Cosmology, particle and nuclear physics

IPhT has a strong activity in the fields of cosmology, particle physics, and nuclear physics. The latter subject has now become a subfield of quantum chromodynamics, since the theory of nuclear structure and low energy nuclear reactions is no longer represented at IPhT. In all of these three domains, our activities range from phenomenological works in direct connection with observations made at particle colliders or by in-space astronomical missions to more formal developments in quantum field theory.

*Permanent staff:* Brando Bellazzini, Francis Bernardeau, Philippe Brax, Ruth Britto, Francois Gelis, Edmond Iancu, Gregory Korchemsky, David Kosower, Stéphane Lavignac, Jean-Yves Ollitrault, Gregory Soyez, Patrick Valageas, Filippo Vernizzi

*Emeritus:* Jean-Paul Blaizot, Robi Peschanski, Mannque Rho

Physics beyond the Standard Model deals with the theoretical problems and observational facts that are left unexplained by the current theory of the elementary particles and their interactions, the so-called Standard Model. These include the nature of dark matter, the origin of neutrino masses, the matter-antimatter asymmetry of the Universe, the origin of the electroweak scale and of the Higgs boson, the possible unification of fundamental interactions and quantum gravity. Physics beyond the Standard Model strives to give answers to these puzzles either in the framework of more fundamental theories involving new particles and interactions, or by employing effective field theory techniques to constrain the underlying new physics. Using both approaches and exploiting experimental data (high-energy collisions, cosmic rays, cosmological observations, neutrino oscillations, rare decays…), our research activities cover a wide range of topics. These include Higgs and collider physics, dark matter phenomenology, neutrino physics and baryogenesis, Grand Unification, as well as more theoretical work on effective field theories and their applications to particle physics and quantum gravity.

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. It is also our main probe of gravity on very large scales and at high energy, and it confronts theoretical physics with numerous puzzles, such as the nature of dark matter and dark energy, the origin of the matter-antimatter asymmetry, the nature of the inflaton, and the space-time cosmological structure. Our research topics include studies of the primordial Universe (statistical properties of the initial fluctuations...), modeling of the gravitational dynamics of large-scale structures in the recent Universe (spatial distribution of matter and galaxies...), and investigations of theories of gravity beyond General Relativity (building of phenomenological models and of systematic theoretical frameworks). These theoretical predictions are confronted with observations to constrain cosmological scenarios and fundamental physics (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, detections of gravitational waves emitted by binary systems).

Quantum chromodynamics (QCD) governs the short-distance interactions of quarks and gluons, which are the constituents of protons and neutrons. The scale dependence of the QCD coupling constant, that grows at large distance, is responsible for the confinement of the quarks inside hadronic bound states. On the other hand, the fact that it becomes weak at short distance leads to a deconfinement phase transition in hadronic systems at large density or temperature. The conditions for this transition can be reproduced in experiments --presently performed at the LHC and at the RHIC-- by colliding large nuclei at high energy. The theoretical study of these collisions relies on a range of tools: perturbative QCD for the short distance phenomena, effective theories (chiral perturbation theory, heavy quark theory, color glass condensate) constructed from the low energy symmetries or by integrating out some unimportant degrees of freedom, kinetic theory and hydrodynamics for the study of relaxation phenomena on even larger distance scales.

As the Large Hadron Collider (LHC) at CERN accumulates increasing amount of data, precise predictions become more and more important. This allows for better determination of standard-model parameters as well as as lower uncertainties and better background estimates, resulting in better potential for (direct and indirect) beyond-standard-model discoveries.

In that context, the IPhT is involved in study of scattering amplitudes. Over the past decade, the field of amplitudes has been tremendously active in finding new tools to compute loop scattering amplitudes (spinor-helicity formalism, generalised unitarity, ...). The lab has contributed major achievements in the understanding of these tools and the automation of next-to-leading order (i.e. 1-loop) and is now actively involved in going to higher loop orders. This field is also connected with other more fundamental studies in conformal field theories (e.g. N=4 SUSY Yang-Mills) and graph theories.

On a different level, the research activities of the IPhT also involve the study of jets and their substructure, another field which has recently gained in importance at the LHC, in particular for searched for new physics. This involves the definition of new substructure tools as well as a first-principles understanding of these tools in QCD. This has deep connections with precision measurements, machine-learning techniques and measurements of the properties of the quark-gluon plasma in heavy-ion collisions.

#865 - Màj : 17/01/2019