We explore direct collider probes of the resonant leptogenesis mechanism for the origin of matter. We work in the context of theories where the Standard Model is extended to include an additional gauged U(1) symmetry broken at the TeV scale, and where the light neutrinos obtain mass through a Type I seesaw at this scale. The CP asymmetry that generates the observed matter-antimatter asymmetry manifests itself in a difference between the number of positive and negative like-sign dileptons N(\ell^+\ell^+)-N(\ell^-\ell^-) that arise in the decay of the new Z' gauge boson to two right-handed neutrinos N, and their subsequent decay to leptons. The relatively low efficiency of resonant leptogenesis in this class of models implies that the CP asymmetry, ε, is required to be sizable, i.e. of order one. In particular, from the sign of the baryon asymmetry of the Universe, \emph{an excess of antileptons is predicted}. We identify the domains in M_{Z'}--M_N space where such a direct test is possible and find that with 300~fb^{-1} of data and no excess found, the LHC can set the $2σ$ exclusion limit ε\lesssim 0.22.
We discuss minimal non-supersymmetric models of resonant leptogenesis, based on an approximate flavour symmetries. As an illustrative example, we consider a resonant tau-leptogenesis model, compatible with universal right-handed neutrino masses at the GUT scale, where the required heavy-neutrino mass splittings are generated radiatively. In particular, we explicitly demonstrate, how a minimum number of three heavy Majorana neutrinos is needed, in order to obtain successful leptogenesis and experimentally testable rates for processes of lepton flavour violation, such as mu --> e gamma and mu --…
Minimal models exist in which thermal, resonant leptogenesis occurs at the electroweak scale. The effects of individual lepton flavours play a crucial role and allow successful leptogenesis with large couplings between some of the charged leptons and the heavy Majorana neutrinos. This leads to potentially observable signals for low energy experiments (such as MEG, MECO and PRIME) and future linear colliders.
In the presence of large extra dimensions, the fundamental scale could be as low as a few TeV. This yields leptogenesis and seesaw at a TeV scale. Phenomenologically two TeV-scale Majorana fermions with a small mass split can realize a resonant leptogenesis whereas a TeV-scale Higgs triplet with a small trilinear coupling to the standard model Higgs doublet can give a verifiable seesaw. We propose an interesting scenario where the small parameters for the resonant leptogenesis and the type-II seesaw can be simultaneously generated by the propagation of lepton number violation from distant bran…
We review the motivations and some results on leptogenesis in seesaw models with an almost conserved lepton number. The paper is based on a talk given at the 5th International Symposium on Symmetries in Subatomic Physics, SSP2012.
The quantum Boltzmann equations relevant for leptogenesis, obtained using non-equilibrium quantum field theory, are described. They manifest memory effects leading to a time-dependent CP asymmetry which depends upon the previous history of the system. This result is particularly relevant in resonant leptogenesis where the asymmetry is generated by the decays of nearly mass-degenerate right-handed neutrinos. The impact of the non-trivial time evolution of the CP asymmetry is discussed either in the generic resonant leptogenesis scenario or in the more specific Minimal Lepton Flavour Violation f…
It has been recently shown that the quantum Boltzmann equations may be relevant for the leptogenesis scenario. In particular, they lead to a time-dependent CP asymmetry which depends upon the previous dynamics of the system. This memory effect in the CP asymmetry is particularly important in resonant leptogenesis where the asymmetry is generated by the decays of nearly mass-degenerate right-handed neutrinos. We study the impact of the nontrivial time evolution of the CP asymmetry in resonant leptogenesis, both in the one-flavour case and with flavour effects included. We show that significant …
We study the resonantly enhanced CP-asymmetry in the decays of nearly mass-degenerate heavy right-handed Majorana neutrinos for which different formulae have been presented in the literature, depending on the method used to calculate it. We consider two different techniques and show that they lead to the same result, thereby reconciling the different approaches.
