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Dynamics and energy dissipation of collisional blast waves in a perpendicular magnetic field
No full textInternational audienceWe experimentally investigate the evolution and dynamics of laser-produced collisional blast waves (BW) under the influence of a perpendicular magnetic field up to 20 T. We show that an external magnetic field causes the BW to diverge from the Taylor–Sedov solution while also impacting its structural morphology. We notably explore the significance of various magnetohydrodynamic (MHD) processes occurring on scales similar to the width of the BW front by comparing their characteristic lengths to it and demonstrate that the downstream plasma's transition from being super- to sub-magnetosonic plays a pivotal role in the overall structure. Our results show that multiple MHD effects can contribute to shaping a magnetized BW, illustrating the complexity of the underlying physics
Measurements of He-collision-induced line-shape parameters of CO2 lines in the ν3 band
No full textInternational audienceHe-collision-induced line-shape parameters of CO2 lines were measured in the 3 band using Fourier transform spectra recorded at room temperature and with pressures ranging from 263 mbar to 1106 mbar. The measured transmission spectra were analyzed with the Voigt profile combined with the first-order line-mixing approximation, accounting for the instrument lineshape function. The He-broadening coefficients, pressure shifts, and first-order line-mixing parameters were determined for 51 lines, from the P(50) to the R(51). The obtained Hebroadening coefficients are in excellent agreement with various literature values. These broadening coefficients, together with data for higher J lines, extrapolated from available hightemperature measurements, allowed us to propose an improved dataset for He-broadening coefficients of CO2 lines. We demonstrated that accounting for line-mixing effects is essential to accurately determine the pressure shifts. The latter, measured for the first time for the 3 band, exhibited a weak rotational dependence, in contrast to the strong dependence observed for air-and self-pressure shifts for CO2. The obtained line-mixing coefficients agree well with those calculated using the Energy Corrected Sudden model. The results of this study significantly enhance the line-shape parameters dataset for CO2 perturbed by He, providing improved data for spectroscopic databases and for studies of planetary and exoplanetary atmospheres
Stochastic Tangential Pareto Dynamics Provably Samples the Whole Pareto Set
No full textThe framework of stochastic multi-objective programming allows for the inclusion of uncertainties in multi-objective optimization problems at the cost of transforming the set of objectives into a set of expectations of random quantities. The stochastic multigradient descent algorithm (SMGDA) gives a solution to these types of problems using only noisy gradient information. However, a bias in the algorithm causes it to converge to only a subset of the whole Pareto front, limiting its use. We analyze the source of this bias and prove the convergence of SMGDA to a stationary point in the nonconvex L-lipschitz smooth case. First, based on this analysis, we propose to reduce the bias of the stochastic multi-gradient calculation using an exponential smoothing technique. We then propose a novel approach to exploring the whole Pareto set by combining the debiased stochastic multigradient with an additive non-vanishing noise that guides the dynamics of the iterates tangential to the Pareto set. We finish by proving that our algorithm, Stochastic Tangential Pareto Dynamics (STPD), generates samples concentrated on the whole Pareto set
Discrete Markov Probabilistic Models
No full textInternational audienceThis paper introduces the Discrete Markov Probabilistic Model (DMPM), a novel algorithm for discrete data generation. The algorithm operates in the space of bits {0, 1} d , where the noising process is a continuous-time Markov chain that can be sampled exactly via a Poissonian clock that flips labels uniformly at random. The time-reversal process, like the forward noise process, is a jump process, with its intensity governed by a discrete analogue of the classical score function. Crucially, this intensity is proven to be the conditional expectation of a function of the forward process, strengthening its theoretical alignment with score-based generative models while ensuring robustness and efficiency. We further establish convergence bounds for the algorithm under minimal assumptions and demonstrate its effectiveness through experiments on low-dimensional Bernoulli-distributed datasets and high-dimensional binary MNIST data. The results highlight its strong performance in generating discrete structures. This work bridges theoretical foundations and practical applications, advancing the development of effective and theoretically grounded discrete generative modeling
Old dog, new tricks: Exact seeding strategy improves RNA design performances
No full textInternational audienceThe Inverse Folding problem involves identifying RNA sequences that adopt a target structure with respect to free-energy minimization, i.e. preferential to all alternative structures. The problem has historically been regarded as challenging, largely due to its proven NP-completeness of an extended version where the base pair maximization energy model is used. In contrast, it has recently been shown that a large subset called m-separable structures, notably including those comprising helices of length 3+, can be solved in linear-time within the same energy model. This permits not only the identification of a single solution, but also the characterization of a language of solutions.In this work, we seek to describe the ``hardness'' of Inverse Folding, bridging (at least heuristically) the gap between a simplified energy model and a more realistic Turner energy model. We used LinearBPDesign to generate seed sequences for RNAinverse, thereby improving the design process in a Turner energy model. To this end, we extended LinearBPDesign to accommodate biseparability and to handle non- or high modulo separable structures by minimalist addition of base pairs.Our study suggests that seeds generated by LinearBPDesign capture long-range interactions, thereby improving the performance of RNAinverse compared to seed focusing on refining the energy model itself. Most surprisingly, a significant number of LinearBPDesign seeds uniquely fold into the target structure in the Turner model, especially when helices are at least of length 2. This observation suggests that the ``hardness'' of design may arise from the intrinsic properties of the structures themselves
