Solar and Stellar Astrophysics
Turbulent Convection: Modal Equations and Energy Pathways
S. Sridhar and Nishant K. Singh
We present a framework for studying high Rayleigh number turbulent convection to better understand
stellar and planetary convection zones. Utilizing the statistical
symmetries of the fully developed turbulent state of Boussinesq convection, we identify relevant mean and fluctuating quantities. After validating these
symmetry assumptions through numerical simulations, we formulate the governing equations. Vertical profiles of key physical quantities in the saturated turbulent state are explored in the simulations. To develop a modal theory, we use
Fourier expansions, review linear theory, and use the Craya-Herring velocity decomposition. The modal equations we derive describe high Rayleigh-number turbulent convection dynamics self-consistently in terms of nonlinear interactions between three mode types: growing
gravity modes, decaying
gravity modes, and horizontal modes. Energy extracted by the growing modes from the superadiabatic background subsequently follows multiple pathways toward dissipation, enabled by the mode couplings. Among these, the traditionally dominant pathway is the turbulent cascade of the growing modes themselves. Reduced modal equations capture this pathway, precisely describing (i) mutual interactions between growing modes, and (ii) the excitation of decaying and horizontal modes, which are subordinate to the growing modes. Determining the relative efficiency of the pathways requires investigating their modal spectra using numerical simulations and kinetic models.
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High Energy Astrophysical Phenomena
Spectral densification and macroscopic phase delay of gravitational echoes from exotic compact objects
Corentin Guigot
Gravitational-wave echoes from Exotic Compact Objects (ECOs) provide an observable probe for horizon-scale physics. Standard phenomenological models for these signals typically assume a constant Free Spectral Range, relying on the
geometric optics approximation. In this work, we demonstrate that wave dispersion at the
photon sphere induces a systematic deviation from this assumption, manifesting instead as a
hyperbolic spectral densification. By employing an analytical framework based on the Riccati equation and macroscopic impedance mapping, we extract the spectrum of these high-finesse resonances without semi-classical approximations. We characterize the structural transition from the eikonal
geometric asymptote ($\ell \gg 1$) down to the wave-tunneling dominated quadrupolar mode ($\ell=2$). In this wave-dominated regime ($\ell \in [2, 10]$), the macroscopic deviation from the semi-classical limit is governed by a phenomenological $\mathcal{L}^{-3/2}$ inverse power law. Finally, we show that this macroscopic densification isolates the structural dispersion of the external spacetime, decoupled from the boundary microphysics, provided the membrane phase shift is frequency-independent.
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High Energy Physics (Phenomenology)
Rare processes in ultrahigh-energy tau-lepton transport
Guo-yuan Huang
In
cosmic neutrino observatories, charged-
lepton transport is a key input for interpreting observables and reconstructing
neutrino events. Charged
leptons propagating through matter undergo several energy loss processes, such as electron pair production and photonuclear interaction. In this work, we investigate several rare processes in tau-
lepton transport, focusing primarily on
muon pair production, $\tau N \to \tau \mu^+\mu^- N$, and Primakoff neutral-pion production, $\tau N \to \tau \pi^0 N$. At EeV energies, we find that the energy loss contributions from
muon pair and Primakoff pion production are only $0.6\%$ and $0.2\%$, respectively, of that from electron pair production. Nevertheless, dimuon production may be relevant to tau
neutrino searches in
underwater and under-ice Cherenkov
telescopes. The interaction length for dimuon emission by an EeV tau is only $6~{\rm km}$ in standard rock. Such events may be identified through the lateral separation of dimuon tracks or through a kebab
topology if tau decays in the detector.
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General Relativity and Quantum Cosmology
The brachistochrone problem for a constant velocity traveler in static and spherically symmetric spacetimes
Longfei Wang and Junji Jia
This work investigates the brachistochrone problem for a traveler with constant local velocity within static and spherically symmetric (SSS) spacetimes. The brachistochrone trajectory (BT) equations for ultra-relativistic travelers are derived for general SSS metrics, and the solution is formally obtained in an integral form. We then apply the result to two representative spacetimes corresponding to the singular isothermal sphere (SIS) with a finite boundary and the relativistic Plummer mass profile, respectively. For the SIS spacetime, the BT inside the boundary is solved analytically and found always to bend. As the equation of state index $w$ increases, the turning radius $r_0$ of the BT, and consequently the total time, also increase. For the BT outside the boundary, it is found that the heavier the central object, the larger the $r_0$ and the total travel time. For the relativistic Plummer model, the BT will be a straight line passing through the origin when the initial and final points' radii are comparable or smaller than the size of the core region of the mass distribution. When the end points lie well outside the core region, the BT bends, exhibiting a larger turning radius for a more concentrated core. We then extend the consideration to travelers with subluminal constant velocity $v$ and show through the generalized Fermat's principle that the BT will be the same geodesic in the optical metric as ultra-relativistic travelers, with the total travel time scaled by a factor of $1/v$.
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Instrumentation and Methods for Astrophysics
The CUSP CubeSat mission for space weather multi-physics analysis, design, and testing
Giovanni Lombardi, Riccardo Di Spirito, Sergio Fabiani et al. (+48 more)
The CUbesat
Solar Polarimeter (CUSP) mission aims to measure the linear polarization of
solar flares in the hard X-ray band by means of a Compton scattering polarimeter. CUSP is a project in the framework of the Alcor Program of the Italian Space Agency aimed at developing new CubeSat missions. We present the outcomes of the CUSP's Phase B study, which is ended on 2 July 2026. The design solutions adopted for the mission's most critical multi-physics design drivers will be discussed, these solutions have been formulated and applied to demonstrate compliance with system requirements at both the spacecraft and platform levels. Moreover, we will discuss the validation of the Payload model based on the environmental testing campaign (e.g., vibration) carried out on a demonstrator.