Within the extension of MSSM by two right handed neutrinos, which masses are degenerate at tree level, we address the issue of leptogenesis. Investigating the quantum corrections in details, we show that the lepton asymmetry is induced at 1-loop level and decisive role is played by the tau lepton Yukawa coupling. On a concrete and predictive neutrino model, which enables to predict the CP violating $δ$ phase and relate it to the cosmological CP asymmetry, we demonstrate that the needed amount of the baryon asymmetry is generated via the resonant leptogenesis.
We present a study of resonant leptogenesis in a radiative seesaw model. We consider the case where two quasi-degenerate right-handed neutrinos realize resonant leptogenesis, and the CP violation necessary to achieve leptogenesis occurs through the CP phases present in the neutrino mixing matrix. A numerical analysis is performed by taking the best-fit values from the current global data for three neutrino mixing angles and two mass-squared differences. We have shown how the predicted value of baryon asymmetry depends on the Dirac and Majorana CP phases. With the particular choice for the mass…
It has been recently shown that the quantum Boltzmann equations may be relevant for the leptogenesis scenario. In particular, they lead to a time-dependent CP asymmetry which depends upon the previous dynamics of the system. This memory effect in the CP asymmetry is particularly important in resonant leptogenesis where the asymmetry is generated by the decays of nearly mass-degenerate right-handed neutrinos. We study the impact of the non-trivial time evolution of the CP asymmetry in the so-called Minimal Lepton Flavour Violation framework where the charged-lepton and the neutrino Yukawa coupl…
We study how lepton flavor violation (LFV) is related to the reactor angle $θ_{13}$, LHC/dark matter (DM) signal with a successful leptogenesis in a radiative seesaw model through $A_4$ flavor symmetry breaking, in the light of the deviation of tri-bimaximal mixing (TBM) angles and the recent precision oscillation data. To do this, we consider the normal mass spectrum of light neutrino giving a successful leptogenesis at electroweak (EW) scale or more with a relatively large $\sinθ_{13}\simeq{\cal O}(0.1)$, where a successful leptogenesis requires LHC/DM signal $m_{-}\lesssim$ a few keV in our…
We consider resonant Dirac leptogenesis in a geometry with three five-dimensional throats in the flat limit. The baryon asymmetry in the universe is generated by resonant decays of heavy Kaluza-Klein scalars that are copies of the standard model Higgs. Discrete exchange symmetries between the throats are responsible for establishing two key features of the model. First, they ensure a near degeneracy of the scalar masses and thus a resonant decay of the scalars. This allows for Dirac leptogenesis at low energies close to the TeV scale. Second, the discrete symmetries connect the observed baryon…
Leptogenesis can successfully explain the matter-antimatter asymmetry via out-of-equilibrium decays of heavy Majorana neutrinos in the early Universe. In this article, we focus on non-resonant thermal leptogenesis and the possibility of lowering its scale. In order to do so, we calculate the lepton asymmetry produced from the decays of one and two heavy Majorana neutrinos using three-flavoured density matrix equations in an exhaustive exploration of the model parameter space. We find regions of the parameter space where thermal leptogenesis is viable at intermediate scales, $T\sim 10^{6}$ GeV.…
We construct a model realizing the inverse seesaw mechanism. The model has two types of gauge singlet fermions in addition to right-handed neutrinos. A required Majorana mass scale (keV scale) for generating the light active neutrino mass in the conventional inverse seesaw can be naturally explained by a "seesaw" mechanism between the two singlet fermions in our model. We find that our model can decrease the magnitude of hierarchy among the mass parameters by $\mathcal{O}(10^4)$ from that in the conventional inverse seesaw model. We also show that a successful resonant leptogenesis occurs for …
An SO(10) grand unified model considered previously by the authors featuring lopsided down quark and charged lepton mass matrices is successfully predictive and requires that the lightest two right-handed Majorana neutrinons be nearly degenerate in order to obtain the LMA solar neutrino solution. Here we use this model to test its predictions for baryogenesis through resonant-enhanced leptogenesis. With the conventional type I seesaw mechanism, the best predictions for baryogenesis appear to fall a factor of three short of the observed value. However, with a proposed type III seesaw mechanism …
