Unité de Recherche Associée au CNRS
The Standard Model describes the elementary constituents of matter (quarks and leptons) and their interactions. Its success has been confirmed, during the last thirty years, in experiments performed at large accelerators. A key element is however still missing: the Higgs boson, which is an excitation of a condensate interacting with elementary particles thereby giving them their observed masses. New high energy structures are required to ensure the quantum stability of this mass scale: supersymmetry, additional new space-time dimensions... Neutrino oscillations, the existence of dark matter, the asymmetry between matter and anti-matter, all point to this as yet undiscovered domain of physics required to understand one day the structure of matter at microscopic scales where the four fundamental forces are unified and at which quantum effects of gravitation are present.
Cosmology aims at retracing the history of our universe since the Big Bang, in order to understand its contents as well as its large scale structure. The astrophysical objects which we observe today (galaxies and clusters of galaxies...) are due to the gravitational collapse of small density irregularities which appeared in the primordial universe. Predictions of the distribution of these objects, as a function of their size, can be confronted with direct observations, or with indirect observations through gravitational lenses. Observational cosmology also confronts particle physics with numerous problems. The nature of dark matter, or dark energy, the origin 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) describes a large number of phenomena, ranging from the binding of nuclei to the confinement of quarks in baryons and mesons. At very high energy, asymptotic freedom (Nobel Prize 2004) makes perturbation theory applicable. At low energy, when a large number of particles is involved, other methods must be used (approximate conformal symmetry for example). The number of quarks and gluons in the hadron wavefunctions increases with energy until it reaches a saturation point. A Color Glass Condensate model has been devised to account for this phenomenon. At high temperature, we expect that quarks and gluons are freed from confinement and that they form a plasma, some properties of which can be studied in ultra-relativistic heavy ion reactions.
It is essential to account for the properties of atomic nuclei starting from the effective interactions between their constituent nucleons. Detailed calculations have analyzed the structure of rotational bands displayed in the spectrum of super-deformed and super-heavy nuclei, as well as the coexistence of different shapes in certain nuclei. Such studies extend naturally to the dense matter which is found, for example, in neutron stars. They are closely linked to properties of hadrons that are dominated at low energy by chiral symmetry breaking: this leads to a prediction of how hadron masses scale when they propagate in a dense medium. Furthermore, effective field theory leads to a theory of interactions between hadrons.
