E.coli bacteria swim by rotating a helical flagellar bundle. The two phenomena that I will discuss are both related to the rotational flows that are generated by rotating propellers. The first example is about swimming of E.coli bacteria in the proximity of liquid-air interfaces. As opposed to swimming above a solid wall, we found bacteria swimming along anticlockwise circular trajectories. We quantitatively account for this through the hydrodynamic interaction of the bacterium with its own mirror image swimming on the opposite side of a perfect-slip boundary. In the second part of the talk I will talk about how swirling flows can couple nearby propellers and lead to phase locking. Using a combination of two-photon polymerization and holographic optical trapping we will discuss hydrodynamic synchronization in a number of micron scale models based on light driven rotators.

We consider diffusion of vibrations in 3d random lattices with translational invariance. Above frequency W_IR corresponding to the Ioffe-Regel crossover notion of phonons becomes meaningless. They cannot propagate through the lattice and transfer the energy. Nevertheless most of the vibrations above this frequency are delocalized. We show that they are similar to diffusons introduced by Allen, Feldman et al., Phil. Mag. B 79, 1715 (1999) for glasses. The frequency W_IR correlates with position of the boson peak. Depending on the rigidity of the lattice W_IR can vary from zero value (when rigidity is zero and there are no phonons) to the typical frequency in the system. We show that above W_IR the energy in the lattice is transferred by means of diffusion of vibrational excitations. At zero rigidity a structure factor of particle displacements S(q; w) coincides well with a structure factor of a random walk on a lattice. We have calculated the diffusivity D(w) of the modes using formula of Edwards and Thouless. It is nearly a constant above W_IR and goes to zero at the localization threshold. The results are in a good agreement with a direct numerical solution of the Newton equations for a sample with 10^6 particles. Our findings may have important consequences for understanding mechanisms of the heat transfer in glasses.

I will present the result of a calculation of the thermodynamic functions of a quark-gluon plasma for general Nc and Nf to three-loop order using hard-thermal-loop perturbation theory. At this order, all ultraviolet divergences can be absorbed into renormalizations of the vacuum energy, the HTL mass parameters, and the strong coupling constant. I will show that at three loops, the results for the pressure, energy density, entropy density, and the trace anomaly are in very good agreement with lattice data down to temperatures T ~ 2Tc. References: arXiv:1103.2528, arXiv:1106.0514, arXiv:1005.1603

Cells and tissues rely on cytoskeletal proteins like actin and myosin when moving, dividing, and changing shape. These proteins are small but together they exert pulling forces over length scales of whole cells and embryos. How can small myosin motors pull together over long distances? We answer this question by observing myosin-driven pulling in in-vitro model cytoskeletal networks. We quantify the length scale over which motors contract by image analysis and compare to percolation theory. Surprisingly, we find a broad range of parameter space that produces cluster sizes exhibiting a power-law distribution, which suggests that motor activity drives networks to critical connectivity.

Combien de temps faut-il à un "chercheur" aléatoire pour atteindre un point "cible" donné  ?  Ce temps de premier passage est en fait une quantité clé qui intervient par exemple dans l'évaluation de la vitesse de propagation d'une épidémie,  de la constante cinétique d'une réaction chimique limitée par la diffusion,  ou encore du temps qu'un animal met pour trouver sa nourriture. Je présenterai dans un premier temps des résultats récents  qui permettent une évaluation explicite du temps moyen de premier passage de marcheurs aléatoires en espace confiné. Cette approche  révèle une dépendance universelle du temps moyen de premier passage, d'une part en fonction du volume du système et d'autre part en fonction de la distance entre les points de départ et d'arrivée de la marche. Je montrerai comment ces résultats s'appliquent au transport dans des structures fractales, en  milieux désordonnés,  ou encore à la diffusion anormale. Je discuterai pour finir l'impact éventuel concernant la réactivité en milieu confiné. Dans un deuxième temps, je montrerai  que les processus de recherche dits intermittents, qui combinent des phases de recherche minutieuse et des phases de déplacement pur, sont des stratégies de recherche efficaces qui permettent de minimiser le temps moyen de premier passage.

