Baryogenesis and lepton number violation

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

Remarks on the high-energy behaviour of cross-sections in weak-scale string theories

We consider the high-energy behaviour of processes involving Kaluza-Klein (KK) gravitons of weak-scale string theories. We discuss how form-factors derived within string theory modify the couplings of KK gravitons and thereby lead to an exponential fall-off of cross sections in the high-energy limit. Further, we point out that the assumption of Regge behaviour for a scattering amplitude in the high energy limit, $T\propto s^{\alpha(t)}$, combined with a linear growth of the total cross-section, $\sigma_{tot}(s)\propto s$, violates elastic unitarity. Regge behaviour leads to a stringent bound on the growth of the total cross-section, \stot (s) \leq 32\pi \alpha' \ln(s/s_0)

Spectator processes and baryogenesis

Spectator processes which are in thermal equilibrium during the period of baryogenesis influence the final baryon asymmetry. We study this effect quantitatively for thermal leptogenesis where we find a suppression by a factor O(1)

Matter Antimatter Asymmetry and Neutrino Properties

The cosmological baryon asymmetry can be explained as remnant of heavy Majorana neutrino decays in the early universe. We study this mechanism for two models of neutrino masses with a large \nu_\mu-\nu_\tau mixing angle which are based on the symmetries SU(5) x U(1)_F and SU(3)_c x SU(3)_L x SU(3)_R x U(1)_F, respectively. In both cases B-L is broken at the unification scale \Lambda_{GUT}. The models make different predictions for the baryogenesis temperature and the gravitino abundance.Comment: latex2e, 14 pages, 2 figures, Contribution to the Festschrift for L. B. Okun, to appear in a special issue of Physics Reports, eds. V. L. Telegdi and K. Winte

Baryon Asymmetry, Neutrino Mixing and Supersymmetric SO(10) Unification

The baryon asymmetry of the universe can be explained by the out-of-equilibrium decays of heavy right-handed neutrinos. We analyse this mechanism in the framework of a supersymmetric extension of the Standard Model and show that lepton number violating scatterings are indispensable for baryogenesis, even though they may wash-out a generated asymmetry. By assuming a similar pattern of mixings and masses for neutrinos and up-type quarks, as suggested by SO(10) unification, we can generate the observed baryon asymmetry without any fine tuning, if $(B-L)$ is broken at the unification scale $\Lambda_{GUT}\sim 10^{16}$GeV and, if m_{\n_\m} \sim 3\cdot 10^{-3} eV as preferred by the MSW solution to the solar neutrino deficit.Comment: latex2e, 39 pages, 15 figures, uses epsfig and pstricks. Additional contribution to the CP-asymmetry added, conclusions unchanged. Final version, to appear in Nucl.Phys.

Hard-Thermal-Loop Corrections in Leptogenesis I: CP-Asymmetries

We investigate hard-thermal-loop (HTL) corrections to the CP-asymmetries in neutrino and, at high temperature, Higgs boson decays in leptogenesis. We pay special attention to the two leptonic quasiparticles that arise at non-zero temperature and find that there are four contributions to the CP-asymmetries, which correspond to the four combinations of the two leptonic quasiparticles in the loop and in the final states. In two additional cases, we approximate the full HTL-lepton propagator with a zero-temperature propagator that employs the thermal lepton mass m_l(T), or the asymptotic thermal lepton mass sqrt{2} m_l(T). We find that the CP-asymmetries in the one-mode approaches differ by up to one order of magnitude from the full two-mode treatment in the interesting temperature regime T \sim M_1. The asymmetry in Higgs boson decays turns out to be two orders of magnitude larger than the asymmetry in neutrino decays in the zero-temperature treatment. The effect of HTL corrections on the final lepton asymmetry are investigated in paper II of this series.Comment: 38 pages, 14 figure

Decay of a Yukawa fermion at finite temperature and applications to leptogenesis

We calculate the decay rate of a Yukawa fermion in a thermal bath using finite temperature cutting rules and effective Green's functions according to the hard thermal loop resummation technique. We apply this result to the decay of a heavy Majorana neutrino in leptogenesis. Compared to the usual approach where thermal masses are inserted into the kinematics of final states, we find that deviations arise through two different leptonic dispersion relations. The decay rate differs from the usual approach by more than one order of magnitude in the temperature range which is interesting for the weak washout regime. We discuss how to arrive at consistent finite temperature treatments of leptogenesis.Comment: 16 pages, 5 figure