Kießig, Clemens (2011): Quasiparticles in Leptogenesis: A hard-thermal-loop study. Dissertation, LMU München: Fakultät für Physik
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### Abstract

We analyse the effects of thermal quasiparticles in leptogenesis using hard-thermal-loop-resummed propagators in the imaginary time formalism of thermal field theory. We perform our analysis in a leptogenesis toy model with three right-handed heavy neutrinos N_1, N_2 and N_3. We consider decays and inverse decays and work in the hierarchical limit where the mass of N_2 is assumed to be much larger than the mass of N_1, that is M_2 >> M_1. We neglect flavour effects and assume that the temperatures are much smaller than M_2 and M_3. We pay special attention to the influence of fermionic quasiparticles. We allow for the leptons to be either decoupled from each other, except for the interactions with neutrinos, or to be in chemical equilibrium by some strong interaction, for example via gauge bosons. In two additional cases, we approximate the full hard-thermal-loop lepton propagators with zero-temperature propagators, where we replace the zero-temperature mass by the thermal mass of the leptons m_l(T) in one case and the asymptotic mass of the positive-helicity mode \sqrt{2} m_l(T) in the other case. We calculate all relevant decay rates and CP-asymmetries and solve the corresponding Boltzmann equations we derived. We compare the final lepton asymmetry of the four thermal cases and the vacuum case for three different initial neutrino abundances; zero, thermal and dominant abundance. The final asymmetries of the thermal cases differ considerably from the vacuum case and from each other in the weak washout regime for zero abundance and in the intermediate regime for dominant abundance. In the strong washout regime, where no influences from thermal corrections are commonly expected, the final lepton asymmetry can be enhanced by a factor of two by hiding part of the lepton asymmetry in the quasi-sterile minus-mode in the case of strongly interacting lepton modes.