Majorana neutrinos and CP violation in the leptonic sector

A set of weak-basis-independent necessary conditions is derived for CP conservation in the leptonic sector of the SU(2) × U(1) gauge theory with an arbitrary number of generations, when both Majorana and Dirac neutrino mass terms are present. In the case of two and three generations of left-handed Majorana neutrinos, weak-basis-independent conditions are given which are necessary and sufficient for CP invariance in the leptonic sector. It is also shown that for three or more generations of Majorana neutrinos, there may be CP violation even in the limit of complete neutrino-mass degeneracy.     

Heavy Dirac and Majorana neutrino production ine + e − collisions

The production of heavy Dirac and Majorana neutrinos ine ⁺ e ^? collisions is investigated. The heavy Dirac and/or Majorana neutrinos can be produced in charged and neutral current processes $(e^ + e^ - \to N_1 \bar N_2 )$ . The production of a single heavy neutrino is possible if it mixes with the light neutrino species. The production of heavy neutrinos in Higgs channels is also studied, since in some specific models the Yukawa couplings could be large enough to make the production of heavy neutrinos through Higgs boson exchanges sufficiently large for detection. The most general left-right symmetric model with possibly complexV orA couplings is used in the analytic calculations of the production cross sections, but the numerical examples are given using simplified left-right symmetric model. The interference terms between different production channels have been studied in great detail and in some cases the interference terms are found to be non-negligible in wide range of production spectrum. The pair production cross section is larger in the Dirac case than it is in the Majorana case, but the single heavy Majorana neutrino production cross section is roughly twice as large as that of a Dirac neutrino.     

Constraint on the heavy sterile neutrino mixing angles in the SO(10) model with double see-saw mechanism

Constraints on the heavy sterile neutrino mixing angles are studied in the framework of a minimal supersymmetric SO(10) model with the use of the double see-saw mechanism. A new singlet matter in addition to the right-handed neutrinos is introduced to realize the double see-saw mechanism. The light Majorana neutrino mass matrix is, in general, given by a combination of those of the singlet neutrinos and the active neutrinos. The minimal SO(10) model is used to give an example form of the Dirac neutrino mass matrix, which enables us to predict the masses and the mixing angles in the enlarged 9×9 neutrino mass matrix. Mixing angles between the light Majorana neutrinos and the heavy sterile neutrinos are shown to be within the LEP experimental bound on all ranges of the Majorana phases.     

On the oscillations of neutrinos with Dirac and Majorana masses

Pontecorvo neutrino oscillations are discussed in the case of Dirac as well as Majorana neutrino mass terms. We prove that none of the possible experiments on neutrino oscillations including those on CP nonconservation, can distinguish between these two possibilities. Oscillations of neutrinos having both Dirac and Majorana mass terms also considered.     

Majorana mass term,Dirac neutrinos and selective neutrino oscillations

In a theory of neutrino mixing via a Majorana mass term involving only the left-handed neutrinos there exist selection rules for neutrino oscillations if true Dirac and/or exactly zero mass eigenstates are present. In the case of three neutrino flavours no oscillation is allowed if the mass spectrum contains one Dirac and one nondegenerate Majorana massive neutrino. The origin of these selection rules and their implications are discussed and the number of possible CP-violating phases in the lepton mixing matrix when Dirac and Majorana mass eigenstates coexist is given.     

Resonant leptogenesis

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 effects on scatterings and the dominant gauge-mediated collision terms are included. We show that resonant leptogenesis can be realized with heavy Majorana neutrinos even as light as ∼1 TeV, in complete accordance with the current solar and atmospheric neutrino data.     

Leptonic CP-violation and mirror leptons

We study leptonic CP-violation within a single fermion generation in an extended electro-weak model where new right-handed leptons, so-called mirror leptons, exist and mix with standard left-handed leptons. Essential differences are pointed out between the mixing of a left-handed neutrino with a mirror neutrino in such a model and the ordinary mixing of two left-handed neutrinos in the standard model. E.g. in the mirror case CP-violation may occur even if neutrinos are Dirac particles. The number of physical CP-violation phases is determined for different mixing schemes and parameterizations of the leptonic weak currents in terms of these phases are presented.     