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All authors Giovanni Lombardi, Riccardo Di Spirito, Sergio Fabiani, Daniele Censi, Giovanni De Cesare, Ettore Del Monte, Riccardo Campana, Enrico Costa, Mauro Centrone, Nicolas De Angelis, Sergio Di Cosimo, Giuseppe Di Persio, Abhay Kumar, Pasqualino Loffredo, Gabriele Minervini, Fabio Muleri, Paolo Romano, Alda Rubini, Emanuele Scalise, Paolo Soffitta, Andrea Alimenti, Enrico Silva, Bruna Bertucci, Giovanni Ambrosi, Giovanni Cucinella, Vito Di Bari, Simone Di Filippo, Andrea Negri, Massimo Perelli, Dario Modenini, Andrea Curatolo, Nicolas Gagliardi, Daniele Pecorella, Alice Ponti, Paolo Tortora, Andrea Del Re, Davide Albanesi, Valerio Campamaggiore, Giulia De Iulis, Adam Ebrahim, Marco Schepis, Alessandro Zambardi, Ilaria Baffo, Pierluigi Fanelli, Costantino Zazza, Valerio Vagelli, Daniele Brienza, Immacolata Donnarumma, Matteo Mergè, Emanuele Zaccagnino, Alessandro Turchi
Soft Condensed Matter
Phoretic flow in a three-dimensional wedge geometry
Abdallah Daddi-Moussa-Ider, Semyon Yakubovich and Maciej Lisicki
Understanding how chemically induced surface transport generates fluid motion in confined geometries is essential for the rational design of microscale pumping devices and active microfluidic systems. Here we develop a theoretical framework for chemically driven phoretic flows in a three-dimensional wedge geometry in the
diffusion-dominated regime. We formulate both the
diffusion and hydrodynamic problems using a
Fourier-Kontorovich-Lebedev spectral representation, exploiting the translational invariance and radial structure of the wedge. Green's functions for the concentration field are derived for reflecting and mixed reflecting-absorbing boundaries, reducing to finite image-like sums or closed-form expressions for commensurate wedge angles. The resulting slip velocity is then used to construct the three-dimensional Stokes flow through the Papkovich-Neuber representation, yielding explicit spectral solutions for the velocity field. These results establish a Green's-function framework for phoretic pumping in wedge-shaped confinement and provide analytical
benchmarks for numerical simulations of chemically driven transport in confined microfluidic systems.
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Fluid Dynamics
A note on the lower boundary condition for Ekman flows
Christian Puntini and Luigi Roberti
We show that the assumption of a stress-free boundary condition at a finite intermediate depth, namely, at the bottom of the Ekman layer, in the analysis of wind-driven ocean flows necessarily leads to an unphysical current profile. Indeed, if the $z$-derivative of the fluid velocity vanishes at a given depth, then this depth necessarily corresponds to a minimum of the velocity profile, with the velocity increasing beneath it. Using a WKB ansatz based on the small variations of the ocean's water density at great depths, we also argue that a no-slip condition at the bottom of the ocean, if sufficiently deep, still effectively implies (up to a very small error) the orthogonality of the Ekman transport and the wind-stress.
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Fluid Dynamics
Nonlinear asymptotic bubble growth in single-mode spherical Rayleigh-Taylor instability
De-Hua Zhang, Shi-Heng Wang, Ke-Jian Qian et al. (+3 more)
We present an analytical model for the nonlinear growth of a single-mode Rayleigh-Taylor instability (RTI) bubble in spherical geometry. The model captures the bubble growth along the polar axis, spanning the linear to nonlinear regimes, for arbitrary Atwood numbers and under both converging- and diverging-
gravity configurations. The model predicts that the bubble acceleration approaches an asymptotic value in the nonlinear stage. The spherical geometry is found to enhance the RTI bubble growth relative to planar and cylindrical configurations with the same effective perturbation wavenumber in the converging-
gravity cases, whereas it mitigates the bubble growth in the diverging-
gravity cases. The model predictions show favorable agreement with direct numerical simulations.
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All authors De-Hua Zhang, Shi-Heng Wang, Ke-Jian Qian, Zhu-Jun Li, Rui Yan, Hang Ding
Soft Condensed Matter
Mechanical isotropy of heterogeneous octahedral materials
Jehoon Moon, Seunghwan Lee, Gisoo Lee et al. (+2 more)
An octahedral network has been widely used as a fundamental building block for diverse architected materials. Here, we demonstrate that heterogeneous octahedral materials can achieve complete elastic isotropy at a critical constituent volume fraction, nearly independent of the constituent stiffness ratio. The anisotropy ratio, a, transitions from a > 1 to a < 1 at the critical constituent volume fraction, due to a change in the dominant deformation modes. Microstructural analysis reveals that the geometry and connectivity of the heterogeneous octahedral materials are very similar to those of heterogeneous materials constructed on combined simple cubic (SC) and body-centered cubic (BCC)
lattices exhibiting opposite elastic anisotropy. More importantly, we demonstrate that varying the constituent volume fractions in the octahedral materials governs elastic anisotropy, similarly to tuning the BCC-to-SC composition ratio in the SC-BCC materials widely employed for designing mechanically isotropic architected materials. The mechanical isotropy of the heterogeneous octahedral materials is further assessed using 3D-printed prototypes.
