krzysztof Sienicki
This article reads two recent tractatus-style texts - Niccolo Covoni and Carlo Rovelli's Tractatus Quanticus (arXiv:2512.06034) and Mikolaj Sienicki and Krzysztof Sienicki's Tractatus de Conscientia (arXiv:2607.05459) - together with Jenann Ismael and Huw Price's interpretive introduction Against Totalitarianism (arXiv:2601.01070), as parts of what may be called a New Tractatus Program. The program is unified by a single pressure point: the demand for a perspective-free description of reality, whether as a final inventory of facts, a world viewed from nowhere, or a self placed outside the physical order. On the relational interpretation of
quantum mechanics, Tractatus Quanticus revises Wittgenstein's opening claim by replacing the world as the totality of facts with the world as what is the case from some perspective. Against Totalitarianism then makes the philosophical stakes of this move explicit by challenging the idea of the world as one closed totality. Tractatus de Conscientia carries the same discipline into the theory of consciousness, where conscious experience is treated neither as a mysterious substance nor as mere behavior, but as an integrated, temporally thick, operationally accessible perspective. Read together, these texts point toward a post-classical philosophy of partiality: reality is not abolished, but de-absolutized; consciousness is not mystified, but constrained by access, coupling,
memory, and evidence; and the limits of
language become inseparable from the limits of perspective. The article does not claim that relational
quantum mechanics is the only viable interpretation of
quantum theory. It instead develops the philosophical consequences of the relational reading. Its aim is programmatic rather than demonstrative: it does not prove a new ontology, but organizes a family of arguments around disciplined perspective-dependence.
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Computational Physics · Cross-listed from astro-ph.EP
Beyond the Cube: Overlapping Grid Methods for Debris Collision Risk Assessment
Yacob Medhin and Simone Servadio
The cube method reduces conjunction screening in orbital debris simulations to $\mathcal{O}(N)$ cost by evaluating only object pairs sharing the same grid
cell at each snapshot, but systematically assigns zero collision probability to pairs separated by a
cell boundary at that epoch, a failure known as boundary blindness. This paper introduces the Double Cube (DC) method, which recovers boundary-crossing conjunctions through a spatially shifted secondary grid using bin-index lookup alone, preserving $\mathcal{O}(N)$ complexity. In an isotropic convergence
benchmark at $L = 50$ km, DC reduces the blindness rate from $\beta_{\mathrm{Cube}} = 7.09\%$ to $\beta_{\mathrm{DC}} = 0.55\%$, and a synchronized experiment separates the temporal component from the
geometric one and leaves DC below $0.01\%$ against $0.80\%$ for the cube. Recovering these pairs changes which pairs are evaluated and leaves the probability formula untouched, so it does not improve calibration by itself, and both uncorrected methods remain substantially miscalibrated, with a reliability slope of $m = 1.1314$ for the cube and $m = 1.0996$ for DC. The formula applies the full
cell volume to every evaluated pair irrespective of separation, which is the same bias characterized in the no-time-counter scheme of Direct Simulation Monte Carlo. Two corrections are derived from the geometry of the uniform
cell alone: a power-law form referenced to the Robbins mean separation $\bar{d} = 0.6617L$ and a Gaussian form using both that mean and the analytically derived standard deviation $\sigma = 0.2494L$, with no quantity fitted to simulation output. The power-law correction moves the reliability slope to $0.9973$ at $k = 1$ and $0.9429$ at $k = 2$, and the Gaussian correction reaches $0.9806$ with an intercept of $0.975$, the only configuration within $3\%$ of unity on both. Both corrections have been implemented in MOCAT-MC.
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Geophysics
Inunda: A GPU-Native, Agent-enabled, Differentiable Solver for High-Resolution Flood Inundation Modeling
Zhi Li
Predicting where floodwater goes and how deep it gets, at high resolution and across large domains, remains computationally expensive with conventional hydraulic solvers, while purely data-driven surrogates are fast but lack physical guarantees and generalize poorly beyond their training events. We present Inunda, a GPU-native flood inundation model that solves the two-dimensional shallow water equations. Inunda uses a mass-conservative local-inertial scheme and runs multi-day events over millions of
cells in minutes on a single GPU. Because every operator is autograd-compatible, the solver is differentiable by construction: model parameters can be estimated by gradient descent against gage observations through reverse-mode automatic differentiation of the full simulation. We demonstrate Inunda on three case studies. For a hindcast of Hurricane Harvey (2017) in Harris County, Texas, Inunda matches surveyed high-water marks to a mean absolute error of 0.67 m, competitive with or better than a suite of established flood models, and reaches a median gage water-level Nash Sutcliffe efficiency of +0.72, more than double the +0.31 of the operational National Water Model v3.0. For the July 2025 Central Texas flash flood, Inunda is driven by an 18-member 1-km convection-allowing precipitation ensemble to produce probabilistic flood forecasts whose skill improves systematically as lead time to the crest shortens. For a post-fire flash-flood application in the Rio Ruidoso burn scar, differentiable calibration recovers the saturated hydraulic conductivity as a spatially explicit field at the model's own resolution and traces its multi-year post-fire recovery. Inunda provides an open, end-to-end pipeline for real-event flood modeling that couples the accuracy of physics-based hydraulics with the calibration and coupling advantages of modern differentiable programming.
