HAL-Polytechnique
Not a member yet
51406 research outputs found
Sort by
Testing strong-field QED with the avalanche precursor
No full textInternational audienceA two-beam high-power laser facility is essential for the study of one of the most captivating phenomena predicted by strong-field quantum electrodynamics (QED) and yet unobserved experimentally: the avalanche-type cascade. In such a cascade, the energy of intense laser light can be efficiently transformed into high-energy radiation and electron-positron pairs. The future 50-petawatt-scale laser facility NSF OPAL will provide unique opportunities for studying such strong-field QED effects, as it is designed to deliver two ultra-intense, tightly focused laser pulses onto the interaction point. In this work, we investigate the potential of such a facility for studying elementary particle and plasma dynamics deeply in the quantum radiation-dominated regime, and the generation of QED avalanches. With 3D particle-in-cell simulations, we demonstrate that QED avalanche precursors can be reliably triggered under realistic laser parameters and layout (namely, focusing , tilted optical axes, and non-ideal co-pointing) with the anticipated capabilities of NSF OPAL. We demonstrate that seed electrons can be efficiently injected into the laser focus by using targets of three types: a gas of heavy atoms, an overcritical plasma, and a thin foil. A strong positron and high-energy photon signal is generated in all cases. The cascade properties can be identified from the final particle distributions, which have a clear directional pattern. At increasing laser field intensity, such distributions provide signatures of the transition, first, to the radiation-dominated interaction regime, and then to a QED avalanche. Our findings can also be used for designing related future experiments
Cross-Analysis of Magnetic and Current Density Field Topologies in a Quiescent High Confinement Mode Tokamak Discharge
No full textInternational audienc
Benchmarking the Benchmarks: Reproducing Climate-Related NLP Tasks
No full textInternational audienceSignificant efforts have been made in the NLP community to facilitate the automatic analysis of climate-related corpora by tasks such as climate-related topic detection, climate risk classification, question answering over climate topics, and many more. In this work, we perform a reproducibility study on 8 tasks and 29 datasets, testing 6 models. We find that many tasks rely heavily on surface-level keyword patterns rather than deeper semantic or contextual understanding. Moreover, we find that 96% of the datasets contain annotation issues, with 16.6% of the sampled wrong predictions of a zero-shot classifier being actually clear annotation mistakes, and 38.8% being ambiguous examples. These results call into question the reliability of current benchmarks to meaningfully compare models and highlight the need for improved annotation practices. We conclude by outlining actionable recommendations to enhance dataset quality and evaluation robustness
A complex logistic equation for universal energy evolution in hadronic elastic scattering
No full textInternational audienceWe introduce a universal evolution equation for elastic scattering of hadrons, derived from Regge field theory (RFT) and solved in closed analytical form. The equation emerges from a complex logistic structure and evolves initial amplitude profiles taken from existing models at fixed energy, reproducing both the differential cross sections and the integrated quantities in a broad energy range. We prove that it admits a unique solution for each initial condition and rigorously satisfies unitarity, the Froissart-Martin bound, and dispersion relations. The dynamics are governed by two physically meaningful parameters: the effective Pomeron mass and the nonlinear coupling , both fitted at a single energy. Our approach offers a minimal, yet predictive framework for saturation and unitarization in elastic scattering and may provide a useful bridge toward small- QCD evolution
Determination of the spin and parity of all-charm tetraquarks
