Active-Sterile Neutrino Oscillations and Leptogenesis

We study coherent active-sterile neutrino oscillations as a possible source of leptogenesis. To this end, we add 3 gauge invariant Weyl_R neutrinos to the Standard Model with both Dirac and Majorana type mass terms. We find that the measured active neutrino masses and mixings, and successful baryogenesis via leptogenesis, may be achieved with fine-tuning, if at least one of the sterile neutrinos has a mass in the approximate range 0.14 to 1.1 GeV.

A simplest seesaw model of the TM1 and μ-τ reflection symmetries for the neutrino masses and leptogenesis

In this paper,following the Occam’s razor principle,we have put forward a very simple form of the Dirac neutrino mass matrix M_(D) in the minimal seesaw model with the right-handed neutrino mass matrix being diagonal M_(R)=diag(M_(1),M_(2));it has one texture zero and only contains three real parameters,whose values can be determined from the neutrino oscillation experimental results.Such a model leads to a neutrino mass matrix M_(v)≃-M_(D)M_(R)^(-1)M_(D)^(T)that obeys the TM1 and μ-τ reflection symmetries simultaneously.In this way all the lepton flavor mixing parameters except for θ_(13) are predicted;the value of θ_(12) is predicted by the TM1 symmetry,while those of θ_(23),δ,ρ and σ by the μ-τ reflection symmetry.And the neutrino masses are predicted to be of the NO case with m_(1)=0,for which all three light neutrino masses will be pinned down with the help of the experimental results for the neutrino mass squared differences.For these results,the effective Majorana neutrino mass∣(M_(ν))_(ee)∣that controls the rate of the neutrinoless double beta decay is predicted to be 1.6 or 3.8 meV in the case of σ=0 or π/2.We have also studied the implications of the model for leptogenesis.It turns out that only in the two-flavor leptogenesis regime(which holds in the temperature range 10^(9)-10^(12) GeV)can leptogenesis have a chance to be successful.And a successful leptogenesis can be achieved at M_(1)≃1.2×10^(11) GeV in the case of σ=π/2,but not in the case of σ=0.

Leptogenesis via a varying Weinberg operator:a semi-classical approach

In this paper, we introduce leptogenesis via a varying Weinberg operator from a semi-classical perspective.This mechanism is motivated by the breaking of an underlying symmetry which triggers a phase transition that causes the coupling of the Weinberg operator to become dynamical. Consequently, a lepton anti-lepton asymmetry arises from the interference of the Weinberg operator at two different spacetime points. Using the semi-classical approach,we treat the Higgs as a background field and show that a reflection asymmetry between leptons and anti-leptons is generated in the vicinity of the bubble wall. We solve the equations of motion of the lepton and anti-lepton quasiparticles to obtain the final lepton asymmetry.

Casas-Ibarra parametrization and leptogenesis

The Casas-Ibarra parametrization is a description of the Dirac neutrino mass matrix MD in terms of the neutrino mixing matrix V , an orthogonal matrix O and the diagonal mass matrices of light and heavy Majorana neutrinos in the type-I seesaw mechanism. Because MDMD is apparently independent of V but dependent on O in this parametrization, a number of authors have claimed that unflavored leptogenesis has nothing to do with CP violation at low energies. Here we question this logic by clarifying the physical meaning of O. We establish a clear relationship between O and the observable quantities, and find that O does depend on V . We show that both unflavored leptogenesis and flavored leptogenesis have no direct connection with low-energy CP violation.

Constraints of gravitational baryo/leptogenesis on Cardassian expansion

In this paper, proceeding from the relation between the Cardassian model and the accelerated expansion of the universe, adopting a parametric method which does not depend on a precise mechanism for gravitational baryo/leptogenesis and using the model parameter of CPT-violating interaction, we study the role of the modified Friedmann equation which plays a role in the matter asymmetry of the early epoch and the accelerated expansion of the present universe. Thus the appropriate Cardassian component in the radiatiomdominated era or in the matter-dominated universe can be obtained. The results indicate that early CPT-violation is included in the Cardassian term. In the same way, the present Cardassian term that belongs to a quintessence-like model can drive the universe towards a flat, matter-dominated and accelerating expansion.

Gravitational leptogenesis in teleparallel and symmetric teleparallel gravities

In this study,we investigate the possibilities of generating baryon number asymmetry under thermal equilibrium within the frameworks of teleparallel and symmetric teleparallel gravities.Through the derivative couplings of the torsion scalar and the non-metricity scalar to baryons,baryon number asymmetry is produced in the radiation dominated epoch.For gravitational baryogenesis mechanisms in these two frameworks,the produced baryon-to-entropy ratio is too small to be consistent with observations.However,the gravitational leptogenesis models within both frameworks have the potential to explain the observed baryon-antibaryon asymmetry.

Cosmological implications of a B−L charged hidden scalar:leptogenesis and gravitational waves

In this study,we investigated the cosmological implications of a complex singlet scalar S with non-trivial B−L charges in the conformal U(1)B−L theory.It was found that,in a sizable region of parameter space,S may disturb the resonant leptogenesis mechanism,which is used to generate baryon asymmetry,and affect the symmetry breaking dynamics in the strong first order phase transition.The stochastic gravitational waves(GWs)produced at the phase transition can be probed in future GW experiments.The GW searches prefer a relatively light S at the TeV-scale;however,this is difficult to detect directly at future high-energy colliders.