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Optics · Cross-listed from nlin.PS
Stabilization of two-dimensional optical continuous-wave states by a potential trough
Thawatchai Mayteevarunyoo and Boris A. Malomed
We consider quasi-one-dimensional (Q1D) continuous waves (CWs) in the two-dimensional (2D) optical system with the cubic-quintic nonlinearity and a Q1D potential trough. In the case of a smooth trough profile, we confirm the known modulational instability (MI) of Q1D CWs with the transverse structure corresponding to the 1D ground state (GS) in the potential trough, and demonstrate the MI of CWs with the dipole-mode (DM) transverse structure, corresponding to the lowest 1D excited state in the potential trough. The CWs of both GS and DM types remain nearly stable close to the edges of their existence regions. Stable stationary states in the form of periodic chains of 2D
solitons, trapped in the potential trough, are produced in a numerical form. The dynamics of the
soliton chains excited by a localized kick is studied too. For the potential trough with the singular delta-functional profile, we find two species of exact analytical solutions for CWs, one of which is completely stable.
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Atomic Physics
Characterisation of a strontium cold atom source using fluorescence spectroscopy and time-of-flight
Kamran Hussain, Hamza Labiad, Anna L. Marchant et al. (+5 more)
We demonstrate a characterisation methodology for a strontium atomic beam, produced by a two-dimensional magneto-optical trap and delivered via a resonant push beam, using fluorescence spectroscopy and time-of-flight (ToF). This provides insight into the beam characteristics of a cold atom source, allowing for direct measurement of the transverse velocity spread, longitudinal velocity distributions, divergence, and the capturable flux for further cooling. From the ToF measurements, we derive a series of flux-per-longitudinal-velocity distributions at varying push saturation parameters ($s_{\mathrm{push}}$) using both a unidirectional and counter-propagating resonant probe beam. A simulation-derived factor is applied to the unidirectional probe longitudinal velocity distribution to account for differences in the scattering rate scaling. The distributions are integrated up to an estimated 3D-MOT capture velocity of \SI{30}{\meter\per\second}. For our system, we find that at $s_{\mathrm{push}} = 0.45$, we obtain a flux of $(1.7 \pm 0.4)\times10^{8}$ atoms/s and $(1.5 \pm 0.4)\times10^{8}$ atoms/s, using a unidirectional probe beam and counter-propagating probe, respectively. These measurements provide a framework for characterising cold atomic sources for applications such as 3D MOT loading and atom interferometers.
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All authors Kamran Hussain, Hamza Labiad, Anna L. Marchant, Jonathan N. Tinsley, Tristan Valenzuela, Jonathon Coleman, David Newbold, Mark G. Bason
Fluid Dynamics
Numerical Simulation of Turbulent Concentric Annular Pipe Flow using One-Dimensional Turbulence (ODT): Part 2: Heat Transfer
Pei-Yun Tsai, Heiko Schmidt and Marten Klein
Turbulent concentric coaxial (annular) pipe flow with passive heat transfer is theoretically analyzed and numerically modeled in an extended parametric range using the
stochastic one-dimensional
turbulence (ODT) model. ODT provides predictive capabilities by fully resolving viscous, conductive, and turbulent advective transport processes along a representative radial coordinate within a dimensionally reduced and
stochastic model formulation. Using a fixed model calibration for moderate and low Prandtl numbers, $Pr=0.71$ and $0.025$, effects of radius ratio, $\eta=R_{\rm i}/R_{\rm o}$ are investigated up to a highly turbulent flow regime. The analytical expression of the inner wall boundary layer yields a logarithmic law of the wall for the passive temperature. These results suggest that a conventional linear expression is inadequate for representing near-wall low-order statistics in the radial gap, in particular at the cylindrical inner wall. Additionally, the log-law region falls short if
curvature and finite Reynolds number effects are not considered. Analytical boundary layer profiles fitting numerical predictions form the basis for heat transfer scaling relations. Heat transfer scalings are parameterized by a Nusselt correlation, which is extended to account for radius ratio effects. The findings demonstrate that the radius ratio has a significant impact on the thermal statistics over the two curved walls and should be considered even at high Reynolds numbers and low Prandtl numbers.
