HAL-Polytechnique
Not a member yet
51406 research outputs found
Sort by
Wavelength selection in the twist buckling of pre-strained elastic ribbons
No full textInternational audienceA competition between short- and long-wavelength twist buckling instabilities has been reported in experiments on thin elastic ribbons having pre-strain concentrated in a rectangular region surrounding the axis. The wavelength of the twisting mode has been reported to either scale (i) as the width of the ribbon when the pre-strain is large (short-wavelength case) or (ii) as the length of the ribbon when the pre-strain is small (large-wavelength case). Existing one-dimensional rod or ribbon models can only account for large-wavelength buckling. We derive a novel one-dimensional model that accounts for short-wavelength buckling as well. It is derived from non-linear shell theory by dimension reduction and captures in an asymptotically correct way both the non-convex dependence of the strain energy on the twisting strain~ (which causes buckling in a first place) and its dependence on the strain gradient . The competition between short- and long-wavelength buckling is shown to be governed by the sign of the incremental elastic modulus associated with the twist gradient . The one-dimensional model reproduces the main features of equilibrium configurations generated in earlier work using 3D finite-element simulations. In passing, we introduce a novel truncation strategy applicable to higher-order dimension reduction, that preserves positiveness of the strain energy even when the gradient modulus is negative
Optimal control under unknown intensity with Bayesian learning
No full textWe consider an optimal control problem inspired by neuroscience, where the dynamics is driven by a Poisson process with a controlled stochastic intensity and an uncertain parameter. Given a prior distribution for the unknown parameter, we describe its evolution according to Bayes' rule. We reformulate the optimization problem using Girsanov's theorem and establish a dynamic programming principle. Finally, we characterize the value function as the unique viscosity solution to a finite-dimensional Hamilton-Jacobi-Bellman equation, which can be solved numerically
Search for rare decays of the Z and Higgs bosons to a J/ or (2S) meson and a photon in proton-proton collisions at = 13 TeV
No full textInternational audienceA search is presented for rare decays of the Z and Higgs bosons to a photon and a J/ or a (2S) meson, with the charmonium state subsequentially decaying to a pair of muons. The data set corresponds to an integrated luminosity of 123 fb of proton-proton collisions at a center-of-mass energy of 13 TeV collected with the CMS detector at the LHC. No evidence for branching fractions of these rare decay channels larger than predicted in the standard model is observed. Upper limits at 95% confidence level are set: (H J/) 2.6 10, (H (2S)) 9.9 10, (Z J/) 0.6 10, and (Z(2S)) 1.3 10. The ratio of the Higgs boson coupling modifiers is constrained to be in the interval (157, 199) at 95% confidence level. Assuming , this interval becomes (166, 208)
Constraints on the photon polarisation in transitions using decays
No full textInternational audienceAn angular analysis of the decay is performed using the proton-proton collision dataset collected between 2011 and 2018 by the LHCb experiment, corresponding to an integrated luminosity of at centre-of-mass energies of 7, 8 and . The analysis is performed in the very low dielectron invariant mass-squared region between and . The longitudinal polarisation fraction of the meson is measured to be less than at confidence level. The observable, which is related to the lepton forward-backward asymmetry, is measured to be , where the first uncertainty is statistical and the second systematic. The transverse asymmetries, and , which are sensitive to the virtual photon polarisation, are found to be and , respectively. The results are consistent with Standard Model predictions
Distribution-Aware Mean Estimation under User-level Local Differential Privacy
No full textInternational audienceWe consider the problem of mean estimation under user-level local differential privacy, where n users are contributing through their local pool of data samples. Previous work assume that the number of data samples is the same across users. In contrast, we consider a more general and realistic scenario where each user u ∈ [n] owns m_u data samples drawn from some generative distribution µ; m_u being unknown to the statistician but drawn from a known distribution M over N ⋆ . Based on a distribution-aware mean estimation algorithm, we establish an M -dependent upper bounds on the worst-case risk over µ for the task of mean estimation. We then derive a lower bound. The two bounds are asymptotically matching up to logarithmic factors and reduce to known bounds when m_u = m for any user u