Under the assumption of hierarchical right-handed neutrino masses, masses of right-handed neutrinos must be larger than $10^8$ GeV in the standard thermal leptogenesis scenario, while the mass can be reduced to around 5 TeV in a neutrinophilic two Higgs doublet model. On the other hand, resonant leptogenesis can work with the masses of TeV-scale. However, necessary degeneracy between the lightest and the second lightest right-handed neutrino masses means unnatural fine-tuning of the order of $10^9$. In this paper, we will investigate the resonant leptogenesis scenario in a neutrinophilic two H…
We study the scenario of thermal leptogenesis in which the leptonic asymmetries are resonantly enhanced through the mixing of nearly degenerate heavy Majorana neutrinos that have mass differences comparable to their decay widths. Field-theoretic issues arising from the proper subtraction of real intermediate states from the lepton-number-violating scattering processes are addressed in connection with an earlier developed resummation approach to unstable particle mixing in decay amplitudes. The pertinent Boltzmann equations are numerically solved after the enhanced heavy-neutrino self-energy ef…
We study resonant leptogenesis at the electroweak phase transition in the singlet Majoron model with right-handed neutrinos. We consider a scenario, where the SM gauge group and the lepton number break down spontaneously during a second-order electroweak phase transition. We calculate the flavour- and temperature-dependent leptonic asymmetries, by including the novel contributions from the transverse polarisations of the W+/- and Z bosons. The required resummation of the gauge-dependent off-shell heavy-neutrino self-energies is consistently treated within the gauge-invariant framework of the P…
Dominant Thermal Resonant Mechanism for Low-Scale Leptogenesis
The University of Osaka（大阪大学）；Marietta Blau Institute for Particle Physics, Austrian Academy of Sciences（奥地利科学院玛丽埃塔·布劳粒子物理研究所）；University of Manchester（曼彻斯特大学）
We investigate the resonant leptogenesis scenario in the minimal B-L extended standard model(SM) with the B-L symmetry breaking at the TeV scale. Through detailed analysis of the Boltzmann equations, we show how much the resultant baryon asymmetry via leptogenesis is enhanced or suppressed, depending on the model parameters, in particular, the neutrino Dirac Yukawa couplings and the TeV-scale Majorana masses of heavy degenerate neutrinos. In order to consider a realistic case, we impose a simple ansatz for the model parameters and analyze the neutrino oscillation parameters and the baryon asym…
We present a class of leptogenesis models where the light neutrinos acquire their observed mass through a symmetry-motivated construction. We consider an extension of the Standard Model, which includes three singlet neutrinos which have mass splittings comparable to their decay widths. We show that this tri-resonant structure leads to an appreciable increase in the observed CP asymmetry over that found previously in typical bi-resonant models. To analyse such tri-resonant scenarios, we solve a set of coupled Boltzmann equations, crucially preserving the variations in the relativistic degrees o…
We propose a model of TeV-scale resonant leptogenesis based upon recent models of the generation of light neutrino masses from supersymmetry-breaking effects with TeV-scale right-handed (rhd) neutrinos, $N_i$. The model leads to naturally large cosmological lepton asymmetries via the resonant behaviour of the one-loop self-energy contribution to $N_i$ decay. Our model addresses the primary problems of previous phenomenological studies of low-energy leptogenesis: a rational for TeV-scale rhd neutrinos with small Yukawa couplings so that the out-of equilibrium condition for $N_i$ decay is satisf…
Vanilla leptogenesis within the type I seesaw framework requires the mass scale of the right-handed neutrinos to be above 10^9 GeV. This lower bound can be avoided if at least two of the sterile states are almost mass degenerate, which leads to an enhancement of the decay asymmetry. Leptogenesis models that can be tested in current and upcoming experiments often rely on this resonant enhancement, and a systematic and consistent description is therefore necessary for phenomenological applications. In this review article, we give an overview of different methods that have been used to study the …
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-of-equilibrium decays is a quite difficult task. We comme…