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Chemical Physics
A Semiclassical Gaussian Wavepacket Method for Non-Adiabatic Molecular Dynamics
Lorenzo Bocchi, Jia-Xi Zeng and Michele Ceotto
We introduce two non-adiabatic semiclassical methods that employ two coupled Gaussian wavepackets, each one traveling on a separate diabatic potential energy surface. The wavepackets take the form of thawed Gaussians and are driven by classical equations of motion which account for the diabatic coupling. The classical equations of motion are derived in one case by enforcing the thawed Gaussian ansatz, while in the other the time-dependent variational principles to the thawed Gaussian ansatz. After a sanity check where both approximations reproduce Rabi oscillations, the methods are applied to two non-adiabatic potential energy scenarios. The first one involves two coupled displaced harmonic oscillators, as in a typical electron transfer reaction. The second one comprises a Morse potential coupled to an upper dissociative state, modeling a photo-dissociation process. In both scenarios, the variational thawed Gaussian approach is quite accurate, while the standard thawed Gaussian one fails to fully capture the non-adiabatic effects. Ultimately, non-adiabatic molecular dynamics is reproduced by means of two classical trajectories without introducing any artificial jump or other ad-hoc non-classical effects.
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Fluid Dynamics
Entropy-Constrained Machine Learning with Residual Data Augmentation for Modeling Chemical Kinetics
Okezzi Ukorigho and Opeoluwa Owoyele
We present a physics-constrained machine learning framework for accelerating the direct numerical simulation (DNS) of turbulent reacting flows. The model replaces the direct evaluation of detailed chemical source terms with a surrogate that predicts reaction rates from a reduced thermochemical state. To improve physical consistency, the second law of thermodynamics is incorporated as a training constraint by enforcing non-negative
entropy generation, which restricts the
evolution of the thermochemical state to physically admissible directions and improves stability during time integration. The approach is demonstrated on DNS of a two-dimensional planar lean premixed methane-air flame interacting with a turbulent flow field. The model reproduces detailed-chemistry results with high fidelity while achieving more than an order-of-magnitude reduction in computational cost. Furthermore, a residual-based synthetic data augmentation strategy enables parametric exploration by constructing new training data from the original
dataset, allowing accurate simulation at new inlet conditions without additional detailed-chemistry CFD runs. These results demonstrate that thermodynamically constrained machine learning can provide reliable and computationally efficient surrogates for detailed chemistry in high-fidelity combustion simulations.
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Chemical Physics · Cross-listed from cond-mat.soft
Delayed Arm Retraction Controls the Nonlinear Oscillatory Response of Long-Chain-Branched Polymer Melts
Dario Nichetti and Alessio Zaccone
Long-chain branching profoundly modifies the nonlinear oscillatory response of entangled
polymer melts by introducing arm-retraction pathways absent in linear
polymers. We present a molecular tube theory that explains the characteristic maximum of the Nonlinearity Index (NLI) observed experimentally in long-chain-branched
polymers. The theory extends the recently developed nonlinear tube-orientation description of linear
polymers by incorporating branch-point force transmission and delayed arm retraction. The backbone initially develops nonlinear orientation as in the corresponding linear
polymer, whereas long-arm retraction subsequently relaxes the stored branch-point tension and progressively erases backbone orientational
memory. This competition produces a characteristic NLI maximum followed by a post-peak decay. The theory predicts two distinct nonlinear regimes corresponding to sparse and dense long-chain branching and introduces an architecture parameter governing the height and width of the nonlinear peak. The resulting framework provides a molecular interpretation of nonlinear
Fourier rheology and directly links the nonlinear harmonic response to
polymer architecture.
