Hard thermal loops for soft or collinear external momenta

We consider finite temperature 1-loop diagrams with hard loop momenta and an arbitrary number of external gauge fields when the external momenta are either soft, or near the light cone and nearly collinear with the loop momentum. We obtain a recursion relation for these diagrams which we translate into an equation for their generating functional. By integrating out the soft fields while keeping two collinear ones we find an integral equation, originally due to Arnold, Moore, and Yaffe, which sums the bremsstrahlung and pair annihilation contribution to the thermal photon production rate.Comment: 17 pages, title corrected, clarifying paragraph added to the appendix, version to appear in JHE

Thermal production of ultrarelativistic right-handed neutrinos: Complete leading-order results

The thermal production of relativistic right-handed Majorana neutrinos is of importance for models of thermal leptogenesis in the early Universe. Right-handed neutrinos can be produced both by 1 2 decay or inverse decay and by 2 -> 2 scattering processes. In a previous publication, we have studied the production via 1 2 (inverse) decay processes. There we have shown that multiple scattering mediated by soft gauge boson exchange also contributes to the production rate at leading order, and gives a strong enhancement. Here we complete the leading order calculation by adding 2 -> 2 scattering processes involving either electroweak gauge bosons or third-generation quarks. We find that processes with gauge interactions give the most important contributions. We also obtain a new sum rule for the Hard Thermal Loop resummed fermion propagator.Comment: 27 pages, 7 figures. Error in the matrix element for the (subdominant) subprocess with s-channel fermion exchange corrected. This changes the corresponding phase space integral and the constant c_V. Numerically it increases the total 2 -> 2 rate by about 2 percent and the complete rate by about 1 percent. The main results and conclusions are unaffecte

Thermal production of relativistic Majorana neutrinos: Strong enhancement by multiple soft scattering

The production rate of heavy Majorana neutrinos is relevant for models of thermal leptogenesis in the early Universe. In the high temperature limit the production can proceed via the 1 2 (inverse) decays which are allowed by the thermal masses. We consider new production mechanisms which are obtained by including additional soft gauge interactions with the plasma. We show that an arbitrary number of such interactions gives leading order contributions, and we sum all of them. The rate turns out to be smooth in the region where the 1 2 processes are kinematically forbidden. At higher temperature it is enhanced by a factor 3 compared to the 1 2 rate.Comment: 26 pages, 8 figures; added references, added comments on 2 to 2 scattering processes, improved appearance of fig. 8, corrected typos; matches published versio

Thermal particle production in the early universe

Besak D. Thermal particle production in the early universe. Bielefeld (Germany): Bielefeld University; 2010.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