Abstract:

We show that the thermal relic abundance of dark matter can be affected by a new type of reaction: semi-annihilation. Semi-annihilation takes the schematic form X_i X_j -> X_k phi, where X_i are stable dark matter particles and phi is an unstable state. Such reactions are generically present when dark matter is composed of more than one species with "flavor" and/or "baryon" symmetries. We give a complete set of coupled Boltzmann equations in the presence of semi-annihilations, and study two toy models featuring this process. Semi-annihilation leads to non-trivial dark matter dynamics in the early universe, often dominating over ordinary annihilation in determining the relic abundance. This process also has important implications for indirect detection experiments, by enriching the final state spectrum from dark matter (semi-)annihilation in the Milky Way.

Abstract:

The Fermi satellite just celebrated its 2-years anniversary in space. With its main instrument, the Large Area Telescope (LAT), Fermi opened a new era in high energy astrophysics and in particular for the study of Gamma-Ray Bursts (GRBs). My thesis work focused primarily on the detection and analysis of GRBs with this instrument. After a description of the standard procedure for the detection and analysis of LAT GRBs, I will provide an overview of the temporal and spectral features observed in the prompt emission of GRBs detected by the LAT after one year and a half of operation. I will also describe how the detection of very high energy photons (>10 GeV) in combination with the knowledge of the lower energy emission can be used to place significant constraints both on Lorentz invariace violation and the extragalactic background light in the Universe.