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Chemical Physics
Disorder-Induced Localization of Molecular Polaritons Despite Spectroscopic Strong Coupling
Amitav Sahu and Tönu Pullerits
Molecular polaritons are hybrid light–matter quasiparticles whose collective character is often associated with molecular excitations extending over many emitters. However, molecular ensembles are intrinsically disordered and dissipative, and spectrally visible polariton peaks do not necessarily imply delocalized molecular character. Here, we theoretically examine how static energetic disorder and finite cavity and molecular linewidths affect the delocalization of electronic polaritons in cavity–coupled molecular ensembles. Using a disordered Tavis–Cummings model, we show that energetic disorder mixes polariton states with the dark-state
manifold, causing a rapid loss of collective molecular character even when polaritonic spectral features remain visible. We quantify this crossover using the molecular participation ratio, a density–
matrix–based coherence measure, and an energy–resolved autocorrelation function. In the lossless electronic model, preserving an extended polaritonic molecular component requires the collective Rabi splitting to exceed the disorder width by more than a factor of five, providing a stricter condition than conventional spectroscopic strong coupling. Extending the analysis to a non–Hermitian
Hamiltonian shows that cavity–molecule linewidth imbalance further reduces disorder tolerance. The resulting delocalization boundary indicates that preserving an extended molecular polariton component requires a collective Rabi splitting larger than roughly eight times the disorder width plus approximately twice the cavity–molecule linewidth mismatch. These results provide a quantitative criterion for polariton delocalization under disorder and loss and show that disorder, dissipation, and collective coupling must be considered together when assessing whether molecular polaritons remain collectively extended in realistic optical cavities.
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Fluid Dynamics
Rigorously justified local time stepping in the unified gas-kinetic wave-particle method for steady multiscale flow simulation
Wenzhi Guo, Junzhe Cao, Wenpei Long et al. (+1 more)
Local time stepping (LTS) can accelerate convergence to steady states in kinetic simulations with large variations in the local time steps across the computational domain. When neighboring
cells advance with unequal time steps, the time-averaged particle flux must be balanced across their common interface. For particle-based or hybrid wave-particle methods under finite volume method (FVM) framework, we rigorously establish a sufficient condition for time-averaged interfacial particle-flux balance: fixed positive local time steps together with proportional particle-mass scaling. When a particle crosses from
cell $L$ to
cell $R$, its mass is scaled by $\Delta t_R/\Delta t_L$. In the unified gas-kinetic wave-particle (UGKWP) implementation, the same ratio is applied to the remaining free-transport time of the crossing particle. LTS also affects the wave-particle decomposition and the time integration of the wave fluxes in UGKWP. The wave-particle decomposition in
cell \(i\) is determined by the local ratio \(\Delta t_i/\tau_i\), which better reflects the local relation between the observation scale and relaxation time for multiscale cases. The
equilibrium and analytic free transport wave fluxes are integrated over and normalized by the corresponding
cell-side time steps to obtain the interfacial time-averaged wave fluxes. The UGKWP-LTS method is used to simulate the hypersonic flow past a cylinder at $\mathrm{Kn}=0.01$ and $0.1$, and a flat plate at $\mathrm{Kn}=0.0169$. In all three cases, the surface quantities obtained with UGKWP-LTS agree well with the reference data. Relative to global time stepping (GTS), UGKWP-LTS achieves step-count speedups of $6.6\times$, $3.8\times$, and $20\times$ for the three cases, respectively. The corresponding wall-clock speedups are $7.1\times$, $4.5\times$, and approximately $21\times$.
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All authors Wenzhi Guo, Junzhe Cao, Wenpei Long, Kun Xu
Chemical Physics · Cross-listed from nlin.PS
Chemical Frequency Combs in Reaction-Diffusion Oscillators
Krishnesh Krishnakumar Nair, Madhurendra Mishra, Zhen Qi et al. (+1 more)
Frequency combs, evenly spaced spectral lines locked to one fundamental frequency, are well known in optics and have also been found in phononic, magnonic, ferroelectric, and cosmological systems, but have not yet been studied in oscillating chemical reactions. In this work, we show that reaction-
diffusion oscillators can also produce frequency combs. We use the Brusselator model and derive its
Hopf bifurcation condition directly from the rate equations. We find that the trimolecular autocatalytic term is the only source of nonlinear harmonic content. Above the
Hopf threshold, our simulations of target-wave patterns show a clear fundamental frequency followed by a long, evenly spaced ladder of harmonics, with each harmonic weaker than the one before it. We then vary the reactant concentrations and kinetic parameters one at a time and find that this comb structure holds across a wide range of values. We also test this idea experimentally using the Belousov-Zhabotinsky reaction. Intensity signals recorded at different points in a target pattern show a shared fundamental frequency with several weakening harmonics, matching the simulated pattern closely. Together, these results show that reaction-
diffusion chemistry is a new platform for generating frequency combs.
