Attention : désormais les séminaires ont lieu tous les lundis à 10h45 en salle  523 du LPTMC - Tour 12-13 


Nicolas Wschebor (Université de la République d'Uruguay, Montevideo)

Proving conformal invariance in critical scalar theories in any dimension

Conformal invariance in three dimension has a tremendous renewed interest due to the surprisingly good results obtained by using the “conformal bootstrap” in last five years. In this talk, the interest of this symmetry is reviewed and its existence in critical (scale invariant) theories in any dimension is discussed. In particular, using Wilson renormalization group, we show that if no integrated vector operator of scaling dimension −1 exists in a given model, then scale invariance implies conformal invariance. By using the Lebowitz inequalities, we prove that this necessary condition (or another similar necessary condition proposed by Polchinski many years ago) is fulfilled in all dimensions less than four for the Ising universality class. This shows, in particular, that scale invariance implies conformal invariance for the three-dimensional Ising model. Finally, the extension of this result to other critical systems is discussed.

Alessandro Codello (INFM Bologna, Italie)

Functional perturbative RG and CFT data in the e-expansion

I will show how the use of standard perturbative RG in dimensional regularization allows for a renormalization group based computation of both the spectrum and a family of coefficients of the operator product expansion (OPE) for the whole family of scalar multi-critical universality classes.

The task is greatly simplified by a straightforward generalization of perturbation theory to a functional perturbative RG approach. I illustrate the procedure in the e-expansion by obtaining the next-to-leading corrections for the spectrum and the leading corrections for the OPE coefficients of Ising and Lee-Yang universality classes and then give several results for the whole family of renormalizable multi-critical models.

Whenever a comparison is possible our RG results explicitly match the ones recently derived using a combination of CFT constraints, Schwinger-Dyson equation and the free theory behavior at the upper critical dimension.

Julien Cividini (Weizmann institute, Israël)

Driven tracer with absolute negative mobility

Instances of negative mobility, where a system responds to a perturbation in a way opposite to naive expectation, have been studied theoretically and experimentally in numerous nonequilibrium systems. After reviewing part of the literature on the topic, we will consider a simple one-dimensional lattice model of a driven tracer in bath. We will show that contrary to previous expectations, Absolute Negative Mobility (ANM), whereby current is produced in a direction opposite to the drive, occurs around an equilibrium state. We derive analytical predictions for the mobility in the linear response regime. The high density regime will help us elucidate the mechanism leading to ANM. The lattice model can be seen as a toy model for hard Brownian discs in a narrow planar channel. Molecular dynamics studies show that the hard discs model exhibits Negative Differential Mobility (NDM), but no ANM.


Vittore Scolari (Institut Pasteur, Paris)

Kinetic signature of cooperativity in the irreversible collapse of a polymer

We investigated the kinetics of a polymer collapse due to the formation of irreversible crosslinks between its monomers. We use the contact probability as a function of the monomeric distance as a scale dependent order parameter and show in simulations the emergence of acooperative pearling instability. This produces a sharp conformational transition in time, inducing a crossover between short and long distance behaviour due to the formation of pearls. The size of pearls and the transition time depends on the equilibrium dynamics of the polymer and the rate at which cross links are formed. We finally confirm experimentally the existence of this transition using a chromosome conformation capture experiment.

Pierre-François Cohadon (Laboratoire Kastler Brossel and Virgo Collaboration)

Gravitational-wave detection and quantum-limited measurements

Detecting gravitational waves required 4 decades of experimental effort to reach a sensitivity at the h~10-21 level, corresponding to mirror displacements below 10-18 m.

I will  review this "noise hunting" effort and give some details about the recent observation of 2 neutron star mergers.

Apart from classical noise (seismic noise, thermal noise...), it was realized as soon as in the late 70s that quantum fluctuations of the light field were responsible for the Standard Quantum Limit, a sensitivity limit that second-generation gravitational-wave interferometers will reach once they operate at their design sensitivity, within a few years. A number of ideas have been considered to beat the SQL: squeezed states of the light field, tailoring the optical response function or taking advantage of the mirror mechanical response to radiation pressure. I will present the first experimental demonstrations of such ideas, either on suspended interferometers or table-top experiments.