Mesoscale dynamics and transport in the North Brazil Current as revealed by the EUREC4A-OA experiment
No full textInternational audienceThe North Brazil Current (NBC) rings are key features for interhemispheric water transport, facilitating exchange between the South Atlantic and North Atlantic Ocean. However, significant uncertainties persist regarding the total volume transported by these structures and the water masses they advect. In this study, we integrate high-resolution in situ observations from the EUREC4A-OA field experiment with satellite altimetry to address these knowledge gaps. Using a novel methodology, we estimate that surface NBC rings transport approximately 1.5 Sv, while subsurface eddies contribute between 0.4 and 9.7 Sv, underscoring their critical role in the regional total transport. Combined, these transports may significantly contribute to closing the Atlantic Meridional Overturning Circulation transport at low latitudes. Surface NBC rings predominantly advect Salinity Maximum Waters and fresh waters from the Amazon River, whereas subsurface NBC rings play a critical role in transporting Eastern South Atlantic Central Waters, Western South Atlantic Central Waters, and Antarctic Intermediate Waters northward. We estimate that the heat transport by surface and subsurface NBC rings is 5.8 and 0.3 TW, respectively, which is significantly lower than previous findings. Overall, these findings underscore the pivotal role of subsurface NBC rings as conduits for South Atlantic Waters across the Equator to the tropical North Atlantic. This study confirms the intricate dynamics of NBC rings and their essential role in interhemispheric water transport
Pitch Spelling Jazz Lead Sheets and Solo Transcriptions
No full textInternational audienceWe present an algorithm for pitch spelling tailored for written jazz music. Receiving some input in a MIDI-like format, including information about note heights (expressed in semitones from a reference lowest note) and boundaries of bars (measures), it estimates appropriate note names, one global key signature, and one local scale for each bar. These related pieces of information are jointly assessed in two optimisation steps. In a first "modal" step, one likely scale is guessed for each bar, by minimising the number of accidentals that shall be printed in the engraved score, in a best-path search. Then, in a second "tonal" step, these local scales are used for estimating the key signature that would give the best note spelling on the whole piece. We report successful experiments on a set of lead sheets from the Real Book as well as transcriptions of Jazz solo recordings and basslines. Our procedure is originally designed for an application to music transcription, in particular the construction of digital collections of written jazz soli from audio recordings, in the context of musical analysis, teaching and cultural heritage preservation. Moreover, we have defined for its purpose new distances between various common jazz scales, which might be of some interest in musicological studies
Multimodal Analysis of Oak Wood Metabolites for a Comprehensive Understanding of the Aging Process in Typical French Spirit Barrels
No full textInternational audienceABSTRACT Oak wood plays a crucial role in barrel aging, significantly influencing the flavor, aroma, and quality of aged spirits. This study integrates liquid chromatography–tandem mass spectrometry (LC–MS/MS), laser desorption/ionization mass spectrometry (LDI‐MS), and time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) to analyze metabolite distribution in oak wood. By comparing freshly cut wood, freshly toasted wood, or historical wood staves of 2‐ to 50‐ or 100‐year‐old, this multimodal approach reveals spatial distribution changes in lignin, polysaccharides, lipids, and extractable compounds such as phenolic ones. Notably, the degradation of polysaccharides and migration of ellagitannins are observed, providing new insights into wood‐aging dynamics. These findings offer potential improvements in barrel aging techniques to enhance the sensory profile of spirits
Dose Rate Effects in Ag-Doped Metaphosphate Glass Radiophotoluminescent Dosimeters Up to MGy Range
No full textInternational audienceThis work presents a comprehensive study of dose rate effects on Ag-doped metaphosphate glass radiophotoluminescence (RPL) dosimeters for doses ranging between 1 Gy and 100 MGy. Data from multiple irradiation campaigns with diverse radiation fields, such as 60Co gamma radiation, X-rays with energy spectrum up to 100 keV, monoenergetic electron beams, and mixed fields offer valuable insights for accurate dosimetry in high-radiation environments. Dose rates ranging over eight decades, from 0.1 kGy/h to 18 MGy/h, are experimentally tested. Low dose rate effects can be observed at low doses; however, for doses higher than 100 kGy, both overestimation and underestimation can occur depending on the dose rate