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All authors Krishnesh Krishnakumar Nair, Madhurendra Mishra, Zhen Qi, Adarsh Ganesan
Plasma Physics
3D Plasma plume characterization of an electrodeless thruster cluster in magnetic arch configuration
Sacha A. Huot, Marco Riccardo Inchingolo, Jaume Navarro-Cavallé et al. (+1 more)
Clustering electrodeless
plasma thrusters in pairs with opposing
magnetic polarities offers an easy means to scale-up the propulsion system of future missions, and also, to mutually cancel their respective
magnetic dipoles. Their
magnetic nozzles merges to form a new
topology, the `
magnetic arch', which can yield a lower
plasma plume divergence than two separate
magnetic nozzles. This work characterizes the
plasma expansion in the fully-closed
magnetic arch of a cluster of two electron-cyclotron resonance thrusters with electrostatic probes (Langmuir probes, Faraday cups, and a Retarding Potential Analyzer). Electrostatic potential,
plasma density, electron temperature, ion current and energy are measured in the two orthogonal planes of
symmetry of the setup for various operating conditions. Results show that a
plasma jet can be extracted even from this
magnetic configuration, albeit with a reduced ion energy. A slight potential hill and hotter electrons exist in the central part of the arch. Ion current profiles are doubly-peaked in the horizontal plane, likely corresponding to the beamlet of each thruster. Trends with xenon mass flow rate and input power are consistent with the expectations of electrodeless
plasma thrusters. The plume experiences an upward or downward deflection depending on the direction of the applied
magnetic field, which could be attributed to the effect of the lateral electron drifts in the
magnetic arch.
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All authors Sacha A. Huot, Marco Riccardo Inchingolo, Jaume Navarro-Cavallé, Mario Merino
Accelerator Physics
Report on the Advanced Linear Collider Study Group (ALEGRO) Workshop 2026
L. Verra, P. Muggli, B. Cross et al. (+59 more)
The 7th ALEGRO workshop, hosted by INFN Frascati National Laboratories from 3rd to 5th March 2026, brought together the international Advanced Novel Accelerators (ANA) community to discuss the role of advanced wakefield accelerators (AWA) in particle and high-energy physics. Organized under the ICFA-ANA panel, the workshop highlighted recent progress in
plasma- and structure-based wakefield acceleration and strengthened international collaboration toward future energy-frontier
colliders. A major focus was the ongoing 10 TeV linear
collider design study, launched in 2025 following the US P5 recommendations. A dedicated session covered accelerator concepts, enabling technologies, and the associated physics case, while recognizing that future priorities will depend on the outcome of the European Strategy for Particle Physics Update. The workshop also reviewed nearer-term applications of advanced accelerators, including fixed-target experiments, injectors for future
colliders and light sources,
plasma-based
Higgs factory concepts, and
proton-driven
plasma wakefield acceleration. Beyond high-energy physics, sessions covered free-electron
lasers, synchrotron light sources, and strong-field QED. Recent demonstrations of FEL lasing with
plasma-accelerated electron beams highlighted significant progress in beam quality and accelerator performance. Operational challenges for reliable user facilities, including high-availability
laser and electron-beam systems, were discussed alongside the growing role of artificial intelligence and machine learning for accelerator optimization and control. This report summarizes the workshop discussions and conclusions from the chairs, together with short contributions from the presenters, providing an overview of the current status and future prospects of advanced wakefield accelerators.