We study minimal scenarios of resonant leptogenesis near the electroweak phase transition. These models offer a number of testable phenomenological signatures for low-energy experiments and future high-energy colliders. Our study extends previous analyses of the relevant network of Boltzmann equations, consistently taking into account effects from out of equilibrium sphalerons and single lepton flavours. We show that the effects from single lepton flavours become very important in variants of resonant leptogenesis, where the observed baryon asymmetry in the Universe is created by lepton-to-bar…
We investigate the viability of thermal leptogenesis in type-I seesaw models with leptonic flavour symmetries that lead to tribimaximal neutrino mixing. We consider an effective theory with an A4 x Z3 x Z4 symmetry, which is spontaneously broken at a scale much higher than the electroweak scale. At the high scale, leptonic Yukawa interactions lead to exact tribimaximal mixing and the heavy Majorana neutrino mass spectrum is exactly degenerate. In this framework, leptogenesis becomes viable once this degeneracy is lifted either by renormalization group effects or by a soft breaking of the A4 sy…
In R-parity violating supersymmetric models both leptogenesis and the correct neutrino masses are hard to achieve together. The presence of certain soft nonholomorphic R-parity violating terms helps to resolve this problem. We consider a scenario where the lightest and the second-lightest neutralino are nearly degenerate in mass and enough CP-asymmetry can be produced through resonant leptogenesis. In this model, the lighter chargino and the lightest neutralino are highly degenerate. We have relatively lighter gauginos which can be produced at the LHC leading to heavily ionizing charged tracks…
We consider a minimal extension of the Standard Model with one singlet neutrino per generation that can realize resonant leptogenesis at the electroweak scale. In particular, the baryon asymmetry in the Universe can be created by resonant lepton-to-baryon conversion of an individual lepton number, for example that of the $τ$-lepton. The solar and atmospheric neutrino data can be explained by a simple CP-violating Yukawa texture. The model has several testable phenomenological implications. Most interestingly, it predicts heavy Majorana neutrinos at the sub-TeV scale, which can be probed at $e^…
Low-Scale Leptogenesis from Resonant Thermal Lepton Flavour Coherences
The University of Osaka（大阪大学）；Marietta Blau Institute for Particle Physics, Austrian Academy of Sciences（奥地利科学院玛丽埃塔·布劳粒子物理研究所）；University of Manchester（曼彻斯特大学）
In this present work, we uphold the standard model (SM) augmented with two right-handed (RH) neutrinos along with two singlet neutral fermions to generate active neutrino masses via (2,2) inverse see-saw mechanism. All entries of the neutrino mass matrix are taken to be complex to make this study a general one. We also investigate if the parameter points compatible with the neutrino oscillation data simultaneously satisfy the experimental bounds coming from the lepton flavour violating (LFV) decays : $μ\to e γ,~ τ\to e γ, ~ τ\to μγ$. This study also explores the prospect of producing the baryo…
We present an S$_4$ flavour symmetric model within a minimal seesaw framework resulting in mass matrices that leads to TM$_1$ mixing. Minimal seesaw is realized by adding two right-handed neutrinos to the Standard Model. The model predicts Normal Hierarchy (NH) for neutrino masses. Using the constrained six-dimensional parameter space, we have evaluated the effective Majorana neutrino mass, which is the parameter of interest in neutrinoless double beta decay experiments. The possibility of explaining baryogenesis via resonant leptogenesis is also examined within the model. A non-zero, resonant…
It is clear that matter is dominant in the Universe compared to antimatter. We call this problem baryon asymmetry. The baryon asymmetry is experimentally determined by both cosmic microwave background and big bang nucleosynthesis measurements. To resolve the baryon number asymmetry of the Universe as well as neutrino oscillations, we study a radiative seesaw model in a modular $A_4$ symmetry. Degenerate heavy Majorana neutrino masses can be naturally realized in an appropriate assignments under modular $A_4$ with large imaginary part of modulus $τ$, and it can induce measured baryon number via…
