Thermal particle production in the early universe
The thesis deals with the production of weakly interacting particles via decay and scattering in the early universe which is in the state of a hot thermal plasma. Already at leading order in the gauge coupling constant multiple rescattering off soft (p ~ gT) gauge bosons has to be included which leads to nontrivial interference terms that must be taken into account. This phenomenon is known under the name "Landau-Pomeranchuk-Migdal (LPM) effect" and has so far only been considered for the computation of photon production rates from a quark-gluon plasma. In this thesis, a conceptually new approach to take gauge interactions consistently into account at leading order is presented. By considering an effective theory for a specific class of diagrams, it is relatively straightforward to derive an integral equation that sums Bremsstrahlung and LPM contribution to the particle production rate. The approach is more general than those that were used previously because the produced particle only has to be specified at the very end. As an application of the new formalism, the production rate of right-handed Majorana neutrinos, relevant for models of thermal leptogenesis, is computed to leading order in the SU(2) and U(1) gauge couplings and numerical results for the production rate are shown. Scattering processes involving top quarks and electroweak gauge bosons are additionally taken into account. While unaffected by the LPM effect, they are still nontrivial due to IR divergences which must be removed in a gauge-invariant way. The corresponding results have also been derived for the first time in this thesis which thus constitutes a major step towards a complete treatment of thermal leptogenesis using thermal field theory. In addition, by using the new approach put forward here, gauge interactions can now also straightforwardly be included in computations of the decay contribution to thermal production rates of other particles, e.g. DM candidates.
Bielefeld University
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