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All authors Jehoon Moon, Seunghwan Lee, Gisoo Lee, Jeongun Lee, Hansohl Cho
Applied Physics
Integrated tunable magnonic devices for beyond 6 GHz signal processing
Maria Cocconcelli, Antonio Angotti, Federico Maspero et al. (+2 more)
In the search for novel technology platforms supporting the transition towards ”beyond 6G” telecommunications, magnonics is emerging as a viable route, primarily due to its intrinsic compatibility with the UWB-FR3 bands and its easy tunability. In this paper, we present a proof-of-concept device based on a CoFeB magnonic waveguide, fully integrated on silicon, which demonstrates all the key features of our integration approach, based on co-integrated hard
magnetic micromagnets, magnonic conduits, reconfigurable soft
magnetic elements and MEMS. The bias field enabling operation up to about 12 GHz is provided by SmCo micromagnets embedded in the silicon substrate. Real-time tunability is implemented in two ways. First, current-driven control is achieved via an integrated current line that generates an additional localized
magnetic field, enabling continuous tuning of Backward Volume
spin-wave propagation. Second, we achieve voltage-controlled tunability by integrating a NiFeMo soft
magnetic element onto a piezoelectric MEMS cantilever and flip-chipping it onto the magnonic device. Upon actuation, the cantilever brings the NiFeMo into proximity with the permanent micromagnets, where it captures the stray field. The resulting voltage-controlled displacement modulates the coupling between the hard and soft
magnetic components, effectively reducing the local bias field on the CoFeB waveguide and enabling fine control of Damon–Eshbach
spin waves.
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All authors Maria Cocconcelli, Antonio Angotti, Federico Maspero, Andrea Cattoni, Riccardo Bertacco
Medical Physics
CBCT-IQ: A Publicly Available Annotated Cone-Beam CT Dataset for Image Quality Assessment and Benchmarking
Sepideh Hatamikia, Anna Breger, Clemens Karner et al. (+14 more)
Medical image quality plays a critical role in diagnostic accuracy, especially in X-ray-based imaging modalities such as cone-beam computed
tomography (CBCT), where image quality must be balanced against radiation dose. While expert visual evaluation remains the clinical standard for image quality evaluation, it is time-consuming, subjective and affected by inter-observer variability, emphasizing the need for reliable quantitative image quality assessment (IQA) methods. However, the development and validation of such IQA methods have been limited by the lack of publicly available CBCT
datasets with expert image quality annotations. In this study, we provide the first open-access CBCT IQA
dataset containing 1,764 annotated image slices acquired using systematic variations in image acquisition and reconstruction parameters. Three clinical experts graded the overall image quality and a predefined regions of interest (ROI) using a four-level scoring scheme. In addition, we
benchmark 26 full reference- and no reference-based IQA measures against expert annotations and introduce an exploratory IQA measure-based ranking capable of distinguishing subtle image quality differences. This
dataset introduced a standardized
benchmark for future CBCT IQA research and provides a valuable resource for the development and validation of new IQA methods, enabling reproducible research and advancing CBCT IQA.
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All authors Sepideh Hatamikia, Anna Breger, Clemens Karner, Birgit Pohn, Poorya MohammadiNasab, Martin Buschmann, Stephanie Nougaret, Laura Haddad, Ali Abbasian Ardakani, Afshin Mohammadi, Paul Apfaltrer, Wolfgang Birkfellner, Alfred Pohl, Ander Biguri, Gernot Kronreif, Carola-Bibiane Schönlieb, Tess Reynolds
Strongly Correlated Electrons
Beyond Stoner-Wohlfarth: Machine-Learning Models and Symbolic Regression of Hard-Magnet Properties
Samuel J. R. Holt, Christina Winkler, Timoteo Colnaghi et al. (+7 more)
Predicting the extrinsic properties from hysteresis loops of a
magnetic grain, namely the coercive field, remanent magnetisation, and maximum energy product, from its intrinsic micromagnetic parameters is a central problem in permanent-magnet modelling. Established analytical models provide useful estimates but often neglect nonuniform magnetisation processes, whereas direct micromagnetic simulations are computationally expensive. In this work, we train machine-learning models on 12012 micromagnetic simulations of an idealised cubic grain, spanning broad ranges of the saturation magnetisation, exchange constant, and uniaxial anisotropy constant. Benchmarked against the analytical models on identical held-out data, the machine-learning models predict all three extrinsic properties with substantially lower errors. Symbolic regression recovers the Kronmüller form of the coercive field, with an effective demagnetising factor that depends on the material, and finds new closed-form expressions for the remanence and maximum energy product. Each law contains at most two fitted constants yet approaches the accuracy of the machine-learning models. We also investigate the inverse problem of recovering the intrinsic parameters from the three extrinsic properties. The saturation magnetisation and anisotropy constant are recovered accurately, whereas the exchange constant is not, because it influences the extrinsic properties only weakly. The trained models are released through the mammos-ai Python package, enabling thousands of candidate parameter sets to be screened in seconds rather than the hours or days required by direct micromagnetic simulation.
