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Comment une idée abstraite devient un dispositif de gestion: Le cas du développement durable
No full textInternational audienceLe développement durable est une idée abstraite. Par quel enchaînement de mécanismes une telle idée peut-elle avoir un effet sur la gestion de firmes, et ce, au niveau le plus opérationnel ? Cet article décrira tout d’abord cet ensemble de dispositifs en mettant notamment l’accent sur le rôle des méta-organisations. Il s’interrogera ensuite sur la complexité organisationnelle qui en découle
Quantitative retardance imaging of biological samples using quadriwave lateral shearing interferometry
No full textInternational audienceWe describe a new technique based on the use of a high-resolution quadri-wave lateral shearing interferometer to perform quantitative linear retardance and birefringence measurements on biological samples. The system combines quantitative phase images with varying polarization excitation to create retardance images. This technique is compatible with living samples and gives information about the local retardance and structure of their anisotropic components. We applied our approach to collagen fibers leading to a birefringence value of (3.4 +/- 0.3) . 10(-3) and to living cells, showing that cytoskeleton can be imaged label-free
Unintended Effects of Anonymous
No full textInternational audienceWe evaluate an experimental program in which the French public employment service anonymized résumés for firms that were hiring. Firms were free to participate or not; participating firms were then randomly assigned to receive either anonymous résumés or name-bearing ones. We find that participating firms become less likely to interview and hire minority candidates when receiving anonymous résumés. We show how these unexpected results can be explained by the self-selection of firms into the program and by the fact that anonymization prevents the attenuation of negative signals when the candidate belongs to a minority
Dépôt d’énergie dans l’air par filamentation laser femtoseconde pour le contrôle des décharges électriques haute-tension
No full textLaser filamentation is a spectacular optical propagation regime appearing for pulses of which peak power exceeds a few GW in air. Filament forms due to the optical Kerr effect, which tends to self-focus the beam until intensity reaches the medium ionization threshold by multiphoton absorption. A complex dynamic competition is then established between the Kerr effect on the one hand, and diffraction, nonlinear absorption and plasma defocusing effect on the other hand. This results in a reorganization of the beam profile, characterized by a thin (100 µm) and intense(10^18 W/m²) core able to propagate over a distance much longer than the Rayleigh length. When the initial pulse peak power largely exceeds filamentation threshold, several co-propagating filaments are formed in the same beam, with each of these multifilaments sharing physical properties of isolated single filaments. While propagating in air, filaments transfer a portion of the laser energy to the medium, mainly through Raman rotational excitation of air molecules, ionization and inverse Bremsstrahlung in the plasma. This energy is redistributed in one nanosecondand almost entirely converted into air molecule translational energy, that is heat. The medium reacts to this rapid heating by launching a cylindrical pressure wave that brings the system back to pressure equilibrium by ejecting matter from the center. This results in the formation of a hot underdense air channel, which slowly resorbs by diffusion at timescales > 1 ms. My work as a Ph. D. student first focused on the study and the optimization of laser energy deposition in air by filamentation. Thus, I investigated the influence of laser parameters such as pulse energy,focusing strength or pulse duration on deposited energy. To this purpose, I used several complementary diagnostics: study of pressure waves using microphones, characterization of the filamentation plasma by means of spectroscopy and time resolved study of underdense air channels using interferometry. I demonstrated in the single filamentation regime that above a given pulse energy, energy deposition becomes so important that the medium generates a shock wave instead of a sound wave, and that underdense channels can last for more than 100 ms. I also studied andcharacterized the high energy multifilamentation regime, showing that moderately focusing the pulse leads to a reorganization of filaments in the focal zone, generating large structures with a resulting plasma ten times denser than filaments. Filamentation-induced hydrodynamic effects lead to a transient reduction of the air breakdown voltage along the path of the laser pulse, enabling one to trigger and guide electric discharges. The second part of my thesis focused on the study and the optimization of such guided discharges for the design of a radio-frequency