Phenomenology and Cosmology of Supersymmetric Grand Unified Theories

The cosmological relic density of a Bino-like Lightest Supersymmetric Particle is calculated in the framework of the Minimal Supersymmetric Standard Model. We include annihilation and coannihilation effects of the Bino with the lightest stau and with the other sleptons, which happen to have comparable masses with it. Coannihilation turns out to be crucial for reducing the Bino relic density to an acceptable level. Phenomenological constraints, also, results from the inclusion of the supersymmetric corrections to the CP-even Higgs boson and b-quark masses and the branching ratio of b -->s gamma. We impose these constraints on two versions of the Minimal Supersymmetric Standard Model, employing radiative electroweak breaking with universal boundary conditions and gauge coupling unification. In the first version of the model we assume Yukawa coupling unification and boundary conditions from gravity-mediated supersymmetry breaking. In the second version of the model we assume boundary conditions from the Horava-Witten theory and we restrict the parameter space by simultaneously imposing the phenomenological and cosmological constraints