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All authors Samuel J. R. Holt, Christina Winkler, Timoteo Colnaghi, Martin Lang, Swapneel A. Pathak, Andrea Petrocchi, Michael Adams, Thomas Schrefl, Andreas Marek, Hans Fangohr
Physics and Society
How null-model constraints affect statistical validation in projected bipartite networks
Alessandro Catalano and Rosario N. Mantegna
Statistical validation of projected bipartite networks depends critically on the null model adopted to describe random co-occurrences. Although several null models have been proposed, their comparison has mainly focused on the validated backbones they produce rather than on the statistical assumptions underlying their construction. Here we compare four widely used null models - the microcanonical configuration model generated by the Curveball
algorithm, the Bipartite Configuration Model (BiCM), the Bipartite Partial Configuration Model (BiPCM), and the Hypergeometric approximation - using three empirical bipartite systems from comparative genomics, international trade and food science. We use the statistically validated links obtained from the microcanonical bipartite configuration model as the reference
benchmark for assessing performances of the other three models. Our central result is that the statistical consequences of relaxing null-model constraints cannot be understood solely from the constraints themselves but must be analyzed through the probability distribution induced for the co-occurrence statistic. In particular, the combined behaviour of the expectation and variance largely explains the observed differences among the statistically validated backbones. We further derive a leading-order sparse approximation for the BiCM expectation, showing that the first correction to the Hypergeometric prediction is controlled by the degree heterogeneity of the non-projected layer. Surprisingly, despite neglecting this heterogeneity, the Hypergeometric model accurately reproduces the co-occurrence variance of the microcanonical ensemble across all
datasets. Our results suggest that null models should be compared not only according to the constraints they preserve but also according to the statistical consequences that these constraints induce on the distribution of the test statistic.
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Plasma Physics
Proof of operation of a micro-hollow cathode discharge driven by a true ns-pulsed high voltage: emission properties and atomic nitrogen production
Nikolaos Chazapis, Dimitrios Stefas, Alexandra Brisset et al. (+5 more)
The implementation of Micro-Hollow Cathode Discharges (MHCDs) for the efficient and controlled dissociation of molecular nitrogen into reactive N-atoms remains a central challenge in the synthesis of strategic materials such as hexagonal boron nitride (h-BN). Traditional MHCDs driven with DC high voltages (HV) can generate N-atoms under low-to-moderate gas pressures. However, they are prone to intrinsic instabilities that eventually induce arcing, increase thermal load and limit significantly their lifetimes. In this work we aim to enhance N2 dissociation efficiency in an Ar/N2 MHCD driven by a true nanosecond (ns) pulsed positive HV which has not been yet considered in MHCDs. A testbed reactor is employed to investigate microplasma behavior under ns-pulsed operation, which is then compared to a conventional DC MHCD. Specifically, we combined optical emission and ns-TALIF diagnostics under suitable operating conditions of each HV regime comprising different gas mixtures, pressures, and electrical parameters. These measurements allowed for the identification of key excited species and mapping of ground-state N-atoms absolute density in the low-pressure chamber of the MHCD reactor. It is demonstrated that ns-pulsed excitation produces a more symmetric discharge expansion on the cathodic surface compared to a standard DC excitation, while generating similar nature of emissive species and consuming up to 2 orders of magnitude lower average power depending on the operating frequency and voltage. Importantly, up to 6-fold enhancement in absolute N-atoms density (maximum value measured: 1.14x10 15 cm -3 ) is achieved with the ns-pulsed MHCD making it very promising for h-BN synthesis.
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All authors Nikolaos Chazapis, Dimitrios Stefas, Alexandra Brisset, Laurent Invernizzi, Noël Girodon-Boulandet, Guillaume Lombardi, Claudia Lazzaroni, Kristaq Gazeli
High Energy Physics (Theory)
A Toy Model for Topological Entanglement Features in 1+1D Integrable Quantum Field Theory
Emmalise J. H. Aalbers and Olalla A. Castro-Alvaredo
In this paper we investigate an
entanglement measure, the Rényi
entropy, in a 1+1D integrable
quantum field theory known as the Federbush model. This is a deformation of the theory of two massive Dirac
fermions by means of a bilinear term in the $U(1)$ currents that couples the two
fermion species. This deformation gives rise to $S$-
matrix elements which are coupling-dependent phases, distinct from $- 1$. These non-trivial phases can be seen as encoding anyon-like statistics. From this viewpoint, the Federbush model is a toy model for topological features of
entanglement in one space dimension. In this paper we show that, for an infinite system, these topological features play no role when computing many known measures of
entanglement at
equilibrium in the ground state. This conclusion applies also to the post-quench dynamics after a small quench of the topological parameter.
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General Physics
Predicting Radial Velocities from Rossiter-McLaughlin Time Series Observations
Vardan Adibekyan, Olivier Demangeon, Diogo Teixeira et al. (+7 more)
The Rossiter-McLaughlin (RM) effect produces apparent radial velocity (RV) shifts through line-profile distortions caused by a transiting
planet blocking different regions of the rotating
stellar surface. Because the underlying orbital RV trend can be estimated from out-of-transit observations, RM sequences provide a controlled laboratory for studying flux-induced RV variations. We compiled a sample of 1171 ESPRESSO observations of 13 targets obtained during 21 RM observing nights and trained machine-learning models to reconstruct a reference RV trend from observed RVs, line-profile diagnostics, and activity indicators. Predictive performance varied substantially among
stars and depended on both the strength of the RM signal and the similarity of the target
star to the training sample. Applications to Sun-as-a-
star observations and Proxima Centauri recovered known periodicities but did not fully remove the activity-induced variability. Larger and more diverse
datasets will be required to assess the potential of this approach for mitigating activity-induced RV signals.
