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...