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.

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.

Abstract. We present a comprehensive review of keV-scale sterile neutrino Dark Matter, collecting views and insights from all disciplines involved -cosmology, astrophysics, nuclear, and particle physics -in each case viewed from both theoretical and experimental/observational perspectives. After reviewing the role of active neutrinos in particle physics, astrophysics, and cosmology, we focus on sterile neutrinos in the context of the Dark Matter puzzle. Here, we first review the physics motivation for sterile neutrino Dark Matter, based on challenges and tensions in purely cold Dark Matter scenarios. We then round out the discussion by critically summarizing all known constraints on sterile neutrino Dark Matter arising from astrophysical observations, laboratory experiments, and theoretical considerations. In this context, we provide a balanced discourse on the possibly positive signal from X-ray observations. Another focus of the paper concerns the construction of particle physics models, aiming to explain how sterile neutrinos of keV-scale masses could arise in concrete settings beyond the Standard Model of elementary particle physics. The paper ends with an extensive review of current and future astrophysical and laboratory searches, highlighting new ideas and their experimental challenges, as well as future perspectives for the discovery of sterile neutrinos.

We show that flavour effects in leptogenesis reduce the region of the seesaw parameter space where the final predictions do not depend on the initial conditions, the strong wash-out regime. In this case the lowest bounds holding on the lightest right-handed (RH) neutrino mass and on the reheating temperature for hierarchical heavy neutrinos do not get relaxed compared to the usual ones in the one-flavour approximation,  GeV. Flavour effects can however relax down to these minimal values the lower bounds holding for fixed large values of the decay parameter K1. We discuss a relevant definite example showing that, when the known information on the neutrino mixing matrix is employed, the lower bounds for are relaxed by a factor 2–3 for fully hierarchical light neutrinos, without any dependence on θ13 and on possible phases. On the other hand, going beyond the limit of hierarchical light neutrinos and taking into account Majorana phases, the lower bounds can be relaxed by one order of magnitude. Therefore, Majorana phases can play an important role in leptogenesis when flavour effects are included.

We study the implications of thermal leptogenesis for neutrino parameters. Assuming that decays of N 1 , the lightest of the heavy Majorana neutrinos, initiate baryogenesis, we show that the final baryon asymmetry is determined by only four parameters: the CP asymmetry ε 1 , the heavy neutrino mass M 1 , the effective light neutrino mass m 1 , and the quadratic mean m of the light neutrino masses. Imposing the CMB measurement of the baryon asymmetry as constraint on the neutrino parameters, we show, in a model independent way, that quasi-degenerate neutrinos are incompatible with thermal leptogenesis. For maximal CP asymmetry ε 1 , and neutrino masses in the range from (∆m 2 sol ) 1/2 to (∆m 2 atm ) 1/2 , the baryogenesis temperature is T B = O(10 10 ) GeV.