In geometrically frustrated magnets, the tendency to order locally is hampered by some structural restriction which gives rise to a macroscopic degeneracy of the ground state. This leads to interesting collective phenomena such as `frozen-to-critical' phase transitions, long range correlations and slow dynamics. In this talk I will focus on a paradigmatic system presenting these features: spin-ice in the square lattice. Very recently, artificial spin-ice samples have been realised by nano-lithography. They provide concrete experimental realisations of vertex models of statistical mechanics (among them the integrable six- and eight-vertex models). I will introduce a stochastic extension of the six-vertex model in order to study this kind of system both by numerical and analytical methods. Then, I shall discuss the equilibrium phase diagram of the model and compare our theoretical results with experimental measurements done in `a-thermal' artificial spin-ice samples. The vertex model reproduces the experimental data when we identify the canonical temperature of our model with the configurational Edwards temperature of the samples. The dynamics of artificial spin ice opens the way to access the motion of topological defects in two dimensions. If time allows it, I will also discuss the slow relaxation dynamics of the system following different quenches.

We present a formulation of naturalness made in the framework of Bayesian statistics, which unravels the conceptual problems related to previous approaches. Among other things, the relative interpretation of the measure of naturalness turns out to be unambiguously established by Jereys' scale. Also, the usual sensitivity formulation (socalled Barbieri-Giudice measure) appears to be embedded in our formulation under an extended form. We derive the general sensitivity formula applicable to an arbitrary number of observables. To illustrate our results, we work out, among other things, the map of combined ne-tuning associated to the gauge hierarchy problem and neutralino dark matter in a classic supersymmetric model.

We study in different experiments the link between viscous dissipation and interfaces. First, we investigate the deformations of viscous threads under compression, which we compare to the buckling of elastic rods. In the second part, we study the dynamics of bubbles in wet foams, close to the jamming point. Bubble rearrangements in coarsening foams are investigated with a multiple light scattering probe. Their duration appears very dependent on interfacial rigidity (tuned by surfactant physico-chemistry) and confinement pressure, i.e. distance to jamming. We also study how bubble layers slide along an inclined plane, providing insight on the mechanisms of wall slip for foams in the wet limit.

le 4 juillet dernier, les expériences ALTAS et CMS du LHC annonçaient du CERN (près de Genève) la découverte d'une nouvelle particule de masse 125 à 126 gigaelectronvolts, découverte relayée par les médias du monde entier. Les publications officielles, fin juillet, évitent de qualifier directement cet évènement de découverte du "boson de Higgs", mais parlent plutôt d'une particule ayant des propriétés consistantes avec ce dernier. Après une brève description de ces expériences et de ce qu'elles ont réellement détecté, nous tenterons dans ce colloque de décrire ce qu'est théoriquement un "boson de Higgs" et ce qu'il faudrait encore expérimentalement, pour affirmer son existence. Nous reviendrons sur le contexte et le principal apport des articles théoriques originaux de 1964 par trois groupes indépendants, où la prédiction d'un tel boson n'apparaissait que de manière accessoire. L'importance de cet apport ne se manifestera clairement que quelques années plus tard, en 1967 et surtout en 1972 avec la construction d'une théorie cohérente décrivant les interactions faibles de courte portée et mariant les effets quantiques et relativistes. Nous aborderons enfin les succès retentissants de ce 'modèle standard', comme il est appelé depuis, mais aussi ses défauts qui ont motivé une grande activité théorique ultérieure prédisant des phénomènes 'au-delà du modèle standard', et des programmes expérimentaux couteux qui tentent de les mettre en évidence. Ceci justifie sans doute l'enjeu scientifique qui sous-tend les précautions d'annonce de la part des expérimentateurs.