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All authors Vardan Adibekyan, Olivier Demangeon, Diogo Teixeira, Nuno C. Santos, Khaled Al Moulla, Eduardo Cristo, Andre Silva, Roman Chertovskih, Garik Israelian, Artur Hakobyan
Soft Condensed Matter
Weak-electrolyte diffusiophoresis for rigid colloids
Subrata Majhi and Huanshu Tan
We develop a model for the diffusiophoresis of a chemically inert, rigid spherical colloid with fixed surface charge in a monovalent weak electrolyte, in which a neutral solute reversibly dissociates into ions. A weak far-field gradient is imposed in the neutral-species concentration. In the fast-reaction limit, local mass action and bulk electroneutrality determine the far-field ionic gradients, while the bulk zero-current condition determines the
diffusion-potential gradient. We solve the coupled Nernst-Planck, Poisson and Stokes equations for arbitrary double-layer thickness, linearising in the gradient strength while retaining the nonlinear Poisson-Boltzmann
equilibrium. In the Debye-Hückel limit, the mobility consists of one half of the matched fully dissociated response and a finite-double-layer correction due to neutral-ion coupling; the correction vanishes in both the Hückel and Smoluchowski limits. Beyond this limit, numerical solutions for the representative systems reveal a branch-selective response as the surface potential magnitude increases. When the counterion is slower than the co-ion, dissociation-association weakens a retarding concentration-polarisation layer, allowing the mobility to exceed the fully dissociated value. When the counterion is faster, the response remains close to the one-half scaling set by mass action. This reaction-polarisation coupling cannot be reproduced by adjusting only the bulk ionic strength, and hence the Debye length, in a fully dissociated model.
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Accelerator Physics
Space-charge compensation of a low-energy He2+ beam with fluorine-containing gases
Takashi Nagatomo, Yasuyuki Morita and Osamu Kamigaito
This study investigates the space-charge compensation (SCC) of a low-energy He2+ beam by introducing fluorine-containing gases into the low-energy beam transport line (LEBT) of the RIKEN Heavy-Ion Linear Accelerator. The introduction of SF6, C3F8, and CHF3 improved beam transmission through the LEBT, and the largest improvement was obtained using SF6. In contrast, the increase in the beam current with Kr was relatively small, and its effect was less pronounced than that of the fluorine-containing gases. The introduction of fluorine-containing gases increased the rise time of the chopped beam to approximately 10 ms. Experiments performed without the prebuncher confirmed that the prebuncher was largely responsible for this slow rise. Furthermore, the substantial difference in the rise time between Kr and the fluorine-containing gases suggests that different charge carriers may contribute to the SCC in these two cases. Based on the results, a new grid-type beam chopper was installed immediately upstream of the
radio-frequency quadrupole linac (RFQ). This configuration allows short beam pulses to be generated immediately before the RFQ while maintaining a direct-current beam in the LEBT, thereby achieving both an effective SCC and a fast beam-current rise. In an acceleration test using the new chopper, a 19.3 micro A He2+ beam was successfully and stably accelerated to the radioisotope-production beamline at a duty factor of 10%. This corresponds to approximately 193 micro A in continuous-wave mode operation, demonstrating that the proposed method is effective for realizing the acceleration of a 200 micro A-class He2+ beam.
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Chemical Physics · Cross-listed from cs.LG
Frugal Bayesian Optimization: Scalable Surrogates for Data- and Resource-Limited Discovery
Panagiotis Krokidas, Christoforos Rekatsinas, Vassilis Sioros et al. (+3 more)
Bayesian Optimization (BO) is widely adopted for data-efficient optimization in scientific and engineering applications, yet its computational cost is rarely evaluated alongside optimization performance. Here we present a systematic, compute-aware study of BO that evaluates surrogate models along two axes: optimization quality and computational frugality. Across eight
benchmark functions and nine real-world
datasets spanning materials science, mechanics, robotics, chemistry, and machine learning, we
benchmark four surrogate models: Gaussian Processes, Random Forests, NGBoost, and
Bayesian Adaptive Spline Surfaces. We show that Gaussian Process-based BO consistently incurs the highest time and
memory overhead without delivering superior optimization or sample efficiency. In contrast, scalable alternatives achieve equal or better performance at a fraction of the computational cost. Motivated by these findings, we introduce a surrogate-recommendation framework that predicts the most suitable BO surrogate from inexpensive
dataset characteristics. Together, these results establish FruBO as a reproducible, compute-aware baseline for
Bayesian Optimization and provide practical guidance for surrogate selection under limited computational and experimental budgets.
