Hierarchical Heavy Neutrinos Research Articles

Neutrino mass generation and leptogenesis via pseudo-Nambu-Goldstone Higgs portal

We consider an extension of the Standard Model with the global symmetry breaking pattern SO(5)/SO(4), where the Higgs boson arises as a pseudo-Nambu-Goldstone boson. The scalar content of the theory consists of a Standard-Model-like Higgs field and an extra real scalar field. The flavour sector of the model is extended by two right-handed neutrinos compatible with the observed light-neutrino phenomenology, and we find that the correct vacuum alignment determines the mass of the heavier neutrino eigenstate to be around 80 TeV. The new singlet-scalar state generates dynamically a Majorana mass term for the heavy neutrino states. We show how the model leads to the correct baryon asymmetry of the universe via leptogenesis in the case of two degenerate or hierarchical heavy neutrinos.

NEUTRINO MASSES AND LEPTOGENESIS FROM EXTRA FERMIONS

Generation of the neutrino masses and leptogenesis (LG) in the standard model extended by the heavy Majorana fermions is considered. Classification of LG scenarios according to the new fermion mass spectra is given, where singlet–triplet LG is considered for the first time. The upper bound on the CP asymmetry relevant for LG with hierarchical heavy neutrinos (Davidson–Ibarra bound) is revised, and shown that in the case of one massless neutrino it essentially depends on the type of the light neutrino mass hierarchy. The resonant scenarios, which help to avoid the problem of extremely high reheating temperature in the early universe, are discussed. In particular, we present new simplified, generalized and detailed formulation of freed LG, which violates Davidson–Ibarra bound in a special class of models.

The minimal scenario of leptogenesis

We review the main features and results of thermal leptogenesis within the type I seesaw mechanism, the minimal extension of the Standard Model explaining neutrino masses and mixing. After presenting the simplest approach, the vanilla scenario, we discuss various important developments of recent years, such as the inclusion of lepton and heavy neutrino flavour effects, a description beyond a hierarchical heavy neutrino mass spectrum and an improved kinetic description within the density matrix and the closed-time-path formalisms. We also discuss how leptogenesis can ultimately represent an important phenomenological tool to test the seesaw mechanism and the underlying model of new physics.

Flavour effects on leptogenesis predictions

We show that flavour effects in leptogenesis reduce the region of the seesaw parameterspace where the final predictions do not depend on the initial conditions, the strongwash-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 getrelaxed compared to the usual ones in the one-flavour approximation, GeV. Flavour effects can however relax down to these minimal valuesthe lower bounds holding for fixed large values of the decay parameterK1. We discuss a relevant definite example showing that, when the known information on theneutrino mixing matrix is employed, the lower bounds for are relaxed by a factor 2–3 for fully hierarchical light neutrinos, without any dependence onθ13 and on possible phases. On the other hand, going beyond the limit of hierarchical lightneutrinos and taking into account Majorana phases, the lower bounds can be relaxed byone order of magnitude. Therefore, Majorana phases can play an important role inleptogenesis when flavour effects are included.

Testing supersymmetry with lepton flavor violatingτandμdecays

In this work the following lepton flavor violating $\ensuremath{\tau}$ and $\ensuremath{\mu}$ decays are studied: ${\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{\ensuremath{-}}{\ensuremath{\mu}}^{\ensuremath{-}}{\ensuremath{\mu}}^{+}$, ${\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{\ensuremath{-}}{e}^{\ensuremath{-}}{e}^{+}$, ${\ensuremath{\mu}}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{\ensuremath{-}}{e}^{\ensuremath{-}}{e}^{+}$, ${\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{\ensuremath{-}}\ensuremath{\gamma}$, ${\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{\ensuremath{-}}\ensuremath{\gamma}$, and ${\ensuremath{\mu}}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{\ensuremath{-}}\ensuremath{\gamma}$. We work in a supersymmetric scenario consisting of the minimal supersymmetric standard model particle content, extended by the addition of three heavy right-handed Majorana neutrinos and their supersymmetric partners, and where the generation of neutrino masses is done via the seesaw mechanism. Within this context, a significant lepton flavor mixing is generated in the slepton sector due to the Yukawa neutrino couplings, which is transmitted from the high to the low energies via the renormalization group equations. This slepton mixing then generates via loops of supersymmetric particles significant contributions to the rates of ${l}_{j}\ensuremath{\rightarrow}3{l}_{i}$ and the correlated ${l}_{j}\ensuremath{\rightarrow}{l}_{i}\ensuremath{\gamma}$ decays. We analyze here in full detail these rates in terms of the relevant input parameters, which are the usual minimal supergravity parameters and the seesaw parameters. For the ${l}_{j}\ensuremath{\rightarrow}3{l}_{i}$ decays, a full one-loop analytical computation of all the contributing supersymmetric loops is presented. This completes and corrects previous computations in the literature. In the numerical analysis compatibility with the most recent experimental upper bounds on all these $\ensuremath{\tau}$ and $\ensuremath{\mu}$ decays, with the neutrino data, and with the present lower bounds on the supersymmetric particle masses are required. Two typical scenarios with degenerate and hierarchical heavy neutrinos are considered. We will show here that the minimal supergravity and seesaw parameters do get important restrictions from these $\ensuremath{\tau}$ and $\ensuremath{\mu}$ decays in the hierarchical neutrino case.

See-saw geometry and leptogenesis

The representation of the see-saw orthogonal matrix in the complex plane establishes a graphical correspondence between neutrino mass models and geometrical configurations, particularly useful to study relevant aspects of leptogenesis. We first derive the CP asymmetry bound for hierarchical heavy neutrinos and then an expression for the effective leptogenesis phase, determining the conditions for maximal phase and placing a lower bound on the phase suppression for generic models. Reconsidering the lower bounds on the lightest right-handed (RH) neutrino mass M 1 and on the reheating temperature T reh , we find that models where one of the two heavier neutrino masses is dominated by the lightest right-handed (RH) neutrinos, typically arising from connections with quark masses, undergo both phase suppression and strong wash-out such that M 1 ( T reh ) ≳ 10 11 ( 10 10 ) GeV . The window 10 9 GeV ≲ M 1 , T reh ≲ 10 10 GeV is accessible only for a class of models where m 1 is dominated by the lightest RH neutrino, with no straightforward connections with quark masses. Within this class we describe a new scenario of thermal leptogenesis where the baryon asymmetry of the Universe is generated by the decays of the second lightest RH neutrino, such that the lower bound on M 1 disappears and is replaced by a lower bound on M 2 . Interestingly, the final asymmetry is independent on the initial conditions. We also discuss the validity of the approximation of hierarchical heavy neutrinos in a simple analytical way.