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All authors L. Verra, P. Muggli, B. Cross, E. Adli, R. Babjak, T. Barklow, F. Bencivenga, C. Benedetti, C. Benedetti, M. Buscher, S. S. Bulanov, A. Caldwell, G. Chen, O. Chubenko, R. D'Arcy, S. Diederichs, K. Downham, J. Farmer, M. Ferrario, A. Ferran Pousa, A. Formenti, M. Fuchs, S. Gessner, L. Giannessi, Z. Gong, J. Grames, T. Grismayer, H. Jaworska, L. Ji, V. Kain, S. Karkare, A. Knetsch, C. Lindstrom, N. Lopes, N. Lopes, F. Massimo, C. Masciovecchio, P. Oliveira, T. Opferkuch, J. Osterhoff, J. Osterhoff, S. Pagan Griso, G. Penco, J. Power, A. Pukhov, T. P. Rakitzis, A. Rastogi, L. Reichwein, C. B. Schroeder, L.O. Silva, A. Sinn, D. Sofikitis, D. Storey, M. Thévenet, M. Trovò, M. Turner, M. Vranic, F. Willeke, T. Wilson, M. Wing, W. Zhang, Chuan Zheng
Plasma Physics
On the generation of astrophysically-relevant intermittent magnetic turbulence in the laboratory
Itamar Cohen, Weipeng Yao, Archie F. A. Bott et al. (+18 more)
Intermittent
magnetic turbulence, namely the presence of non-ordered and clusterized fields, is a ubiquitous phenomenon in space and astrophysical
plasmas. It is currently understood that it plays a crucial role in the dynamics of astrophysical systems at all scales, from influencing the
evolution of the cosmos as a whole to governing local particle acceleration. While there is direct evidence of
turbulence in the
solar wind, and despite progress obtained through multi-wavelength observations, most of our knowledge of it outside the
solar system derives from indirect evidence, through modeling. Here we show that
magnetic turbulence, that quantitatively matches that measured in space, can be reproduced in the laboratory. Starting from a homogeneous magnetized
plasma, we randomly perturb it using a speckled
laser beam. Using
proton radiography, we can follow the development and quantitatively characterize the produced intermittent
turbulence from its inception.
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All authors Itamar Cohen, Weipeng Yao, Archie F. A. Bott, Sophia N. Chen, Nikola Mirkovic, Jerome Beard, Petrisor Gabriel Bleotu, Georgiana Giubegal, Anda-Maria Talposi, Yoav Heller, Clement Lacoste, Patrizio Antici, Damiano Caprioli, Emmanuel DHumieres, Victor Malka, Alexandre Marcowith, Ovidiu Tesileanu, Mateusz Ruszkowski, Philipp Kempski, Olga Alexandrova, Julien Fuchs
Geophysics · Cross-listed from cond-mat.stat-mech
The Statistical physics of unsaturated soil water: kinetic theory and non commutative pore water dynamics
Riccardo Rigon
We develop a statistical-mechanical theory of water in unsaturated soil whose outcome is a continuum field equation for the pore-occupancy g(r,x,t), the fraction of pores of radius r that are water-filled at position x and time t. The theory is built across three scales: microscopic inter-pore transfers set by Hagen-Poiseuille rates and a driving potential (the difference of pore-class chemical potentials, taken in capillary-gravitational form but open to adsorptive, osmotic, or thermal refinement); a mesoscale master equation relaxing the occupancy toward the
equilibrium step g_eq=H(r*-r); and, on contracting the averaging volume to a point, the continuum balance d_t g + div F = C[g] - E - T, of which everything else is a limit, a moment, or a boundary resolution. The kinetic equation is an Onsager gradient flow descending the Gibbs free energy, with an H-
theorem for the isothermal unforced system and mass conservation as its zeroth moment. A single dimensionless group, the pore-resolved Damkohler number Da(r,x), organizes the behavior and unifies phenomenologies long modelled separately. A Chapman-Enskog reduction identifies Richards' equation as the quasi-static (Da->0) limit, with matric potential and hydraulic conductivity K emerging only there and K vanishing below the percolation threshold; capillary-bundle and critical-path models are its diagonal and spectral limits. Hysteresis is the holonomy of a forcing bundle, a
geometric phase rather than per-pore bistability, with a falsifiable loop-area law H ~ I^2. Preferential flow is what the same equation does where Da>1, so the Richards/preferential-flow dichotomy becomes a continuous Da-controlled crossover. Out of the quasi-static limit g(r) is the irreducible state variable. All inputs are
geometric properties of the pore network, measurable from micro-CT and calibrated against no macroscopic data.
