Right-handed Neutrino Mass Spectrum Research Articles

Low-scale resonant leptogenesis in SU(5) GUT with T13 family symmetry

We investigate low-scale resonant leptogenesis in an $SU(5) \times \mathcal{T}_{13}$ model where a single high energy phase in the complex Tribimaximal seesaw mixing produces the yet-to-be-observed low energy Dirac and Majorana ${CP}$ phases. A fourth right-handed neutrino, required to generate viable light neutrino masses within this scenario, also turns out to be necessary for successful resonant leptogenesis where $CP$ asymmetry is produced by the same high energy phase. We derive a lower bound on the right-handed neutrino mass spectrum in the $\mathrm{GeV}$ range, where part of the parameter space, although in tension with Big Bang Nucleosynthesis constrains, can be probed in planned high intensity experiments like DUNE. We also find the existence of a curious upper bound ($\text{TeV}$-scale) on the right-handed neutrino mass spectrum in majority of the parameter space due to significant washout of the resonant asymmetry by lighter right-handed neutrinos. While in most of the parameter space of our model classical Boltzmann equations are sufficient, when right-handed neutrino masses are below the electroweak sphalerons freeze-out temperature we resort to the more general density matrix formalism to take into account decays and oscillations of the right-handed neutrinos as well as their helicities which are relevant when they are relativistic.

Neutrino parameters in the Planck-scale lepton number breaking scenario with extended scalar sectors

Two-loop effects on the right-handed neutrino masses can have an impact on the low-energy phenomenology, especially when the right-handed neutrino mass spectrum is very hierarchical at the cut-off scale. In this case, the physical masses of the lighter right-handed neutrinos can be dominated by quantum effects induced by the heavier ones. Further, if the heaviest right-handed neutrino mass is at around the Planck scale, two-loop effects on the right-handed neutrino masses generate, through the seesaw mechanism, an active neutrino mass which is in the ballpark of the experimental values. In this paper we investigate extensions of the Planck-scale lepton number breaking scenario by additional Higgs doublets (inert or not). We find that under reasonable assumptions these models lead simultaneously to an overall neutrino mass scale and to a neutrino mass hierarchy in qualitative agreement with observations.

Revisiting gravitino dark matter in thermal leptogenesis

In this paper, we revisit the gravitino dark matter scenario in the presence of the bilinear R-parity violating interaction. In particular, we discuss a consistency with the thermal leptogenesis. For a high reheating temperature required for the thermal leptogenesis, the gravitino dark matter tends to be overproduced, which puts a severe upper limit on the gluino mass. As we will show, a large portion of parameter space of the gravitino dark matter scenario has been excluded by combining the constraints from the gravitino abundance and the null results of the searches for the superparticles at the LHC experiments. In particular, the models with the stau (and other charged slepton) NLSP has been almost excluded by the searches for the long-lived charged particles at the LHC unless the required reheating temperature is somewhat lowered by assuming, for example, a degenerated right-handed neutrino mass spectrum.

Squeezing out predictions with leptogenesis from SO(10)

We consider the see-saw mechanism within a nonsupersymmetric SO(10) model. By assuming the SO(10) quark-lepton symmetry, and after imposing suitable conditions that ensure that the right-handed neutrino masses are at most mildly hierarchical (compact right-handed spectrum) we obtain a surprisingly predictive scenario. The absolute neutrino mass scale, the Dirac and the two Majorana phases of the neutrino mixing matrix remain determined in terms of the set of already measured low energy observables, modulo a discrete ambiguity in the signs of two neutrino mixing angles and of the Dirac phase. The right-handed neutrino mass spectrum is also predicted, as well as the size and sign of the leptogenesis $CP$ asymmetries. We compute the cosmological baryon asymmetry generated through leptogenesis and obtain the correct sign, and a size compatible with observations.

A 4 flavor models in split seesaw mechanism

A seesaw mechanism in an extra-dimension, known as the split seesaw mechanism, provides a natural way to realize a splitting mass spectrum of right-handed neutrinos. It leads to one keV sterile neutrino as a dark matter candidate and two heavy right-handed neutrinos being responsible for leptogenesis to explain the observed baryon asymmetry of the Universe. We study models based on $A_4$ flavor symmetry in the context of the split seesaw mechanism. It is pointed out that most of known $A_4$ flavor models with three right-handed neutrinos being $A_4$ triplet suffer from a degeneracy problem for the bulk mass terms, which disturbs the split mechanism for right-handed neutrino mass spectrum. Then we construct a new $A_4$ flavor model to work in the split seesaw mechanism. In the model, the experimentally observed neutrino masses and mixing angles can be realized from both type I+II seesaw contributions. The model predicts the $\mu-\tau$ symmetry in the neutrino mass matrix at the leading order, resulting in the vanishing $\theta_{13}$ and maximal $\theta_{23}$. The flavor symmetry $A_4$ is broken via the flavon vacuum alignment which can be obtained from the orbifold compactification. The model can be consistent with all data of neutrino oscillation experiments, cosmological discussions of dark matter abundance, leptogenesis, and recent astrophysical data.

