The Standard Model describes the fundamental building blocks of matter (quarks and leptons) and their interactions. Particle accelerator experiments have been testing it for the last thirty years with unprecedented accuracy. One of the key elements of this theory, the Higgs field, whose role is to provide a mass to the elementary particles, is under high scrutiny at the Large Hadron Collider (LHC) at CERN. Despite its success in describing a host of experimental results, the Standard Model fails to explain observations such as the existence of dark matter, the matter-antimatter asymmetry, the acceleration of the expansion of the Universe, and the smallness of neutrino masses. Moreover, certain theoretical conundrums find no explanation within the Standard Model, e.g. the quantum instability between the electroweak scale and the Planck scale, at which gravitational interactions become strong. Physics beyond the standard model strives to give answers to these puzzles by envisaging more fundamental theories involving new particles and their interactions, or employing model-independent techniques with which one can extract information relevant to those theories directly from experimental data (high-energy collisions, cosmic rays, cosmological observations, neutrino oscillations, rare decays...). Several key phenomena such as the dynamics of the electroweak symmetry breaking, the nature and properties of dark matter, the origin of the mass of neutrinos, and also the nature of dark energy are under study using these complementary approaches.
Cosmology aims to retrace the history of our Universe since the Big Bang, in order to understand its content as well as its large scale structure. The astrophysical objects that we observe today (galaxies, clusters of galaxies...) are due to the gravitational collapse of small density irregularities that appeared in the primordial universe. Our research topics range from studies of the primordial universe (statistical properties of the initial fluctuations, generation of gravitational waves or magnetic fields...) to studies of the gravitational dynamics of large-scale structures in the recent universe (spatial distribution of galaxies...). These theoretical predictions are confronted with observations to constrain cosmological scenarios (measurements of the temperature fluctuations of the cosmic microwave background, of the distribution of clusters of galaxies, of the distortion of the images of background galaxies by the gravitational potential along the line of sight...). Observational cosmology also confronts particle physics with numerous problems. The nature of dark matter, of dark energy, the origin of the matter-antimatter asymmetry, the nature of the inflaton and the space-time cosmological structure, are all questions which physicists are attempting to answer.
Quantum chromodynamics (QCD) is the fundamental theory that underlies the strong nuclear interactions. It governs the short-distance interactions of quarks and gluons, which are the constituents of protons and neutrons. QCD explains the confinement of quarks and gluons inside these particles. Understanding the strong interactions is crucial to the analysis of data collected by experiments running at the Large Hadron Collider (LHC) at CERN. QCD interactions have the remarkable property of becoming weaker at shorter distances, which allows the use of a perturbative approach to study high-energy processes. Conversely, it becomes stronger at long distances. This strong-coupling regime motivates the study of effective theories, or of related theories with additional symmetry, such as maximal supersymmetry, and of dual string pictures. QCD may also require non-perturbative techniques in situations involving a large number of particles, even at weak coupling. This happens in the wavefunction of a proton or nucleus: the gluon density increases rapidly with energy until it saturates, a situation that can be handled by the so-called ‘color glass condensate’. At high temperatures, nuclear matter undergoes a deconfinement transition to form a quark-gluon plasma. Its properties can be studied in ultra-relativistic heavy-ion collisions at the LHC and at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven.