The long-lived stau as a thermal relic

We consider physics beyond the Standard Model which implies the existence a of long-lived electromagnetically charged massive particle species (CHAMP) which we denote by X±. We discuss in detail the unique sensitivity the early Universe exhibits on the mere presence and on the decay of such a particle. We carry out a detailed study of gravitino (G) dark matter scenarios in which the lighter scalar tau (stau, τ1) is the lightest Standard Model superpartner so that τ1=X. We also provide a thorough investigation of the thermal freeze-out process of τ1. The thesis is divided into three parts: Part I: In this part we consider a generic but weak-scale CHAMP. In Chapter 1 we set the stage for the coming investigations by shortly reviewing the framework of Big Bang Nucleosynthesis (BBN), by working out the typical CHAMP freeze-out abundance, and by reviewing the stringent constraints arising from such a decaying component during/after BBN. In Chapter 2 we discuss the physics which emerges when the light elements fused in BBN are captured by X- at the time of primordial nucleosynthesis. In the remainder of this chapter we focus on the catalytic production of 6Li and 9Be. The second part is devoted to scenarios in which G is the lightest supersymmetric particle (LSP) and τ1 is the next-to-lightest SUSY particle (NLSP). In Chapter 3 we focus on the gravitino LSP as a dark matter candidate. In Chapter 4 we then focus on gravitino dark matter scenarios in which τ1 is the NLSP. In Part III we take an in-depth look into the chemical decoupling process of the long-lived τ1 from the primordial plasma. The quantity of interest is the thermal freeze-out abundance of the stau. We identify its dependence on the crucial SUSY parameters and also show that it sensitively depends on the details of the Higgs sector. Stau annihilation into final state Higgses as well as resonant annihilation via the heavy CP even Higgs boson can substantially deplete the decoupling yield. (orig.)