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A localisation phase transition for the catalytic branching random walk
No full textWe show the existence of a phase transition between a localisation and a non-localisation regime for a branching random walk with a catalyst at the origin. More precisely, we consider a continuous-time branching random walk that jumps at rate one, with simple random walk jumps on , and that branches (with binary branching) at rate everywhere, except at the origin, where it branches at rate . We show that, if is large enough, then the occupation measure of the branching random walk localises (i.e.~when normalised by the total number of particles, it converges almost surely without spatial renormalisation), whereas, if is close enough to , then the occupation measure delocalises, in the sense that the proportion of particles in any finite given set converges almost surely to zero. The case (when branching only occurs at the origin) has been extensively studied in the literature and a transition between localisation and non-localisation was also exhibited in this case. Interestingly, the transition that we observe, conjecture, and partially prove in this paper occurs at the same threshold as in the case~.One of the strengths of our result is that, in the localisation regime, we are able to prove convergence of the occupation measure, whilst existing results in the case give convergence of moments instead
Hamiltonian hydrodynamic reductions of one-dimensional Vlasov equations
No full textWe investigate Hamiltonian fluid reductions of the one-dimensional Vlasov-Poisson equation. Our approach utilizes the hydrodynamic Poisson bracket framework, which allows us to systematically identify fundamental normal variables derived from the analysis of the Casimir invariants of the resulting Poisson bracket. This framework is then applied to analyze several well-established Hamiltonian closures of the onedimensional Vlasov equation, including the multi-delta distribution and the waterbag models. Our key finding is that all of these seemingly distinct closures consistently lead to the formulation of a unified form of parametric closures: When expressed in terms of the identified normal variables, the parameterization across all these closures is revealed to be polynomial and of the same degree. All these parametric closures are uniquely generated from one of the moments, called µ2, a cubic polynomial in the normal variables. This result establishes a structural connection between these different physical models, offering a path toward a more unified and simplified description of the one-dimensional Vlasov-Poisson dynamics through its reduced hydrodynamic forms with an arbitrary number of fluid variables
Universal Audio Generation
No full textThis report describe the research done during the third ESPERANTO/JSALT workshop from the 10th June 2024 to the 2nd of August 2024
Search for Magnetic Monopoles with the Complete ANTARES Dataset
No full textInternational audienceThis study presents a novel search for magnetic monopoles using data collected over a 14 year period (2008-2022) by the ANTARES neutrino telescope. The interaction of magnetic monopoles with matter was modeled according to Kazama, Yang, and Goldhaber cross-section. Upper limits on the flux of magnetic monopoles are obtained for velocities both above and below the Cherenkov threshold. No events consistent with the passage of magnetic monopoles were detected, enabling the setting of an upper flux limit for relativistic magnetic monopoles of the order of
Next generation neural mass model with dopamine modulation mediated by D1-type receptors
No full textInternational audienceNeuromodulation is a complex process in which chemical substances modulate brain activity, allowing for the rich repertoire of behavior that it showcases. Among these substances, called neurotransmitters, dopamine (DA) has a preponderant role, being involved in several mechanism, from motor control to processing of rewarding stimuli. Dysfunctions in the dopamine connections within different brain regions has been observed in pathology, such as Parkinson’s disease and Schizophrenia. Thus, DA effect, and in general of different neurostransmitters, has to be taken into account when building models of brain activity. For the sake of investigating this mechanism, we expand a previously proposed mean-field formalism, that describe the average activity of a neural population in the thermodynamic limit by adding the effect of dopamine modulation, considering that its timescale is much slower than that of spiking activity. Mean-field derivation consists of a Lorentzian ansatz combined with the moment closure approach to arrive at a closed set of equations. Moreover, this formulation allows for a direct comparison of the mean-field reduction with the underlying neural network to test the validity of the reduced model in qualitatively reproducing the population behavior. The resulting mathematical framework is able to reproduce the population activity in distinct dynamical regimes, together with transitions between them. Furthermore, we evince how neuraldynamics, described by the mean-field, is effectively shaped by the effect of DA modulation, hence putting forward the present formulation as a good candidate to investigate influence of DA and other chemicals, such as serotonin or acetylcholine, on neural population dynamical behavior
The North Balearic Front as an ecological boundary: zooplankton fine-scale distribution patterns in late spring
No full textInternational audienceObservations, models and theory have suggested that ocean fronts are ecological hotspots, generally associated with higher diversity and biomass across many trophic levels. Nutrient injections are often associated with higher chlorophyll concentrations at fronts, but the response of the zooplankton community is still insufficiently understood. The present study investigates mesozooplankton stocks and composition during late spring, northeast of Menorca, along two north-south transects that crossed the North Balearic Front separating central waters of the Northwestern Mediterranean Sea gyre from peripheral waters originating from the Algerian basin. During the BioSWOT-Med campaign, vertical triple-net tows with 200 and 500 µm meshes were carried out at three depths (100, 200, and 400 m), and the samples were processed with ZooScan to classify organisms into eight taxonomic groups. Zooplankton distributions were analyzed for the surface layer (0–100 m), a mid-depth layer (100–200 m), and a deeper layer (200–400 m). The results did not show a significant increase in biomass in the front in any layers. The NBF appears to act as a boundary between communities rather than a pronounced area of active or passive zooplankton accumulation. Analyses of stratified vertical distributions of zooplankton highlighted distinct taxonomic compositions in the three layers, and a progressive homogenization of community structure with depth, reflecting a weaker impact of hydrological processes on deeper communities. The clearest impact of the front was within the upper 100 m, where the mesozooplanktonic taxonomic composition differed between the front and adjacent water masses, with a decrease in all taxonomic groups except Cnidaria, which increased dramatically. In the two deeper layers, the front also influenced community composition, although to a lesser extent, with marked increases in Foraminifera and Cnidaria. Moreover, the northern water mass and the front were dominated by large copepods, while the southern water mass exhibited higher zooplankton diversity and smaller-sized copepods. The results of this study highlight the complexity of processes shaping planktonic communities over time and space in the NBF zone and its adjacent waters. These processes include zooplankton stock reduction in the transitional post-bloom period, marked effect of diel variation linked to vertical migrations, and potentially the impact of storm-related mixing in the surface layer that can disrupt established ecological patterns
Surface oscillations of a liquid-solid fluidized bed
No full textInternational audienceWe report experimental investigations of an unconfined liquid-solid fluidized bed at low to moderate Reynolds number. The fluidization velocity U f is measured when an upward flow is imposed, and the sedimentation velocity U sed is measured when the flow is stopped. Only when the level of inlet flow fluctuation is sufficiently low will these two velocities coincide, as expected for an unbound system. A systematic comparison between U f and U sed should therefore be made when attempting to determine fluidization laws. Comparisons with previous studies show that U f measurements gather on a single function of the ratio Φ/Φ pack between the particle volume fraction Φ and its packing value Φ pack , except for a factor K that encompasses all effects other than concentration:Stokes number, Reynolds number, confinement and inlet condition. Time-resolved measurements of the fluidized bed surface show two contributions: uncorrelated high-frequency fluctuations and low-frequency oscillations that remain correlated over minutes. The coherent oscillations correspond to the arrival of upward concentration waves at the bed surface. Their wavelength, which characterizes the length scale of large-scale heterogeneities, was known to diverge as the packing state is approached. Here, we found that it also increases with decreasing concentration, suggesting that the loss of global bed stability observed in previous work at high expansions may be related to the development of heterogeneous structures of a size comparable to that of the column.</div