During the process of thermal leptogenesis temperature decreases by about one order of magnitude while the baryon asymmetry is generated. We present an analytical description of this process so that the dependence on the neutrino mass parameters becomes transparent. In the case of maximal CP asymmetry all decay and scattering rates in the plasma are determined by the mass M 1 of the decaying heavy Majorana neutrino, the effective light neutrino mass m 1 and the absolute mass scale m of the light neutrinos. In the mass range suggested by neutrino oscillations, m sol ≃ 8 × 10 −3 eV m 1 m atm ≃ 5 × 10 −2 eV, leptogenesis is dominated just by decays and inverse decays. The effect of all other scattering processes lies within the theoretical uncertainty of present calculations. The final baryon asymmetry is dominantly produced at a temperature T B which can be about one order of magnitude below the heavy neutrino mass M 1 . We also derive an analytical expression for the upper bound on the light neutrino masses implied by successful leptogenesis.
There are theoretical and phenomenological motivations that there may exist additional heavy ZЈ bosons with family nonuniversal couplings. Flavor mixing in the quark and lepton sectors will then lead to flavor changing couplings of the heavy ZЈ, and also of the ordinary Z when Z-ZЈ mixing is included. The general formalism of such effects is described, and applications are made to a variety of flavor changing and CPviolating tree and loop processes. Results are described for three specific cases motivated by a specific heterotic string model and by phenomenological considerations, including cases in which all three families have different couplings, and those in which the first two families, but not the third, have the same couplings. Even within a specific theory the results are model dependent because of unknown quark and lepton mixing matrices. However, assuming that typical mixings are comparable to the Cabibbo-Kobayashi-Maskawa matrix, processes such as coherent -e conversion in a muonic atom, K 0 -K 0 and B-B mixing, ⑀, and ⑀Ј/⑀ lead to significant constraints on ZЈ bosons in the theoretically and phenomenologically motivated range M Z Ј ϳ1 TeV. PACS number͑s͒: 12.15.Mm, 13.20.Ϫv, 13.35.Ϫr 1 Models with an extra ZЈ often include additional exotic fermions ͓i.e., with nonstandard SU͑2͒ assignments͔ as well. Mixing of ordinary and exotic fermions could lead to flavor changing ZЈ and Z couplings even in the absence of family nonuniversal charges, and to nonuniversal W couplings ͓18͔. We do not consider such effects in this paper.
Interactions of heavy Majorana neutrinos in the thermal phase of the early universe may be the origin of the cosmological matter-antimatter asymmetry. This mechanism of baryogenesis implies stringent constraints on light and heavy Majorana neutrino masses. We derive an improved upper bound on the CP asymmetry in heavy neutrino decays which, together with the kinetic equations, yields an upper bound on all light neutrino masses of 0.1 eV. Lepton number changing processes at temperatures above the temperature T B of baryogenesis can erase other, pre-existing contributions to the baryon asymmetry. We find that these washout processes become very efficient if the effective neutrino mass m 1 is larger than m * ≃ 10 −3 eV. All memory of the initial conditions is then erased. Hence, for neutrino masses in the range from ∆m 2 sol ≃ 8 × 10 −3 eV to ∆m 2 atm ≃ 5 × 10 −2 eV, which is suggested by neutrino oscillations, leptogenesis emerges as the unique source of the cosmological matter-antimatter asymmetry.
The cosmological baryon asymmetry can be explained by the nonperturbative electroweak reprocessing of a lepton asymmetry generated in the out-of-equilibrium decay of heavy right-handed Majorana neutrinos. We analyze this mechanism in detail in the framework of a SO(10)-subgroup. We take three right-handed neutrinos into account and discuss physical neutrino mass matrices.
We study CP asymmetries in lepton-number violating two-body scattering processes and show how they are related to CP asymmetries in the decays of intermediate massive Majorana neutrinos. Self-energy corrections, which do not contribute to CP asymmetries in two-body processes, induce CP violating couplings of the intermediate Majorana neutrinos to lepton-Higgs states. We briefly comment on the implications of these results for applications at finite temperature.
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