The high-energy universe as a laboratory for particle physics

This manuscript presents part of the work that I have conducted since I joined the Particle Physics Department of the Institute of Research into the Fundamental Laws of the Universe (Irfu), at CEA Saclay. The common thread of my work is based around the idea of advancing particle physics and cosmology by performing measurements of particles in their natural environment. In a sense this approach corresponds to doing in vivo particle physics, whereas scientists working on controlled experiments such as those using particle accelerators are doing in vitro particle physics. This might be considered very ambitious but it actually recovers the tradition that led until the 50's to the discovery and the study of new particles. In 2012 a new particle has been discovered, with properties that resemble a lot to that of the Higgs particle, which is supposed to explain the masses of elementary particles. The next fundamental particle to be discovered might be the dark matter particle that could explain most of the mass of the universe, or the axion that can solve some of the inconsistencies of the standard model of particle physics. In this report it is argued that natural 'astrophysical' environments, in particular those involving high-energy phenomena, can play an essential role in this endeavor. This will mainly be illustrated by the search for a solution to one of the greatest Nature's puzzle, namely the identification of the missing mass of the universe often referred to as the dark matter problem. Over the years, the research presented in this essay has represented an excellent opportunity for me to initiate many pleasant collaborations with physicists mainly from Saclay, Paris, Annecy, Zurich, Erlangen, Tubingen, Heidelberg, Padova, Barcelona and Santa Cruz. The first chapter is a very short preamble setting the framework of my research. In particular, I introduce the reason why new yet undiscovered fundamental components might exist in the universe. The following three chapters describe part of the research activities that I conducted in that perspective. Chapter 1 is devoted to searches for particle dark matter with charged cosmic rays; some constraints on new physics models are obtained and some limitations of the method are outlined. Chapter 2 mainly deals with searches for new particles using very high-energy gamma rays. This includes dark matter in the form of weakly-interacting massive particles and axion-like particles. In chapter 3, the difficult and essential issue of building instruments capable of breakthrough in the area is addressed. This is done through the example of the development of new kinds of mirrors for ground-based gamma-ray astronomy. Of course important parts of my work do not appear in the present report. This is the case for all the ideas that turned out to be dead ends and on-going projects that are not yet fully mature. I have been involved in activities such as refereeing for peer-reviewed journals and in scientific committees, particularly with the French INSU national program PNHE (Programme National Hautes Energies) and the European Strategy for Particle Physics conducted by CERN. I have also devoted a significant fraction of my time to teaching and outreach, as well as supervising research students. I have enjoyed giving numerous seminars for the general public and secondary school students. I have also been very involved in teaching general physics to adults returning to full time education at the Universite Paris 11, as well as to the students of Ecole Nationale des Techniques Avancees and Ecole des Mines de Paris. (author)