No full textInternational audienceThe traditional quark model accounts for the existence of baryons, such as protons and neutrons, which consist of three quarks, as well as mesons, composed of a quark-antiquark pair. Only recently has substantial evidence started to accumulate for exotic states composed of four or five quarks and antiquarks. The exact nature of their internal structure remains uncertain. This paper reports the first measurement of quantum numbers of the recently discovered family of three all-charm tetraquarks, using data collected by the CMS experiment at the Large Hadron Collider from 2016 to 2018. The angular analysis techniques developed for the discovery and characterization of the Higgs boson have been applied to the new exotic states. The quantum numbers for parity and charge conjugation symmetries are found to be +1. The spin of these exotic states is consistent with 2, while 0 and 1 are excluded at 95% and 99% confidence level, respectively. The assignment implies particular configurations of constituent spins and orbital angular momenta, which constrain the possible internal structure of these tetraquarks
Cohomologically tropical varieties
No full textInternational audienceGiven the tropicalization of a complex subvariety of the torus, we define a morphism between the tropical cohomology and the rational cohomology of their respective tropical compactifications. We say that the subvariety of the torus is cohomologically tropical if this map is an isomorphism for all closed strata of the tropical compactification.We prove that a schön subvariety of the torus is cohomologically tropical if and only if it is wunderschön and its tropicalization is a tropical homology manifold. The former property means that the open strata in the boundary of a tropical compactification are all connected and the mixed Hodge structures on their cohomology are pure of maximum possible weight; the latter property requires that, locally, the tropicalization verifies tropical Poincaré duality.We study other properties of cohomologically tropical and wunderschön varieties, and show that in a semistable degeneration to an arrangement of cohomologically tropical varieties, the Hodge numbers of the smooth fibers are captured in the tropical cohomology of the tropicalization. This extends the results of Itenberg, Katzarkov, Mikhalkin and Zharkov.</div
Sélection de maillage basée sur les erreurs pour des simulations numériques efficaces en présence de paramètres variables
No full textAdvanced numerical simulations often depend on mesh refinement techniques to manage discretization errors in complex models and reduce computational costs. This work concentrates on Adaptive Mesh Refinement (AMR) for steady-state solutions, which uses error estimators to iteratively refine the mesh locally and gradually tailor it to the solution. AMR requires evaluating the solution across a series of meshes. When solving the model for multiple operating conditions, such as in uncertainty quantification studies, full systematic adaptation can cause significant computational overhead. To mitigate this, the Error-based Mesh Selection (EMS) method is introduced to decrease the cost of adaptation. For each operating condition, EMS seeks to choose, from a library of pre-adapted meshes, the one that minimizes the discretization error. A key feature of this approach is the use of Gaussian Process models to predict the solution errors for each mesh in the library. These error models are built solely from the library's meshes and their solutions, using restriction errors as proxies for discretization errors, thereby avoiding additional model evaluations. The EMS method is tested on an analytical shock problem and a supersonic scramjet configuration, showing near-optimal mesh selection. The influence of library size on the resulting error level is also examined.Les simulations numériques avancées s’appuient souvent sur des techniques de raffinement de maillage pour contrôler les erreurs de discrétisation de modèles complexes et réduire les coûts de calcul. Ce travail porte sur le raffinement adaptatif de maillage (AMR) pour des solutions stationnaires, utilisant des estimateurs d’erreur pour raffiner itérativement le maillage localement et l’adapter progressivement à la solution. L’AMR nécessite d’évaluer la solution sur une série de maillages. Lorsqu’on résout le modèle pour de multiples conditions de fonctionnement, comme dans des études de quantification d’incertitudes, une adaptation systématique complète peut entraîner un surcoût de calcul important. Pour y remédier, la méthode de sélection de maillage basée sur l’erreur (EMS) est introduite afin de réduire le coût de l’adaptation. Pour chaque condition de fonctionnement, EMS cherche à choisir, dans une bibliothèque de maillages pré-adaptés, celui qui minimise l’erreur de discrétisation. Une caractéristique clé de cette approche est l’utilisation de modèles à processus gaussiens pour prédire les erreurs de solution pour chaque maillage de la bibliothèque. Ces modèles d’erreur sont construits uniquement à partir des maillages de la bibliothèque et de leurs solutions, en utilisant les erreurs de restriction comme substituts des erreurs de discrétisation, évitant ainsi toute évaluation supplémentaire du modèle. La méthode EMS est testée sur un problème analytique de choc et sur une configuration de statoréacteur supersonique (scramjet), montrant une sélection de maillage quasi optimale. L’influence de la taille de la bibliothèque sur le niveau d’erreur obtenu est également examinée
