Leptogenesis is a class of scenarios in which the cosmic baryon asymmetry originates from an initial lepton asymmetry generated in the decays of heavy sterile neutrinos in the early Universe. We explain why leptogenesis is an appealing mechanism for baryogenesis. We review its motivations and the basic ingredients and describe subclasses of effects, like those of lepton flavours, spectator processes, scatterings, finite temperature corrections, the role of the heavier sterile neutrinos, and quantum corrections. We then address leptogenesis in supersymmetric scenarios, as well as some other popular variations of the basic leptogenesis framework.

The focus of this chapter lies on the possible experimental tests of leptogenesis scenarios. We consider both leptogenesis generated from oscillations, as well as leptogenesis from out-of-equilibrium decays. As the Akhmedov-Rubakov-Smirnov (ARS) mechanism allows for heavy neutrinos in the GeV range, this opens up a plethora of possible experimental tests, e.g. at neutrino oscillation experiments, neutrinoless double beta decay, and direct searches for neutral heavy leptons at future facilities. In contrast, testing leptogenesis from out-ofequilibrium decays is a quite difficult task. We comment on the necessary conditions for having successful leptogenesis at the TeV-scale. We further discuss possible realizations and their model specific testability in extended seesaw models, models with extended gauge sectors, and supersymmetric leptogenesis. Not being able to test high-scale leptogenesis directly, we present a way to falsify such scenarios by focusing on their washout processes. This is discussed specifically for the left-right symmetric model and the observation of a heavy WR, as well as model independently when measuring ∆L = 2 washout processes at the LHC or neutrinoless double beta decay. 3

If leptonic unitarity is violated by new physics at an energy scale much lower than the electroweak scale, which we call low-scale unitarity violation, it has different characteristic features from those expected in unitarity violation at high-energy scales. They include maintaining flavor universality and absence of zero-distance flavor transition. We present a framework for testing such unitarity violation at low energies by neutrino oscillation experiments. Starting from the unitary 3 active plus N (arbitrary positive integer) sterile neutrino model we show that by restricting the active-sterile and sterilesterile neutrino mass squared differences to 0.1 eV 2 the oscillation probability in the (3 + N ) model becomes insensitive to details of the sterile sector, providing a nearly modelindependent framework for testing low-scale unitarity violation. Yet, the presence of the sterile sector leaves trace as a constant probability leaking term, which distinguishes lowscale unitarity violation from the high-scale one. The non-unitary mixing matrix in the active neutrino subspace is common for the both cases. We analyze how severely the unitarity violation can be constrained in ν e -row by taking a JUNO-like setting to simulate medium baseline reactor experiments. Possible modification of the features of the (3 + N ) model due to matter effect is discussed to first order in the matter potential.

We consider SO(10) Grand Unified Theories (GUTs) with vacuum expectation values (vevs) for fermion masses in the 10 + 126 representation. We show that the baryon asymmetry generated via leptogenesis is completely determined in terms of measured low energy observables and of one single high energy parameter related to the ratio of the 10 and 126 SU(2) doublet vevs. We identify new decay channels for the heavy Majorana neutrinos into SU(2) singlet leptons e c which can sizeably affect the size of the resulting baryon asymmetry. We describe how to equip SO(10) fits to low energy data with the additional constraint of successful leptogenesis, and we apply this procedure to the fits carried out in ref. [1]. We show that a baryon asymmetry in perfect agreement with observations is obtained.

We present a comprehensive study of the three-active plus N sterile neutrino model as a framework for constraining leptonic unitarity violation induced at energy scales much lower than the electroweak scale. We formulate a perturbation theory with expansion in small unitarity violating matrix element W while keeping (non-W suppressed) matter effect to all orders. We show that under the same condition of sterile state masses 0.1 eV 2 m 2 J (1-10) GeV 2 as in vacuum, assuming typical accelerator based long-baseline neutrino oscillation experiment, one can derive a very simple form of the oscillation probability which consists only of zeroth-order terms with the unique exception of probability leaking term C αβ of O(W 4 ). We argue, based on our explicit computation to fourth-order in W , that all the other terms are negligibly small after taking into account the suppression due to the mass condition for sterile states, rendering the oscillation probability sterile-sector model independent. Then, we identify a limited energy region in which this suppression is evaded and the effects of order W 2 corrections may be observable. Its detection would provide another way, in addition to detecting C αβ , to distinguish between low-scale and high-scale unitarity violation. We also solve analytically the zerothorder system in matter with uniform density to provide a basis for numerical evaluation of non-unitary neutrino evolution.