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Medical Physics
Two observables of one wall: how surface relaxivity can bias the diffusion intra-axonal fraction and the myelin water fraction
Rutger H.J. Fick
Surface relaxivity and time-dependent
diffusion are two readouts of the same wall collisions on one substrate: the transverse rate that microstructure imaging fits is a bulk rate plus a surface rate $\rho\,(S/V)$, and intra- and extra-axonal water carry different $S/V$, so their $T_2$ differ and any compartment estimate that normalises by a TE-weighted $b=0$ is biased. We derive closed forms for the interior (Brownstein-Tarr) and exterior (Novikov-Burcaw) surface rates over myelinated cylinders, validate them with wall-counting Monte Carlo, and quantify the resulting bias on the
diffusion intra-axonal signal fraction $f_{\mathrm{intra}}$ and on the myelin water fraction (MWF). The interior/exterior $S/V$ ratio is $(1-\mathrm{VF})/(g\,\mathrm{VF})$, independent of the calibre distribution and crossing unity at fibre volume fraction $\mathrm{VF}^{\ast}=1/(1+g)$. Physiological white matter sits above this crossover, so surface relaxivity over-weights the intra-axonal signal, biasing $f_{\mathrm{intra}}$ by $\approx 12\%$ at clinical PGSE ($\mathrm{TE}=80\,\mathrm{ms}$). That fractions are TE-dependent is known; new here are the surface-relaxivity and packing attribution, a closed-form sign law, and a testable packing-dependent TE drift. The same physics reads through relaxometry as a smaller MWF bias: the thinnest axons' water crosses below the myelin window and is counted as myelin, so fine white matter reads myelin-richer ($\sim 0.33\,\mathrm{pp}$, beneath single-voxel noise but super-linear in the uncertain $\rho$), while in primary lumen-preserving demyelination this bias is nearly constant and cancels in the longitudinal change. One $S/V$ physics thus biases both
diffusion and relaxometry microstructure estimates, the $f_{\mathrm{intra}}$ bias being first-order and packing-dependent, the MWF bias small and structural.
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Chemical Physics
Quantum phases in endofullerene zigzag chains
Tobias Serwatka, Muhammad Shaeer Moeed, Roger G. Melko et al. (+1 more)
We employ large-scale density
matrix renormalization group calculations to study the
quantum phases of dipolar molecules confined in bent (zigzag) endofullerene chains, as a function of the chain angle $\gamma$. For LiF, ferroelectric order persists across the full range $60^\circ < \gamma 180^\circ$, with the critical effective dipole moment increasing as the chain bends and parallel
alignment becomes less favorable. Near the equilateral configuration ($\gamma = 60^\circ$),
geometric frustration drives a transition to an antiferroelectric Néel-ordered phase in which neighboring dipoles anti-align along the chain axis. We show that capturing this reorientation requires including dipolar couplings beyond the nearest-neighbor approximation, since next-nearest-neighbor interactions become equally strong at $\gamma = 60^\circ$. For confined water, o-D$_2$O reproduces both ordered phases, whereas p-H$_2$O – owing to its large rotational constants – develops no order at any chain angle despite the enhanced coordination of the bent geometry. Because a zigzag chain is the narrowest stripe of a two-dimensional
lattice, these results suggest that engineered endofullerene layers could host a rich variety of dipole-ordered
quantum phases beyond the ferroelectric ordering observed in previous work.
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All authors Tobias Serwatka, Muhammad Shaeer Moeed, Roger G. Melko, Pierre-Nicholas Roy
Space Physics
Reformation of Supercritical Perpendicular Shock
Yuri V. Khotyaintsev, Domenico Trotta and Daniel B. Graham
Super-critical collisionless shocks are not static structures but evolve continuously as they reflect incoming ions back upstream. The physical process responsible for this non-stationarity – whether it is dominated by wave-like corrugation of the shock surface (rippling) or by a cyclic rebuilding of the shock transition (reformation) – remains debated. We combine Magnetospheric Multiscale (MMS) observations of a nearly perpendicular ($\theta_{Bn}\approx89^\circ$), supercritical ($M_A\approx6$) bow shock with high-resolution two-dimensional hybrid simulations to address this question. MMS reveals repeated ion phase-space holes and intense, localized
Hall electric fields. A virtual-spacecraft analysis of the simulation reproduces these signatures and shows that they arise from a self-regulating feedback cycle: strong
Hall-field ion reflection builds a reflected-ion foot, which weakens the
Hall field and suppresses further reflection until the foot decays and the cycle restarts. This reformation cycle, spatially organized by the two-dimensional shock structure, explains most of the observed non-stationarity.
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Geophysics
Acceleration and Strain-Rate Elastic Wave Equation and its Finite-Difference Solution for Distributed Acoustic Sensing Seismic Simulations
Saeed Izadian
Distributed Acoustic Sensing (DAS) is increasingly being adopted across various seismic disciplines due to its cost-effective acquisition capabilities and its high-resolution spatial and temporal sampling. Fiber-optic cables offer exceptional resistance in high pressure and temperature environments, facilitating both rapid-deployment and long-term continuous monitoring. Unlike traditional sensors, fibre optic cables record seismic vibrations as strain or strain-rate along the cables longitudinal axis. To effectively integrate this data format, the governing equations for seismic wave propagation must be adjusted. This study introduces a formulation based on acceleration and strain-rate, analogous to the conventional velocity-stress framework used for describing elastic waves. The resulting system of first order, coupled partial differential equations closely mirrors existing velocity and stress models, allowing current theoretical frameworks and computational tools to be updated with minimal modification. Numerical validation using finite-difference simulations confirms that the proposed formulation produces strain-rate wavefields in complex media with accuracy equivalent to that of traditional velocity-stress methods, thereby effectively eliminating the requirement for data conversion or extra computations.