Lepton Flavor Violating Z→lIlJ in SO(3) Gauge Extension of SM

The SO(3) gauge extension of SM, which is proposed to present a successfulexplanation for the observed small masses of neutrino and the nearly tri-bimaximal neutrino mixing, predicted the vector-like SO(3) triplet Majorana neutrinos and SU_L(2) double Higgs bosons. In this work we calculate branching ratios of the charged lepton flavor violating decays l_Il_JV (V=γ,Z) induced by these Majorana neutrinos and Higgs bosons. We find that under the model parameters constrained by experimental bounds on the decays Z→l_Il_J, the branching ratio of decays l_I→l_Jγ can be up to 10^-10, which may be accessible at the future experiments.     

On neutrino mass matrices in the three-family model

We study some general features of real neutrino mass matrices in the case of the three-family Electroweak Model with three right-handed neutrinos. The eigenvalue equation is expressed in terms of invariants of the submatrices (Dirac and Majorana) of the neutrino mass matrix and a geometrical interpretation of quantities which enter there is given.     

Leptogenesis with linear,inverse or double seesaw

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, with or without resonant effect, is allowed in the decays of the right-handed neutrinos.     

Distinguishable RGE Running Effects Between Dirac Neutrinos and Majorana Neutrinos with Vanishing Majorana CP-Violating Phases

In a novel parametrization of neutrino mixing and in the approximation of τ-lepton dominance, we show that the one-loop renormalization-group equations （RGEs） of Dirac neutrinos are different from those of Majorana neutrinos even if two Majorana CP-violating phases vanish. As the latter can keep vanishing from the electroweak scale to the typical seesaw scale, it makes sense to distinguish between the RGE running effects of neutrino mixing parameters in Dirac and Majorana cases. The differences are found to be quite large in the minimal supersymmetric standard model with sizable tan β, provided the masses of three neutrinos are nearly degenerate or have an inverted hierarchy.     

Lepton-number-violating decays of singly-charged Higgs bosons in the type-(Ⅰ+Ⅱ) seesaw model

The lepton-number-violating decays of singly-charged Higgs bosons H± are investigated in the minimal type-(Ⅰ+Ⅱ) seesaw model with one SU(2)L Higgs triplet Δ and one heavy Majorana neutrino N1 at the TeV scale.We find that the branching ratios B(H+ → lα+ ν)(for α = e,μ,τ) depend not only on the mass and mixing parameters of three light neutrinos νi(for i=1,2,3) but also on those of N1.Assuming that the mass of N1 lies in the range of 200 GeV to 1 TeV,we figure out the generous interference bands for the contributions of νi and N1 to B(H+ →lα+ ν).We illustrate some salient features of such interference effects by considering three typical mass patterns of νi,and show that the relevant Majorana CP-violating phases can affect the magnitudes of B(H+ →l+αν) in this parameter region.     

Dirac vs. Majorana neutrino masses from a TeV interval

We investigate the nature (Dirac vs. Majorana) and size of left-handed neutrino masses in a supersymmetric five-dimensional model compactified in the interval [0,πR], where quarks and leptons are localized on the boundaries while the gauge and Higgs sectors propagate in the bulk of the fifth dimension. Supersymmetry is broken by Scherk–Schwarz boundary conditions and electroweak breaking proceeds through radiative corrections. Right-handed neutrinos propagate in the bulk and have a general five-dimensional mass M, which localizes the zero modes towards one of the boundaries, and arbitrary boundary terms. We have found that for generic boundary terms left-handed neutrinos have Majorana masses. However for specific boundary configurations left-handed neutrinos are Dirac fermions as the theory possesses a conserved global U(1) symmetry which prevents violation of lepton number. The size of neutrino masses depends on the localization of the zero-modes of right-handed neutrinos and/or the size of the five-dimensional neutrino Yukawa couplings. Left-handed neutrinos in the sub-eV range require either MR10 or Yukawa couplings 10⁻³R, which make the five-dimensional theory perturbative up to its natural cutoff.     

Search for heavy sterile neutrinos in trileptons at the LHC

We present a search strategy for both Dirac and Majorana sterile neutrinos from the purely leptonic decays of W~±→e~±e~±μ~?ν and μ~±μ~± e~?ν at the 14 TeV LHC. The discovery and exclusion limits for sterile neutrinos are shown using both the Cut-and-Count(CC) and Multi-Variate Analysis(MVA) methods. We also discriminate between Dirac and Majorana sterile neutrinos by exploiting a set of kinematic observables which differ between the Dirac and Majorana cases. We find that the MVA method, compared to the more common CC method, can greatly enhance the discovery and discrimination limits. Two benchmark points with sterile neutrino mass m N =20 GeV and 50 GeV are tested. For an integrated luminosity of 3000 fb~(-1), sterile neutrinos can be found with 5σ significance if heavy-to-light neutrino mixings |U_(Ne)|~2~|U_(Nμ)|~2~10~(-6), while Majorana vs. Dirac discrimination can be reached if at least one of the mixings is of order 10~(-5).     