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All authors Panagiotis Krokidas, Christoforos Rekatsinas, Vassilis Sioros, Grigorios M. Chatziathanasiou, Efi-Maria Papia, George Giannakopoulos
Fluid Dynamics
A global mass-preserving numerical method for low-Mach-number real-gas flows in closed systems
Sanath Kotturshettar, Gonçalo Alcobia, Pietro Carlo Boldini et al. (+2 more)
A mass-preserving low-Mach-number framework is proposed for closed-system real-fluid flows governed by general nonlinear equations of state. The formulation enforces consistency between the spatially uniform thermodynamic pressure, the equation of state, and global mass conservation. Moreover, the numerical
algorithm employs a segregated strategy in which the thermodynamic state is updated before the momentum equations, and the velocity field is advanced using a pressure-correction method. This approach enables an efficient solution procedure by decoupling the thermodynamic and momentum updates and retaining the use of FFT-based solvers for the pressure correction. The resulting formulation is implemented with second-order spatial accuracy. The implementation is first verified using the method of manufactured solutions, in which the thermodynamic state is prescribed through analytical density and thermodynamic-pressure fields, enabling verification of the nonlinear equation of state, thermodynamic-pressure
evolution, and the low-Mach-number divergence constraint. The framework is subsequently validated against
benchmark laminar and turbulent flows for both ideal and real fluids, particularly transcritical CO$_2$ channel flow, demonstrating its accuracy and robustness in the presence of strong thermodynamic nonlinearities.
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All authors Sanath Kotturshettar, Gonçalo Alcobia, Pietro Carlo Boldini, Rene Pecnik, Pedro Costa
High Energy Physics (Phenomenology)
Pentaquark Bound States and Regge Trajectories in QCD via Bethe Salpeter Formalism
M.Ghaderi, N. Tazimi and M. Monemzadeh
We present a comprehensive calculation of pentaquark masses and Regge trajectories within the framework of the Godfrey-Isgur relativized
quark model. The pentaquark is treated as a
meson-
baryon molecular system, and the bound state is solved using the Bethe-Salpeter equation with the effective
meson-
baryon interaction motivated by GI, including smearing, running coupling, and
spin-
spin interactions. Using the latest PDG data for the known Pc and Pcs states, we compute the ground state and first two radial excitations for four pentaquark candidates: Pc(4440), Pc(4457), Pcs(4338), and Pcs(4459). The calculated masses are in excellent agreement with experiment. We also construct radial Regge trajectories in the (n, M^2) plane. Within the three calculated radial levels, the trajectories are approximately linear, with slopes comparable to those reported for ordinary
hadrons. These results are consistent with a common confinement-driven pattern, but do not establish its universality.
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Quantum Physics
Fisher-Orthogonal Memory in Quantum Reservoir Computing
Ce Wang and Xingze Qiu
Quantum reservoir computing processes temporal information through driven many-body dynamics, but its performance is ultimately limited by how accurately past inputs can be extracted from finite measurements. Here we formulate this limitation as a local multiparameter estimation problem and introduce a delay-space
quantum Fisher information
matrix to quantify the distinguishability of information stored at different delays. This perspective identifies Fisher-orthogonal
memory as a measurement-efficient design principle: different delays should perturb the reservoir state along mutually Fisher-orthogonal directions. We first analyze the single-
qubit limit using the Gill–Massar bound, revealing an optimal write-store-routing trade-off. Guided by this structure, we construct solvable multi-
qubit reservoirs based on Clifford routing orbits and Singer-cycle Pauli
algebra. The resulting dynamics yield diagonal delay-space QFIMs with analytically programmable fading profiles. Under finite-shot local Pauli readout, these reservoirs retain sharp
memory windows that are absent in a validation-selected Ising baseline. Their product-task behavior is governed by second-order responses inherited from the same Pauli-routing
algebra. Our results provide an analytically controlled route toward measurement-efficient
quantum reservoir computing.
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Solar and Stellar Astrophysics
Pulsations along the Asymptotic Giant Branch (AGB): a global view
Pierre Darriulat, Pham Thi Tuyet Nhung and Do Thi Hoai
In three preceding articles, we studied the relation between the light curves of Mira variables and their
stellar properties. The analysis was restricted to a sample of
stars offering high density and quality of observations in the visible, allowing for the definition of simple parameterizations of the light curves. Clear relations were revealed between the
evolution of the
stars along the AGB and parameters such as the width of the light pulse or the deviation of its ascending branch from a sine wave. However, the restriction to Mira variables was found to be a major drawback preventing a complete picture to be drawn. The present article overcomes this deficiency by reviewing a large sample of semi-regular and irregular variables. It gives evidence for two families of
stars having different progenies. The first family enters the AGB already pulsating with amplitudes at the scale of one unit of magnitude. It keeps cooling down and expanding, while pulsating with increasing amplitudes and periods and decreasing width of the light pulse; its progeny is dominated by some of the Mira variables studied in the three preceding articles. The second family enters the AGB before displaying significant pulsations; some of its progeny may leave the AGB without entering the Thermally Pulsing Branch and experiencing strong TDU events; others may instead become Mira variables studied in the three preceding articles and pulsating with a broader light pulse. The possible transition of both families from oxygen-rich to carbon-rich is studied and described. Comments on the interpretation of these results in the context of current state-of-the-art knowledge are presented. While the
stars of the first family tend to have larger initial masses than the
stars of the second family, details of the properties of the convective envelope seem also to play a role in defining which families a
star belongs to.