Supersymmetric leptogenesis and the gravitino bound

Supersymmetric thermal leptogenesis with a hierarchical right-handed neutrino mass spectrum requires the mass of the lightest right-handed neutrino to be heavier than about 109 GeV. This is in conflict with the upper bound on the reheating temperature which is found by imposing that the gravitinos generated during the reheating stage after inflation do not jeopardize successful nucleosynthesis. In this Letter we show that a solution to this tension is actually already incorporated in the framework, because of the presence of flat directions in the supersymmetric scalar potential. Massive right-handed neutrinos are efficiently produced non-thermally and the observed baryon asymmetry can be explained even for a reheating temperature respecting the gravitino bound if two conditions are satisfied: the initial value of the flat direction must be close to Planckian values and the phase-dependent terms in the flat direction potential are either vanishing or sufficiently small.

Right-handed neutrinos in cosmology: light versus heavy

I review the role of right-handed (RH) neutrinos in cosmology, discussing how they can provide interesting solutions to some of the existing puzzles. I focus on the particularly relevant case represented by the see-saw limit, that gives rise to a host of cosmological applications, making a comparison between the two options of a 'light' and a 'heavy' RH neutrino mass spectrum.

On the impact of flavour oscillations in leptogenesis

When lepton flavour effects in thermal leptogenesis are active, they introduceimportant differences with respect to the case in which they are neglected,the so-called one-flavour approximation. We investigate analytically andnumerically the transition from the one-flavour to the two-flavour case when theτ-lepton flavour becomes distinguishable from the other two flavours. We study the impactof the oscillations of the asymmetries in lepton flavour space on the final leptonasymmetries, for the hierarchical right-handed neutrino mass spectrum. Flavouroscillations project the lepton state on the flavour basis very efficiently. We concludethat flavour effects are relevant typically for M1 1012 GeV, where M1 is the mass of the lightest right-handed neutrino.

Leptogenesis beyond the limit of hierarchical heavy neutrino masses

We calculate the baryon asymmetry of the Universe in thermal leptogenesisbeyond the usual lightest right-handed (RH) neutrino dominated scenario(N1DS) and in particular beyond the hierarchical limit (HL), , for the RH neutrino mass spectrum. After providing some orientationamong the large variety of models, we first revisit the central role of theN1DS, with new insights on the dynamics of the asymmetry generation, and then discuss the mainroutes departing from it, focusing on models beyond the HL. We study in detail two examplesof ‘strong–strong’ wash-out scenarios: one with ‘maximal phase’ and the limit of very largeM3, studying the effects arising when is small. We extend analytical methods already applied to theN1DS showing, for example, that, in the degenerate limit (DL), the efficiency factors of the RHneutrinos become equal, with the single decay parameter replaced by the sum. Bothcases disprove the misconception that close RH neutrino masses necessarily leadto a final asymmetry enhancement and to a relaxation of the lower bounds onM1 and on the initial temperature of the radiation-dominated expansion. We also explain whyleptogenesis tends to favour normal hierarchy compared to inverted hierarchy for theleft-handed neutrino masses.

Enlarging the window for radiative leptogenesis

We investigate the scenario of resonant thermal leptogenesis, in which the leptonic asymmetries are generated through renormalization group corrections induced at the leptogenesis scale. In the framework of the standard model extended by three heavy Majorana neutrinos with masses M1=M2≪M3 at some high scale, we show that the mass splitting and CP-violating effects induced by renormalization group corrections can lead to values of the CP asymmetries large enough for a successful leptogenesis. In this scenario, the low-energy neutrino oscillation data can also be easily accommodated. The possibility of having an underlying symmetry behind the degeneracy in the right-handed neutrino mass spectrum is also discussed.

Seesaw neutrino masses and mixing with extended democracy

In the context of a minimal extension of the Standard Model with three extra heavy right-handed neutrinos, we propose a model for neutrino masses and mixing based on the hipothesis of a complete alignment of the lepton mass matrices in flavour space. Considering a uniform quasi-democratic structure for these matrices, we show that, in the presence of a highly hierarchical right-handed neutrino mass spectrum, the effective neutrino mass matrix, obtained through the seesaw mechanism, can reproduce all the solutions of the solar neutrino problem.