Ultrafast dynamics of hot carriers: Theoretical approaches based on real-time propagation of carrier distributions
No full textInternational audienceIn recent years, computational approaches which couple density functional theory (DFT)-based description of the electron–phonon and phonon–phonon scattering rates with the Boltzmann transport equation have been shown to obtain the electron and thermal transport characteristics of many 3D and 2D semiconductors in excellent agreement with experimental measurements. At the same time, progress in the DFT-based description of the electron–phonon scattering has also allowed to describe the non-equilibrium relaxation dynamics of hot or photo-excited electrons in several materials, in very good agreement with time-resolved spectroscopy experiments. In the latter case, as the time-resolved spectroscopy techniques provide the possibility to monitor transient material characteristics evolving on the femtosecond and attosecond time scales, the time evolution of photo-excited, nonthermal carrier distributions has to be described. Similarly, reliable theoretical approaches are needed to describe the transient transport properties of devices involving high energy carriers. In this review, we aim to discuss recent progress in coupling the ab initio description of materials, especially that of the electron–phonon scattering, with the time-dependent approaches describing the time evolution of the out-of-equilibrium carrier distributions, in the context of time-resolved spectroscopy experiments as well as in the context of transport simulations. We point out the computational limitations common to all numerical approaches, which describe time propagation of strongly out-of-equilibrium carrier distributions in 3D materials, and discuss the methods used to overcome them
Macroscopic limit from a structured population model to the Kirkpatrick-Barton model
No full textInternational audienceWe consider an ecology model in which the population is structured by a spatial variable and a phenotypic trait. The model combines a parabolic operator on the spatial variable with a kinetic operator on the trait variable. We prove the existence of solutions to that model, and show that these solutions are unique. The kinetic operator present in the model, that represents the effect of sexual reproductions, satisfies a Tanaka-type inequality: it implies a contraction of the Wasserstein distance in the space of phenotypic traits. We combine this contraction argument with parabolic estimates controlling the spatial regularity of solutions to prove the convergence of the population size and the mean phenotypic trait to solutions of the Kirkpatrick-Barton model, which is a well-established model in evolutionary ecology. Specifically, at high reproductive rates, we provide explicit convergence estimates for the moments of solutions of the kinetic model
Search for long-lived heavy neutral leptons in proton-proton collision events with a lepton-jet pair associated with a secondary vertex at = 13 TeV
No full textInternational audienceA search for long-lived heavy neutral leptons (HNLs) using proton-proton collision data corresponding to an integrated luminosity of 138 fb collected at = 13 TeV with the CMS detector at the CERN LHC is presented. Events are selected with a charged lepton originating from the primary vertex associated with the proton-proton interaction, as well as a second charged lepton and a hadronic jet associated with a secondary vertex that corresponds to the semileptonic decay of a long-lived HNL. No excess of events above the standard model expectation is observed. Exclusion limits at 95% confidence level are evaluated for HNLs that mix with electron and/or muon neutrinos. Limits are presented in the mass range of 1-16.5 GeV, with excluded square mixing parameter values reaching as low as 2 10. For masses above 11 GeV, the presented limits exceed all previous results in the semileptonic decay channel, and for some of the considered scenarios are the strongest to date
Double DVCS amplitudes including kinematic twist-3 and 4 corrections
No full text5 pages, 2 figures, conference proceedings for the 31st International Workshop on Deep Inelastic Scattering (DIS2024)International audienceGeneralized parton distributions (GPDs) are off-forward matrix elements of quark and gluon operators that work as a window to the total angular momentum of partons and their transverse imaging (nucleon tomography). To access GPDs one needs to look into exclusive processes which are usually studied in a kinematic regime known as the Bj\"orken limit. In this limit, the photon virtualities are much larger than the hadron mass , and the kick to the hadron measured by the Mandelstam's variable . It turns out that this is not enough for the purposes of a precise GPD extraction and, in particular, of nucleon tomography for which measurements in a sizable range of are required. Deviation with respect to the Bj\"orken limit induces kinematic higher-twist corrections which enter the amplitudes with powers of and , where denotes the scale of the process (basically, the sum of photon virtualities in the case of DDVCS). There are also corrections by the name of "genuine" higher twists which are a separate topic and are not the subject of this research study. In this manuscript, we present novel calculations of DDVCS amplitudes off a (pseudo-)scalar target including up to kinematic twist-4 corrections. These results are important for measuring DDVCS, DVCS and TCS through the Sullivan process and off helium-4 target at the future Electron-Ion Collider (EIC) and JLab experiments. Preliminary numerical estimates for the pion target are provided
Magnetized Strange Stars and Signals of Gravitational Waves
No full textInternational audienceWe study the emission of gravitational waves from spheroidal magnetized strange stars for both an isolated slowly rotating star and a binary system. In the first case, we compute the quadrupole moment and the amplitude of gravitational waves that may be emitted. For the binary system, the tidal deformability is obtained by solving simultaneously the system of spheroidal structure equations and the Love number equation. These results are compared with the data inferred from the GW170817 event which is also used to calculate the mass and tidal deformability of the companion star in the binary system. Our model supports binary systems formed by magnetized strange stars describing reasonable signals of gravitational waves contrasted with other models of binary systems composed of magnetized hadronic stars and non-magnetized quark stars