Théorie

Hidden symmetries and Goldstone bosons from higher dimensions?

Free massless scalars have a shift symmetry. This is usually broken by interactions, such that quantum corrections induce a quadratically divergent mass term. In the Standard Model this leads to the hierarchy problem, the question why the Higgs mass is so much smaller than the Planck mass. We present an example where a large scalar mass term is avoided by coupling the scalar to an infinite tower of massive states, obtained from a six-dimensional theory compactified on a torus with magnetic flux.

Chern-Weil theorem, Lovelock Lagrangians in critical dimensions and boundary terms in gravity actions

We show how to translate into tensorial language the Chern-Weil theorem for the Lorentz symmetry, which equates the difference of the Euler densities of two manifolds to the exterior derivative of a transgression form. For doing so we need to introduce an auxiliary, hybrid, manifold whose geometry we construct explicitely. This allows us to find the vector density, constructed out of spacetime quantities only, whose divergence is the exterior derivative of the transgression form.

Stochastic effective action for a spectator scalar in inflation

We discuss the IR dynamics of a light scalar field in inflation. We demonstrate from a functional integral perspective how the full quantum theory gives rise to Starobinsky's semi-classical and local stochastic description when the field is smoothed on scales comparable to the Hubble horizon. We then apply the functional renormalization group to the stochastic dynamics by progressively integrating out frequencies. The resulting effective action determines the approach to equilibrium and allows for the computation of unequal time correlators for large time separations.

A classical computation of (pre)Hawking radiation backreaction

In this talk I will discuss how a gravitationally collapsing object emits classical radiation at a rate equal to the quantum radiation rate if suitable initial conditions are chosen for two classical fields. The coupled dynamics of gravitational collapse and radiation are then solved for in a simple toy model, illustrating how the classical system may be used to gain insight into Hawking evaporation.

Thermalization and hydrodynamics of the quark-gluon plasma

There is experimental evidence that the quark-gluon plasma produced in ultra-relativistic heavy ion collisions is well described by viscous hydrodynamics, with a low value of the viscosity (relative to the entropy density). This observation, added to the recent discovery that the same description works well also for high energy proton-nucleus or high multiplicity proton-proton collisions, is raising a number of interesting theoretical questions:  How does the system of gluons freed in the early stage of a collision evolve towards local thermal equilibrium?

Revisiting the infrared physics of particles of arbitrary spin

After a brief overview of the recent developments relating different aspects of the infrared physics of spin-1 and spin-2 particles, I will revisit Weinberg's constraints on the S-matrix of massless higher-spin particles from the viewpoint of asymptotic symmetries in flat space. I will also discuss the hints that this analysis provides on possible higher-spin extensions of the BMS symmetry.

Correlation functions in fully developed turbulence

Turbulence is an ubiquitous phenomenon in natural and industrial fluid flows. Yet, it still lacks a  satisfactory theoretical description. One of the main open issues is to calculate the statistical properties of the turbulent steady state, and in particular what is generically called intermittency effects, starting from the fundamental description of the fluid dynamics provided by Navier-Stokes equation. In this presentation, I will focus on isotropic and homogeneous turbulence in three-dimensional incompressible flows.

Collective Neutrino Oscillations in Core-Collpase Supernovae and Neutron Star Mergers

Neutrino oscillations in dense neutrino media is a very nonlinear and rich phenomenon.
Such a dense neutrino gas could be found in very extreme astrophysical sites such as 
core collapse supernovae and neutron star mergers. Studying collective neutrino oscillations 
in the aforementioned settings is very essential since neutrinos could play a key role both 
in the dynamics and, in the nucleosynthesis. In this seminar, I will talk about our current understanding 
of this topic which has tremendously changed in the past few years.
 

Massless elementary particles with continuous spin: Wigner's exotic representation of the Poincaré group

Old results and recent developments on the theoretical description of elementary particles with "continuous" spin will be reviewed. At free level, these fields are described by unitary irreducible representations of the isometry group (either Poincare or anti de Sitter group) with an infinite number of physical degrees of freedom per spacetime point. We will mention a list of new results, in particular on their cubic interactions, as well as important issues that remain open.

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