Right-handed neutrinos offer an elegant solution to two well established phenomena beyond the Standard Model (SM) - masses and oscillations of neutrinos, as well as the baryon asymmetry of the Universe. It is also a minimalistic solution since it requires only singlet Majorana fermions to be added to the SM particle content. If these fermions are nearly degenerate, the mass scale of right-handed neutrinos can be very low and accessible by the present and planned experiments. There are at least two well studied mechanisms of the low-scale leptogenesis: baryogenesis via oscillations and resonant…
We consider sub-TeV scale flavoured resonant leptogenesis within the minimal type-I seesaw scenario with two right-handed singlet neutrinos $N_{1,2}$ forming a pseudo-Dirac pair, concentrating on the case of masses of the pseudo-Dirac pair having values $M_{1,2} \lesssim 100$ GeV. The case when the CP violating asymmetries in the individual lepton charges $L_l$, $l=e,μ,τ$, and in the total lepton charge $L$ of the Universe are generated in $1 \leftrightarrow 2$ decay processes is investigated. We show that successful leptogenesis is possible for $M_{1,2}$ lying in the interval $M_{1,2} = (0.3 …
We investigate the resonant leptogenesis scenario in the minimally $U(1)_{B-L}$ extended standard model with minimal flavor violation. In our model, the $U(1)_{B-L}$ gauge symmetry is broken at the TeV scale and standard model singlet neutrinos gain Majorana masses of order TeV. In addition, we introduce a flavor symmetry on the singlet neutrinos at a scale higher than TeV. The flavor symmetry is explicitly broken by the neutrino Dirac Yukawa coupling, which induces splittings in the singlet neutrino Majorana masses at lower scales through renormalization group evolutions. We call this setup "…
Leptogenesis via Resonant Sequential Dominance and TBC3 Mixing in a Type-I Seesaw Model
Graduate School of Science and Technology, Tokai University（东海大学理学院）；Micro/Nano Technology Center, Tokai University（东海大学微纳技术中心）；Department of Physics, School of Science, Tokai University（东海大学理学院物理系）
We present a low-scale type-I seesaw scenario with discrete flavor and CP symmetries. This scenario not only explains the measured values of the lepton mixing angles, but also makes predictions for leptonic CP violation, and connects the low-energy CP phases relevant for neutrino oscillation and neutrinoless double beta decay experiments with the high-energy CP phases relevant for leptogenesis. We show that the three right-handed Majorana neutrinos in this scenario have (almost) degenerate masses and their decays can explain the observed baryon asymmetry of the Universe via resonant leptogenes…
We study a class of leptogenesis models where the light neutrinos acquire their observed small masses by a symmetry-motivated construction. This class of models may naturally include three nearly degenerate heavy Majorana neutrinos that can strongly mix with one another and have mass differences comparable to their decay widths. We find that such a tri-resonant heavy neutrino system can lead to leptonic CP asymmetries which are further enhanced than those obtained in the usual bi-resonant approximation. Moreover, we solve the Boltzmann equations by paying special attention to the temperature d…
We consider a TeV scale resonant lepogenesis within type-I seesaw mechanism. We show that a concrete model based on $A_4$ flavor symmetry with a scaling ansatz in the neutrino Dirac mass matrix and a retro-circulant heavy Majorana mass matrix can be realized. The full Yukawa coupling matrix $Y_ν$ can be fully reconstructed from the low energy neutrino oscillations data and the quasi-degenerate heavy Majorara neutrino masses. We have carried out a detailed numerical analysis to constrain the Dirac neutrino mass matrix elements. In particular, it was found that its diagonal elements lie near or …
We study a two-loop induced radiative neutrino model at TeV scale with global $U(1)$ symmetry, in which we analyze dark matter and resonant leptogenesis. The model includes two kinds of dark matter candidates. We discuss what kind of dark matter can satisfy the observed relic density as well as the current direct detection bound, and be simultaneously compatible with the leptogenesis. We also discuss whether our resonant leptogenesis can be differentiated from the other scenarios at TeV scale or not.
Leptogenesis is generally challenging to directly test due to the very high energy scales involved. In this work we propose a new probe for leptogenesis with cosmological collider physics. With the example of a cosmological Higgs collider, we demonstrate that during inflation leptogenesis models can produce detectable primordial non-Gaussianity with distinctive oscillatory patterns that encode information about the lepton-number violating couplings, the Majorana right-hand neutrino masses, and the CP phases, which are essential to leptogenesis.