Entropy, baryon asymmetry and dark matter from heavy neutrino decays

The origin of the hot phase of the early universe remains so far an unsolved puzzle. A viable option is entropy production through the decays of heavy Majorana neutrinos whose lifetimes determine the initial temperature. We show that baryogenesis and the production of dark matter are natural by-products of this mechanism. As is well known, the cosmological baryon asymmetry can be accounted for by leptogenesis for characteristic neutrino mass parameters. We find that thermal gravitino production then automatically yields the observed amount of dark matter, for the gravitino as the lightest superparticle and typical gluino masses. As an example, we consider the production of heavy Majorana neutrinos in the course of tachyonic preheating associated with spontaneous B−L breaking. A quantitative analysis leads to constraints on the superparticle masses in terms of neutrino masses: For a light neutrino mass of ¹⁰−5 eV the gravitino mass can be as small as 200 MeV, whereas a lower neutrino mass bound of 0.01 eV implies a lower bound of 9 GeV on the gravitino mass. The measurement of a light neutrino mass of 0.1 eV would rule out heavy neutrino decays as the origin of entropy, visible and dark matter.     

Bimaximal mixing in an SO(10) minimal Higgs model

An SO(10) SUSY GUT model was previously presented based on a minimal set of Higgs fields. The quark and lepton mass matrices derived fitted the data extremely well and led to large ν_μ−ν_τ mixing in agreement with the atmospheric neutrino data and to the small-angle MSW solution for the solar neutrinos. Here we show how a slight modification leading to a non-zero up quark mass can result in bimaximal mixing for the atmospheric and solar neutrinos. The “just-so” vacuum solution is slightly favored over the large-angle MSW solution on the basis of the hierarchy required for the right-handed Majorana matrix and the more nearly-maximal mixing angles obtained.     

Resurrection of grand unified theory baryogenesis

A "new" scenario is proposed for baryogenesis. We show that the delayed decay of colored Higgs particles in grand unified theories may generate an excess baryon number of the empirically desired amount, if the mass of the heaviest neutrino is in the range 0.02 eV相似文献   

Signals of CPT violation and non-locality in future neutrino oscillation experiments

We investigate the sensitivities of future neutrino oscillation experiments for measuring the neutrino mass squared differences and leptonic mixing angles independently with neutrinos and anti-neutrinos. We update the expected sensitivities of Neutrino Factories to the “atmospheric” (anti-)neutrino parameters using an optimized setup. A dedicated β-Beam facility, in combination with a SPMIN reactor experiment, could give excellent sensitivities also to the “solar” parameters, for neutrinos and anti-neutrinos respectively. A signal of a different mass matrix for neutrinos and anti-neutrinos would imply CPT violation and non-locality of the underlying particle theory.     

Heavy fourth-generation neutrino and flavour-changing neutral currents

We study the flavour-changing neutral currents in the case that the fourth-generation neutrino exists and the known three left-handed neutrino masses are at the experimental limits of the direct measurements. The fourth-generation neutrino has the mass of order a few ten GeV and the flavour-changing processes of the heavy neutrinos are expected to be observed onZ ⁰ ine ⁺ e ^? collisions. The heavy fourth-generation neutrino is significant to reveal the nature of the neutrino; Dirac or Majorana, the see-saw mechanism and the right-handed scale.     

Theory of neutrinoless double beta decay—A brief review

Neutrinoless double decay (0νββ-decay) is a unique probe for lepton number conservation and neutrino properties. This is a process with long and interesting history with important implications for particle physics and cosmology, but its observation is still elusive. The search for the 0νββ-decay represents the new frontiers of neutrino physics, allowing to determine the Majorana nature of neutrinos and to fix the neutrino mass scale and possible CP violation effects, which could explain the matter-antimatter asymmetry in the Universe. At present a complete theory is missing and, thus, to motivate and guide the experiments the mechanism mediated by light neutrinos is mostly considered. The subject of interest is an effective mass of Majorana neutrinos, which can be deduced from the measured half-life, once this process is definitely observed. The accuracy of the determination of this quantity is mainly determined by our knowledge of the nuclear matrix elements. There is a request to evaluate them with high precision, accuracy and reliability. Recently, there is an increased interest to the resonant neutrinoless double electron capture, which may also establish the Majorana nature of neutrinos. This possibility is considered as alternative and complementary to searches for the 0νββ-decay.