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Materials Science
Quadratic piezoelectricity from stacking-engineered interference in multilayer sliding ferroelectrics
Jiaxuan Fan, Yuexing Zhao, Xiao-Ping Li et al. (+3 more)
Designing nonlinear piezoelectricity requires suppressing the linear piezoelectric coefficient without extinguishing higher-order electromechanical response, yet a general and reconfigurable route remains lacking. Here we introduce stacking-engineered piezoelectric interference as such a mechanism in multilayer sliding ferroelectrics. Combining first-principles calculations with a generalized Ginzburg–Landau framework, we show that each interlayer gap acts as a local piezoelectric channel whose sign and magnitude are determined by stacking. Constructive interference between same-signed channels produces a linear-dominated response, whereas destructive interference between oppositely signed channels suppresses the linear coefficient while preserving a finite quadratic response. Representative MoS$_2$ and NiTe$_2$ multilayers approach the
parabolic limit, with BAAC-stacked MoS$_2$ reducing the linear-to-quadratic crossover strain by a factor of 25 relative to CBA-stacked MoS$_2$. Experimentally accessible tetralayer MoS$_2$ sliding pathways further connect linear-dominated, quadratic-dominated and sign-inverted states. Here, we identify stacking-engineered interference as a design principle for programmable nonlinear electromechanics in layered materials.
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All authors Jiaxuan Fan, Yuexing Zhao, Xiao-Ping Li, Yurong Yang, Xing-Qiu Chen, Lei Wang
Computational Physics
Multistability and state-switching in series-coupled resonant tunneling diodes
Jannis Waldmann, Jonnel Jaurigue and Kathy Lüdge
Resonant tunneling diodes (RTDs) embedded in an electrical
circuit are known for their
neuron-like response characteristics, which makes them promising candidates for neuromorphic applications. This paper investigates the dynamical response of series-coupled RTDs and systematically analyzes the impact of coupling and inhomogeneities on the solution structure. We further propose a scheme for controlled switching between coexisting stable states which allows to realize tunable
memory elements in these
circuits. The coupled RTD system exhibits a rich bifurcation structure, showing different degrees of multistability between symmetric and antisymmetric solutions. Limit-cycle branches and their dependence on the system parameters are analyzed using numerical continuation methods. A central focus is placed on the role of
symmetry. For two identical RTDs, the system possesses a $\mathbb{Z}_2$ exchange
symmetry, which governs the emergence of
symmetry-breaking bifurcations and multistable states. The analysis is further generalized to $N$ coupled RTDs, revealing the underlying $S_N$
symmetry structure and its influence on the organization of
equilibrium branches, paving the way for neuromorphic network operation.
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Quantum Physics
Martijn Dols, Carlos Gonzalez-Ballestero, Mikhail Cherkasskii et al. (+3 more)
We propose a protocol where
chiral magnons mediate a state transfer between two distant
spin qubits. The protocol is implemented by varying the coupling between the
spin qubits and the
magnons in time, such that an arbitrary state is transferred from one
qubit to the other. The modulation of the coupling is performed such that the two-
spin-
qubit state is kept as a dark state of the
magnon bath, bypassing the associated losses. We show that the protocol can be realized on a hybrid system composed of two nitrogen-vacancy (NV) centers coupled to the nonreciprocal and
chiral magnon modes of an yttrium iron garnet (YIG) stripe. We propose two methods to achieve the time modulation of the NV-
magnon coupling: i) the NV-magnet distance of both NV centers is varied; ii) the external
magnetic field and the NV-magnet distance of one NV center are varied. We evaluate the implementability of both methods numerically, including the constraints on the temperature, and the dephasing time and minimal lifetime of the
spin qubits required for high-fidelity state transfer. We find that using realistic experimental parameters, a state transfer between NV centers at a distance of several microns can be achieved with a fidelity $\gtrsim 0.95$. Our findings expand the toolbox of magnonics for
quantum information purposes.
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All authors Martijn Dols, Carlos Gonzalez-Ballestero, Mikhail Cherkasskii, Christian L. Degen, Victor A. S. V. Bittencourt, Silvia Viola Kusminskiy
High Energy Physics (Experiment)
Constraining Axion-Like-Particles with germanium detector at the Kuo-Sheng Reactor Neutrino Laboratory
TEXONO Collaboration: Greeshma C., L. Singh, H. T. Wong et al. (+22 more)
We present results of a search for
Axion-Like Particles (ALPs) produced $via$ Primakoff and Compton-like scattering channels, using data acquired with TEXONO experiment at the Kuo-Sheng
Nuclear Power Station. The analysis is based on 278.91 days of reactor-ON and 43.60 days of reactor-OFF data. These
datasets were collected using a 1.06 kg high-purity germanium detector located 28 m from a 2.9 GW reactor core. No significant excess is observed in the residual spectrum from the reactor-ON and reactor-OFF data subtraction. Using data acquired with low-background germanium detectors, upper limits on both the ALP-
photon ($g_{a\gamma\gamma}$) and ALP-electron ($g_{aee}$) couplings are derived for ALP masses ranging from 1 eV to 3 MeV at 90% confidence level. Since both $g_{a\gamma\gamma}$ and $g_{aee}$ couplings contribute to ALP production and detection, a combined analysis is performed by treating both channels as active parameters.