Within the low scale resonant leptogenesis scenario, the cosmological CP asymmetry may arise by radiative corrections through the charged lepton Yukawa couplings. While in some cases, as one expects, decisive role is played by the $λ_{τ}$ coupling, we show that in specific neutrino textures only by inclusion of the $λ_{μ}$ the cosmological CP violation is generated at 1-loop level. With the purpose to relate the cosmological CP violation to the leptonic CP phase $δ$, we consider an extension of MSSM with two right handed neutrinos (RHN), which are degenerate in mass at high scales. Together wi…
Extending the Minimal Supersymmetric Standard Model with three right-handed neutrino superfields is one of the best motivated scenarios for physics beyond the Standard Model. However, very little is known from observations about the high energy parameters of this model. In this paper we show, under the plausible assumptions that the neutrino Yukawa eigenvalues are hierarchical and the absence of cancellations, that there exists an upper bound on the smallest Yukawa eigenvalue stemming from the non-observation of the rare lepton decay mu --> e gamma. Furthermore, we show that this bound implies…
In a class of Type-II seesaw dominated $SO(10)$ models proposed recently with heavy neutrinos, extra $Z'$ boson, and resonant leptogenesis, at first we show that the lightest first generation sterile neutrino that mediates dominant contributions to neutrinoless double beta decay also generates the displaced vertex leading to verifiable like-sign di-electron as well as di-muon production events outside the LHC detectors having suppressed standard model back-ground and missing energy. Resonant leptogenesis in this case is implemented by a pair of quasi-degenerate sterile neutrinos of the second …
We present a new realization of inverted neutrino mass hierarchy based on $S_3 \times {\cal U}(1)$ flavor symmetry. In this scenario, the deviation of the solar oscillation angle from $π/4$ is correlated with the value of $θ_{13}$, as they are both induced by a common mixing angle in the charged lepton sector. We find several interesting predictions: $\te_{13}\geq 0.13$, $\sin^2\te_{12}\geq 0.31$, $\sin^2\te_{23}\simeq 0.5$ and $0\leq \cos \de \leq 0.7$ for the neutrino oscillation parameters and $0.01 {\rm eV}\stackrel{<}{_\sim}m_{\bt \bt}\stackrel{<}{_\sim} 0.02 {\rm eV}$ for the effective n…
The left-right symmetric model with doublet and bi-doublet Higgs scalars can accommodate linear, inverse or double seesaw for generating small neutrino masses in the presence of three singlet fermions. If the singlet fermions have small Majorana masses, they can form three pairs of quasi-degenerate Majorana fermions with three right-handed neutrinos. The decays of the quasi-degenerate Majorana fermions can realize the resonant leptogenesis. Alternatively, the right-handed neutrinos can obtain seesaw suppressed Majorana masses if the singlet fermions are very heavy. In this case leptogenesis, w…
We study the impact of minimal non-supersymmetric models of resonant leptogenesis on charged lepton flavour violation and the neutrino mixing angle theta(13). Possible low-scale flavour realisations of resonant tau-, mu- and e-leptogenesis provide very distinct and predictive frameworks to explain the observed baryon asymmetry in the Universe by sphaleron conversion of an individual tau-, mu- and e-lepton-number asymmetry which gets resonantly enhanced via out-of-equilibrium decays of nearly degenerate heavy Majorana neutrinos. Based on approximate flavour symmetries, we construct viable scena…
The lepton asymmetry generated by the out-of-equilibrium decays of heavy Majorana neutrinos with a quasi-degenerate mass spectrum is resonantly enhanced. In this work, we study this scenario within a first-principle approach. The quantum field theoretical treatment is applicable for mass splittings of the order of the width of the Majorana neutrinos, for which the enhancement is maximally large. The non-equilibrium evolution of the mixing Majorana neutrino fields is described by a formal analytical solution of the Kadanoff-Baym equations, that is obtained by neglecting the back-reaction. Based…