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Optics · Cross-listed from cond-mat.soft
Quantifying nanoparticle size effect on the photoacoustic generation efficiency
Arthur Billon, Sarah Boumati, Lorenzo Mancini et al. (+9 more)
Photoacoustic (PA) signal generation in colloidal suspensions of optically absorbing nanoparticles is dominated by the thermal expansion of water for gold nanoparticles, but remains mostly unexplored for organic nanoparticles. Here, we derive a model where the PA generation efficiency scales with particle size and thermoelastic contrast with water. The model is validated using solid lipid nanoparticles labeled with several BODIPY dyes. This experimental validation paves the way for quantitative PA characterization of nanomaterials and rational design of PA contrast
agents.
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All authors Arthur Billon, Sarah Boumati, Lorenzo Mancini, Charaf Eddine Benaddi, Elise Michel, Clément Linger, Colin Schotté, Rachel Méallet, Frédéric Gobeaux, Gilles Clavier, Nicolas Tsapis, Jérôme Gateau
Computational Physics
Scale-Bridging Phase-Field Modeling of Microstructure Evolution by FE$^2$ Computational Homogenization
Geralf Hütter, Fenil Lathiya, Vincent von Oertzen et al. (+1 more)
Phase-field models have become a standard tool for simulating complex microstructure
evolution in materials, but their application to engineering-scale components is often hindered by prohibitive computational costs arising from the need to resolve fine-scale features. To address this challenge, we propose a consistent homogenization framework for phase-field theory. By enforcing a Hill-Mandel-type condition of micro-homogeneity formulated in terms of Gurtin's microforces, a well-posed boundary value problem is derived for the representative volume element (RVE), establishing rigorous micro-macro relations for both the order parameter and its gradient. The theory is implemented within a computational two-scale (FE$^2$) scheme and validated against direct numerical simulations. Two distinct examples are investigated: a minimal Allen-Cahn model and a mechanically-coupled model for stress-driven martensitic phase transformation. The results demonstrate that the proposed framework can reliably predict the spatial and temporal
evolution of the macroscopically averaged fields with reasonable accuracy.
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All authors Geralf Hütter, Fenil Lathiya, Vincent von Oertzen, Bjoern Kiefer
Plasma Physics
The fixed boundary plasma equilibrium basis for a one Gigawatt electric stellarator power plant
Samuel A. Lazerson, António J. Coelho, Dana Douqa et al. (+9 more)
A fixed boundary stellarator
equilibrium capable of producing 3 GW of fusion power (1 GW-electric) is presented as the design basis for the GIGA fusion power plant being developed by Gauss Fusion GmbH. The stellarator concept provides a steady-state, transient free, low recirculating power approach to a fusion power plant, which builds on 50 years of progress in
plasma physics. A set of requirements for a fixed boundary
equilibrium were determined through application of 0.5 D modeling. Optimization of a modified Wendelstein 7-X (W7-X)
equilibrium was performed to achieve these requirements including alpha power confinement greater than $85\%$, neoclassical effective ripple below 0.01, bootstrap current below 50 kA, and reduced turbulent heat fluxes. In order to fix the
plasma volume of $1500~m^3$ during optimization, the VMEC code was modified to renormalize the boundary coefficient to the desired
plasma volume. The STELLOPT stellarator optimization code was modified as well to include new bootstrap current targets, a new target for the radial electric field, and the capability to hold the
magnetic field on axis at a fixed value. An intermediary conceptual design
plasma and final evolved fixed boundary
equilibria are compared to the original modified W7-X
equilibrium. The final evolved
equilibrium is shown to achieve all the necessary requirements for the GIGA fusion power plant through more detailed modeling of stability, fast ion confinement, and transport.
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All authors Samuel A. Lazerson, António J. Coelho, Dana Douqa, Adrian Asmund Fessler, Luisa Hübner, Matteo Moscheni, Eoin Hodge, Richard Kembleton, Joona Sissonen, Konsta Särkimäki, Antti Snicker, the Gauss Fusion Team