Twin cogenesis

We investigate a cogenesis mechanism within the twin Higgs setup that can naturally explain the nature of dark matter,the cosmic coincidence puzzle,little hierarchy problem,leptogenesis,and the tiny neutrino masses.Three heavy Majorana neutrinos are introduced to the standard model sector and the twin sector respectively,which explain the tiny neutrino masses and generate the lepton asymmetry and the twin lepton asymmetry at the same time.The twin cogenesis mechanism applies to any viable twin Higgs model without an explicit Z_(2)breaking in the leptonic sector and evading theΔN_(eff) constraint.We illustrate the twin cogenesis mechanism using the neutrino-philic twin two Higgs doublet model,a newly proposed model to lift the twin neutrino masses with spontaneous Z_(2)breaking.The dark photon with a Stueckelberg mass O(10)MeV ensures the energy in the twin sector as well as the symmetric component of twin sector particles can be depleted.The lightest twin baryons are the dark matter candidates with masses of approximately 5.5 GeV,which explains naturally the amount of dark matter and visible matter in the Universe are of the same order.We also demonstrate twin cogenesis in the fraternal twin Higgs setup,in which the dark matter candidate is the twin bottom bound stateΩ’_(b’b’b’).

Cosmological imprints of Dirac neutrinos in a keV-vacuum 2HDM

The Dirac neutrino masses could be simply generated by a neutrinophilic scalar doublet with a vacuum being dramatically different from the electroweak one.While the case with an eV-scale vacuum has been widely explored previously,we exploit in this work the desert where the scalar vacuum is of O(keV)scale.In this regime,there would be rare hope to probe the keV-vacuum neutrinophilic scalar model via the lepton-flavor-violating processes,which makes it distinguishable from the widely considered eV-scale vacuum.Although such a keV-vacuum scenario is inert in the low-energy flavor physics,we show that the baryogenesis realized via the lightest Dirac neutrino can be a natural candidate in explaining the baryon asymmetry of the Universe.Furthermore,the Dirac neutrinos with a keV-vacuum scalar can generate a shift of the effective neutrino number within the range 0.097≤ΔN_(eff)≤0.112,which can be probed by the future Simons Observatory experiments.In particular,the model with a minimal valueΔN_(eff)=0.097 can already be falsified by the future CMB Stage-Ⅳand Large Scale Structure surveys,providing consequently striking exploratory avenues in the cosmological regime for such a keV-vacuum scenario.

Dark sector production and baryogenesis from not quite black holes

Primordial black holes have been considered attractive dark matter candidates,whereas some of the predictions rely heavily on the near-horizon physics that remains to be tested experimentally.As a concrete alternative,thermal 2-2-holes closely resemble black holes without event horizons.Being a probable endpoint of gravitational collapse,they provide a solution to the information loss problem but also naturally result in stable remnants.Previously,we have considered primordial 2-2-hole remnants as dark matter.Owing to the strong constraints from a novel phenomenon associated with remnant mergers,only small remnants with mass approximate to the Planck mass can constitute all dark matter.In this paper,we examine the scenario in which the majority of dark matter consists of particles produced by the evaporation of primordial 2-2-holes,whereas the remnant contribution is secondary.The products with sufficiently light mass may contribute to the number of relativistic degrees of freedom in the early universe,which we also calculate.Moreover,2-2-hole evaporation can produce particles that are responsible for the baryon asymmetry.We observe that baryogenesis through direct B-violating decays or through leptogenesis can both be realized.Overall,the viable parameter space for the Planck remnant scenario is similar to that of primordial black holes with Planck remnants.However,heavier remnants result in different predictions,and the viable parameter space remains large even when the remnant abundance is small.

Broken S_(3L)×S_(3R) flavor symmetry and leptonic CP violation

In the framework of the canonical seesaw model,we present a simple but viable scenario to explicitly break an S3L×S3R flavor symmetry in the leptonic sector.It turns out that the leptonic flavor mixing matrix is completely determined by the mass ratios of the charged leptons（i.e.,me/mμand mμ/mτ） and those of light neutrinos（i.e.,m1/m2 and m2/m3）.The latest global-fit results of the three neutrino mixing angles {θ12,θ13,θ23}and two neutrino mass-squared differences {？m212,？m312} at the 3σ level are used to constrain the parameter space of {m1/m2,m2/m3}.The predictions for the mass spectrum and flavor mixing are highlighted：（1） the neutrino mass spectrum shows a hierarchical pattern and a normal ordering,e.g.,m1≈2.2meV,m2≈8.8 meV and m3≈52.7 meV;（2） only the first octant of θ23 is allowed,namely,41.8？ θ23 43.3？;（3） the Dirac C P-violating phase δ ≈-22？deviates significantly from the maximal value-90？.All these predictions are ready to be tested in ongoing and forthcoming neutrino oscillation experiments.Moreover,we demonstrate that the cosmological matter-antimatter asymmetry can be explained via resonant leptogenesis,including the individual lepton-flavor effects.In our scenario,leptonic C P violation at low-and high-energy scales is closely connected.

Hyperbolic Cardassian universe

We propose a hyperbolic function form of the Cardassian component in the Cardassian model. Using the repartition of this Cardassian component, we can obtain a non-zero gravitational interaction between the time derivative of Ricci scalar curvature and the baryon/lepton number current in the radiation-dominated universe. Furthermore, the other term that acts like a non-zero cosmological constant would give an accelerated expansion of current universe and the features of this model do not violate our desired requirements.