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All authors TEXONO Collaboration: Greeshma C., L. Singh, H. T. Wong, J.-W. Chen, M. Deniz, H. B. Li, S. Karadağ, S. Karmakar, J. Li, F. K. Lin, S. T. Lin, C.-P. Liu, S. K. Liu, H. Ma, D. K. Mishra, M. K. Pandey, K. Rani, R. Raj, V. Sharma, M. K. Singh, M. K. Singh, V. Singh, C.-P. Wu, L. T. Yang, Q. Yue
Medical Physics
Intestinal peristalsis and wrinkling: A novel paradigm
René Thierry Djoumessi, Christopher Miller, Nipuni D. Nagahawatte et al. (+3 more)
A new computational framework for modeling the intestinal wall as a multi-layered fiber-reinforced continuum is presented. The framework reproduces for the first time physiological motility and overcoming large-displacements limitations (self-contact and volume locking) occurring in classical hyperelastic formulations of soft tissues. We introduce: i) layer-specific functions, segregating active circumferential and longitudinal muscle fibers while maintaining homogeneous passive reinforcement, and ii) a quasi-incompressible volumetric contribution, to handle large peristaltic contractions.
Cell electrophysiology is further extended to reproduce both slow waves and spike bursting activities thus mimicking for the first time a localized neural excitation in a three-dimensional geometry of small intestine segment. We introduce a spatio-temporal modulation of contractility to accurately capture activation driven by both slow waves and spike bursts. The overall coupled nonlinear electromechanical boundary valued problem is modeled following the active strain approach. A robust augmented-Lagrangian contact
algorithm is also embedded to avoid self-penetration and geometrical instabilities under large displacements. The 8-variables nonlinear governing equations are then discretized using in house P1-P2-P4 finite elements codes implemented within the GetFEM library. Numerical experiments demonstrate the ability of the proposed framework to reproduce physiological peristalsis, i.e., wall contraction greater than 80%, thus allowing full occlusion matching in vivo endoscopic images, and naturally generating wrinkling patterns consistent with experimental observations. We show that an active electromechanics anisotropic heterogeneous modeling strategy is critical for a numerically stable and physiologically accurate representation of gastrointestinal motility.
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All authors René Thierry Djoumessi, Christopher Miller, Nipuni D. Nagahawatte, Marco Paggi, Leo K. Cheng, Alessio Gizzi
Soft Condensed Matter
Coalescence-induced alignment of anisotropic particles in drying sessile droplets
Johannes Schöttner, Qingguang Xie and Jens Harting
Alignment of anisotropic particles strongly governs the functional properties of printed materials, yet most studies have focused on particle
alignment in single evaporating droplets. In droplet- based printing, however, neighboring droplets can coalesce, generating rapid capillary flows that redistribute material and markedly affect the final morphology. Here, we use mesoscale simulations to investigate how droplet coalescence and subsequent evaporation jointly determine
alignment and redistribution in sessile droplets with different contact angles and volumes. During the early stages of coalescence, the mean nematic order along the coalescence direction increases for all combinations of contact-angle and volume asymmetries of the droplets. At later times, the mean nematic order either continues to increase or decreases, depending on the droplet geometry. We derive a
geometric scaling based on
curvature and volume asymmetry and show that the simulation results collapse onto a master curve, identifying an effective
geometric asymmetry parameter that governs the mean nematic order at the end of coalescence. During evaporation, the contact angle strongly influences how the coalescence-induced orientational structure is transferred to the final deposit. For small contact angles, the contact line remains pinned for a longer duration, better preserving
alignment. In contrast, larger contact angles promote contact-line motion, which weakens
alignment, as reflected by a reduced mean nematic order, while simultaneously generating stronger concentration gradients in the final deposit.
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Cosmology and Nongalactic Astrophysics
A Quantitative Framework for Testing the Hubble Tension in a Bianchi Type I Cosmological Background
Luigi Tedesco
The Hubble tension is usually formulated as a disagreement between two determinations of a single scalar parameter, $H_0$, within an isotropic FLRW model. We develop a quantitative framework treating the tension as a consistency test of the scalar FLRW compression of cosmological data in a homogeneous, anisotropically expanding Bianchi type I background. Beyond synthesizing established results on Bianchi I kinematics, null geodesics, and optical propagation, our original contribution is a worked weak-shear, axisymmetric calculation mapping a specified shear history into a low-
redshift luminosity-distance quadrupole. The calculation explicitly separates the direction-dependent
redshift–affine-parameter mapping from the Jacobi-focusing contribution, propagating the resulting distance quadrupole through an analytic polar-cap toy window. For freely decaying shear, we obtain $A_D(z) = -B_{H0} + (2q_0-1)B_{H0}z/2 + (5-q_0-18q_0^2+6j_0)B_{H0}z^2/12 + O(z^3, B_{H0}^2)$, where $B_{H0}=(H_{\parallel 0}-H_{\perp 0})/H_0$ and $j_0$ is the mean jerk parameter. A representative BBN limit, $\Omega_{\sigma 0} \le 10^{-23}$, implies $\vert{}B_{H0}\vert{} \le 9.5 \times 10^{-12}$ and a distance-modulus quadrupole below $2.4 \times 10^{-11}$ mag at $z=0.15$. The early-Universe bound used is adopted from prior work; the novelty lies in propagating it through the derived Sachs–Jacobi mapping into limits on the luminosity-distance quadrupole and catalogue-window bias. By contrast, a 1% directional shift requires $\Omega_{\sigma 0} \approx 2.5 \times 10^{-5}$, while matching the Planck 2018–SH0ES 2022 separation requires $\Omega_{\sigma 0} \approx 1.8 \times 10^{-3}$. Thus, the minimal shear-only model cannot resolve the tension, though the framework supplies a falsifiable programme for testing late-time anisotropy with SNe, BAO, and standard sirens.
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