plasma antenna,contactless high-voltage switches or a laser lightning rod. To this purpose I developed and built an interferometric plasma diagnostic, allowing to measure the lifetime of generated plasmas. I also contributed to the proof of principle for a filament induced plasma antenna emitting RF signal. Finally, I took part to prospective experimental studies for the development of a laser lightning rod.La filamentation laser est un régime de propagation optique spectaculaire atteint pour des impulsions dont la puissance crête excède quelques gigawatts dans l’air. Le filament se forme sous l’action de l’effet Kerr optique du milieu traversé qui tend à auto-focaliser le faisceau jusqu’à ce que l’intensité résultante atteigne le seuil d’ionisation du milieu par absorption multiphotonique. Une compétition dynamique complexe s’établit alors entre l’effet Kerr, d’une part, et la diffraction, l’absorption non-linéaire de l’énergie laser et l’effet défocalisant du plasma d’autre part. Il en résulte une réorganisation du profil du faisceau, caractérisée par un coeur mince (100 µm) et intense (10^18 W/m²) pouvant se maintenir sur une distance égale à plusieurs longueurs de Rayleigh. Lorsque la puissance initiale de l’impulsion dépasse largement le seuil de filamentation, on assiste à la formation de plusieurs filaments co-propagatifs au sein du même faisceau, chacun de ces multifilaments possédant des caractéristiques physiques proches de monofilaments isolés. Au cours de sa propagation dans l’air, le filament transfère une partie de l’énergie laser au milieu, principalement via l’excitation rotationnelle Raman des molécules d’air, l’ionisation de l’air et l’effet deBremsstrahlung inverse au sein du plasma. Cette énergie est redistribuée au cours de la nanoseconde suivant le passage du laser, principalement sous forme d’énergie translationnelle des molécules d’air, c’est-à-dire de chaleur. Le milieu réagit à ce chauffage rapide par la formation d’une onde de pression cylindrique, qui ramène le système à l’équilibre de pression en éjectant de la matière du centre. Il en résulte la formation d’un canal d’air sous-dense et chaud, qui se résorbe par diffusion à des échelles de temps supérieures à la milliseconde. Ma thèse s’est en premier lieu focalisée sur l’étude et l’optimisation du dépôt d’énergie dans l’air par filamentation. J’ai ainsi étudié l’influencedes différents paramètres laser, comme l’énergie de l’impulsion, la focalisation employée et la durée d’impulsion sur la densité d’énergie déposée. Pour ce faire, j’ai employé plusieurs diagnostics complémentaires : mesure des ondes de pression à l’aide de microphones, analyse du plasma de filament par spectroscopie et mesure résolue en temps des canaux sous-dense par interférométrie. J’ai ainsi montré en régime de monofilamentation qu’au-delà d’une certaine énergie laser initiale, le dépôt d’énergie devient si important qu’une onde de choc est générée en lieu et place d’une onde sonore, et que les canaux sous-denses résultant ont des durées de vie de l’ordre de 100 ms. J’aiégalement étudié et caractérisé le régime de multifilamentation à haute énergie, montrant qu’en focalisant modérément l’impulsion, les filaments se réorganisent dans la zone focale pour former des structures plus larges générant un plasma dix fois plus dense que les filaments. Les effets hydrodynamiques engendrés par filamentation entraînent un abaissement transitoire du seuil de claquage électrique de l’air le long du trajet de l’impulsion laser, permettant ainsi de déclencher et de guider des décharges électriques. La seconde partie de ma thèse avait pour objet l’étude et l’optimisation de telles décharges guidées pour la mise au point d’une antenne plasma radio-fréquence, de commutateurs haute tension sans contact ou encore d’un paratonnerre laser. Pour ce faire, j’ai développé et construit un diagnostic plasma interférométrique à deux couleurs permettant de caractériser ladurée de vie des plasmas générés. J’ai également participé à une expérience de principe démontrant la possibilité de réaliser une antenne plasma RF à partir d’un filament laser. Enfin, j’ai participé à diverses études expérimentales prospectives dans l’optique du développement d’un paratonnerre laser
Demonstration of a Circularly Polarized Plasma-Based Soft-X-Ray Laser
No full textInternational audienceWe report the first experimental demonstration of a laser-driven circularly polarized soft-x-ray laser chain. It has been achieved by seeding a 32.8 nm Kr ix plasma amplifier with a high-order harmonic beam, which has been circularly polarized using a four-reflector polarizer. Our measurements testify that the amplified radiation maintains the initial polarization of the seed pulse in good agreement with our Maxwell-Bloch modeling. The resulting fully circular soft-x-ray laser beam exhibits a Gaussian profile and yields about 1010 photons per shot, fulfilling the requirements for laboratory-scale photon-demanding application experiments
