Abstract
In many realizations of leptogenesis, heavy right-handed neutrinos play the main role in the generation of an imbalance between matter and antimatter in the early Universe. Hence, it is relevant to address quantitatively their dynamics in a hot and dense environment by taking into account the various thermal aspects of the problem at hand. The strong washout regime offers an interesting framework to carry out calculations systematically and reduce theoretical uncertainties. Indeed, any matter–antimatter asymmetry generated when the temperature of the hot plasma [Formula: see text] exceeds the right-handed neutrino mass scale [Formula: see text] is efficiently erased, and one can focus on the temperature window [Formula: see text]. We review recent progress in the thermal field theoretic derivation of the key ingredients for the leptogenesis mechanism: the right-handed neutrino production rate, the CP asymmetry in the heavy-neutrino decays and the washout rates. The derivation of evolution equations for the heavy-neutrino and lepton-asymmetry number densities, their rigorous formulation and applicability are also discussed.
Highlights
In the standard picture, the RH neutrinos are produced by thermal scatterings in the early Universe and decay out of equilibrium either into SM leptons or antileptons in different amounts due to the CP-violating phases
The strong washout scenario is usually defined in terms of the decay parameter for the k-th RH neutrino, Kk = Γk/H [4, 5], which is the ratio between the neutrino decay rate and the Hubble rate
Any initial lepton asymmetry possibly present before the onset of leptogenesis is erased [4, 5] and the neutrino dynamics is close to equilibrium
Summary
The sections that follow review the recent progress made in the calculation of the various CP-even and -odd rates needed to obtain quantitatively accurate estimates of the final asymmetry in thermal leptogenesis. The relevant processes for the N1-dominated scenario of thermal leptogenesis are the decays and inverse decays of the lightest RH neutrino, as well as the leptonnumber-violating scattering processes that it mediates. For the heavy-neutrino yield, the evolution is dominated by decays and inverse decays, and we obtain the rate equation sHN dYN1 = − z dz YN1 YNeq. It would appear that the asymmetry does not vanish, as it should, when the RH neutrinos are in equilibrium It is for this reason that the s-channel ∆L = 2 scattering terms appear with a prime: we must only include those scattering terms that do not count processes already accounted for through the decay and inverse decay terms. We will see how the current state-of-the-art goes beyond the simplistic analysis and approximations that we have detailed above
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