Provable non-accelerations of the heavy-ball method
No full textInternational audienceIn this work, we show that the heavy-ball (\HB) method provably does not reach an accelerated convergence rate on smooth strongly convex problems. More specifically, we show that for any condition number and any choice of algorithmic parameters, either the worst-case convergence rate of \HB on the class of -smooth and -strongly convex \textit{quadratic} functions is not accelerated (that is, slower than ), or there exists an -smooth -strongly convex function and an initialization such that the method does not converge. To the best of our knowledge, this result closes a simple yet open question on one of the most used and iconic first-order optimization technique. Our approach builds on finding functions for which \HB fails to converge and instead cycles over finitely many iterates. We analytically describe all parametrizations of \HB that exhibit this cycling behavior on a particular cycle shape, whose choice is supported by a systematic and constructive approach to the study of cycling behaviors of first-order methods. We show the robustness of our results to perturbations of the cycle, and extend them to class of functions that also satisfy higher-order regularity conditions
Accessing Generalized Parton Distributions through 2 to 3 exclusive processes
No full text7 pages, 1 figure, Presented by S. Wallon at the 31st International Workshop on Deep Inelastic Scattering (DIS2024).International audienceWe review our results on a new class of 2 to 3 exclusive processes, as a probe of both chiral-even and chiral-odd quark GPDs. We consider the exclusive photoproduction of a photon-meson pair, in the kinematics where the pair has a large invariant mass, described in the collinear factorization framework. We cover the whole kinematical range from medium energies in fixed target experiments to very large energies of colliders, by considering the experimental conditions of JLab 12-GeV, COMPASS, future EIC and LHC (in ultra-peripheral collisions) cases. Our analysis covers neutral and charged rho-mesons, as well as charged pions. The case of the rho-meson, depending on its polarization, provides access to either chiral-even or chiral-odd GPDs, at leading twist. We find that the order of magnitude of the obtained cross sections are sufficiently large for a dedicated experimental analysis to be performed, especially at JLab. Furthermore, we compute the linear photon beam polarization asymmetry, which we find to be sizeable, in the case of a longitudinally polarized -meson or of a charged pion. These predictions are obtained for both asymptotic distribution amplitude (DA) and holographic DA
Bottom quark energy loss and hadronization with B and B nuclear modification factors using pp and \PbPb collisions at = 5.02 TeV
No full textInternational audienceThe production cross sections of B and B mesons are reported in proton-proton (pp) collisions recorded by the CMS experiment at the CERN LHC with a center-of-mass energy of 5.02 TeV. The data sample corresponds to an integrated luminosity of 302 pb. The cross sections are based on measurements of the B J/(1020)(KK) and B J/K decay channels. Results are presented in the transverse momentum () range 7-50 GeV/ and the rapidity interval 2.4 for the B mesons. The measured -differential cross sections of B and B in pp collisions are well described by fixed-order plus next-to-leading logarithm perturbative quantum chromodynamics calculations. Using previous PbPb collision measurements at the same nucleon-nucleon center-of-mass energy, the nuclear modification factors, , of the B mesons are determined. For 10 GeV/, both mesons are found to be suppressed in PbPb collisions (with values significantly below unity), with less suppression observed for the B mesons. In this range, the values for the B mesons are consistent with those for inclusive charged hadrons and D mesons. Below 10 GeV/, both B and Bs are found to be less suppressed than either inclusive charged hadrons or D mesons, with the B value consistent with unity. The values found for the B and B are compared to theoretical calculations, providing constraints on the mechanism of bottom quark energy loss and hadronization in the quark-gluon plasma, the hot and dense matter created in ultrarelativistic heavy ion collisions