Shower development of particles with momenta from 15 GeV to 150 GeV in the CALICE scintillator-tungsten hadronic calorimeter
No full text35 pages, 21 figures, 8 tablesInternational audienceWe present a study of showers initiated by electrons, pions, kaons, and protons with momenta from 15 GeV to 150 GeV in the highly granular CALICE analogue scintillator-tungsten hadronic calorimeter. The data were recorded at the CERN Super Proton Synchrotron in 2011. The analysis includes measurements of the calorimeter response to each particle type as well as measurements of the energy resolution and studies of the longitudinal and radial shower development for selected particles. The results are compared to Geant4 simulations (version 9.6.p02). In the study of the energy resolution we include previously published data with beam momenta from 1 GeV to 10 GeV recorded at the CERN Proton Synchrotron in 2010
Search for a charged Higgs boson in pp collisions at sqrt(s) = 8 TeV
No full textSubmitted to J. High Energy Phys ; see paper for full list of authorsInternational audienceA search for a charged Higgs boson is performed with a data sample corresponding to an integrated luminosity of 19.7 +/- 0.5 inverse-femtobarns collected with the CMS detector in proton-proton collisions at sqrt(s) = 8 TeV. The charged Higgs boson is searched for in top quark decays for m(H+/-) t (b) H+/- for m(H+/-) > m(t) - m(b). The H+/- -> tau+/- nu[tau] and H+/- -> t b decay modes in the final states tau[h]+jets, mu tau[h], l+jets, and ll' (l = e, mu) are considered in the search. No signal is observed and 95\% confidence level upper limits are set on the charged Higgs boson production. A model-independent upper limit on the product branching fraction B( t -> H+/- b ) B( H+/- -> tau+/- nu[tau] )= 1.2-0.15\% is obtained in the mass range m(H+/-) = 80-160 GeV, while the upper limit on the cross section times branching fraction sigma( pp -> t (b) H+/- ) B( H+/- -> tau+/- nu[tau] ) = 0.38-0.025 pb is set in the mass range m(H+/-) = 180-600 GeV. Here, cross section sigma( pp -> t (b) H+/- ) stands for the sum sigma( pp -> tbar (b) H+ ) + sigma( pp -> t (bbar) H- ). Assuming B( H+/- -> t b ) = 1, an upper limit on sigma ( pp -> t (b) H+/- ) of 2.0-0.13 pb is set for m(H+/-) = 180-600 GeV. The combination of all considered decay modes and final states is used to set exclusion limits in the m(H+/-)-tan(beta) parameter space in different MSSM benchmark scenarios
Non-parametric Stochastic Approximation with Large Step sizes
No full textInternational audienceWe consider the random-design least-squares regression problem within the reproducing kernel Hilbert space (RKHS) framework. Given a stream of independent and identically distributed input/output data, we aim to learn a regression function within an RKHS , even if the optimal predictor (i.e., the conditional expectation) is not in . In a stochastic approximation framework where the estimator is updated after each observation, we show that the averaged unregularized least-mean-square algorithm (a form of stochastic gradient), given a sufficient large step-size, attains optimal rates of convergence for a variety of regimes for the smoothnesses of the optimal prediction function and the functions in
HUXLEY-SIMMONS MODEL REVISITED
No full textInternational audienceMuscle contraction is a largely mechanical process taking place at the sub-cellular level. While being an intrinsically active system the contractile apparatus also displays some intriguing passive mechanical properties including negative stiffness and a fundamental nonequivalence of isometric and isotonic loading protocols. We reveal the origin of this unusual behavior by analyzing a conceptual model which represents a delicate generalization of the Huxley-Simmons model. Our analytically explicit study sheds light on the crucial role of long-range interactions in this system. The model can be easily adapted to a wide class of biological phenomena involving cooperative switching mediated by effective backbones, from muscle power-stroke to gating, binding and folding
Pseudorapidity distribution of charged hadrons in proton-proton collisions at sqrt(s) = 13 TeV
No full textsee paper for full list of authorsInternational audienceThe pseudorapidity distribution of charged hadrons in pp collisions at sqrt(s) =13 TeV is measured using a data sample obtained with the CMS detector, operated at zero magnetic field, at the CERN LHC. The yield of primary charged long-lived hadrons produced in inelastic pp collisions is determined in the central region of the CMS pixel detector (abs(eta)<2) using both hit pairs and reconstructed tracks. For central pseudorapidities (abs(eta)<0.5), the charged-hadron multiplicity density is dN/d(eta)[charged,abs(eta) < 0.5] = 5.49 +/- 0.01 (stat) +/- 0.17 (sys), a value obtained by combining the two methods. The result is compared to predictions from Monte Carlo event generators and to similar measurements made at lower collision energies