Duality in left-right symmetric seesaw mechanism

We consider type I + II seesaw mechanism, where the exchanges of both right-handed neutrinos and isotriplet Higgs bosons contribute to the neutrino mass. Working in the left-right symmetric framework and assuming the mass matrix of light neutrinos m(v) and the Dirac-type Yukawa couplings to be known, we find the triplet Yukawa coupling matrix f, which carries the information about the masses and mixing of the right-handed neutrinos. We show that in this case there exists a duality: for any solution f, there is a dual solution [symbol: see text] = m(v)/nu(L) - f, where nu(L) is the vacuum expectation value of the triplet Higgs boson. Thus, unlike in pure type I (II) seesaw, there is no unique allowed structure for the matrix f. For n lepton generations the number of solutions is 2(n). We develop an exact analytic method of solving the seesaw nonlinear matrix equation for f.     

Decay and decoupling of heavy right-handed Majorana neutrinos in the L–R model

Heavy right-handed neutrinos are of current interest. The interactions and decay of such neutrinos determine their decoupling epoch during the evolution of the universe. This in turn affects various observable features like the energy density, nucleosynthesis, CMBR spectrum, galaxy formation and baryogenesis. Here, we consider reduction of right-handed electron-type Majorana neutrinos, in the left–right symmetric model, by the channel and the channel originating from an anomaly, involving the gauge group, as well as decay of such neutrinos. We study the reduction of these neutrinos for different ranges of left–right model parameters, and find that, if the neutrino mass exceeds the right-handed gauge boson mass, then the neutrinos never decouple for realistic values of the parameters, but, rather, decay in equilibrium. Because there is no out-of-equilibrium decay, no mass bounds can be set for the neutrinos. Received: 1 November 2000 / Published online: 23 February 2001     

Leptogenesis: The other cuts

For standard leptogenesis from the decay of singlet right-handed neutrinos, we derive source terms for the lepton asymmetry that are present in a finite density background but absent in the vacuum. These arise from cuts through the vertex correction to the decay asymmetry, where in the loop either the Higgs boson and the right-handed neutrino or the left-handed lepton and the right-handed neutrino are simultaneously on-shell. We evaluate the source terms numerically and use them to calculate the lepton asymmetry for illustrative points in parameter space, where we consider only two right-handed neutrinos for simplicity. Compared to calculations where only the standard cut through the propagators of left-handed lepton and Higgs boson is included, sizable corrections arise when the masses of the right-handed neutrinos are of the same order, but the new sources are found to be most relevant when the decaying right-handed neutrino is heavier than the one in the loop. In that situation, they can yield the dominant contribution to the lepton asymmetry.     

Theoretical and experimental update on a model featuring a second Z-boson

An update is provided on theoretical and experimental aspects of a simple extension of the standard model featuring right-handed neutrinos and a second neutral gauge boson (Z′). We identify an ambiguity which a priori exists in the definition of electric charge in this model, and show spontaneous symmetry breaking can resolve it. We then re-analyse the experimental bounds on the mass of Z′ in the light of recent measurements of the standard Z boson mass and width at LEP and SLC. We find that the lower bound at 90% confidence level for the Z′ mass is 460 GeV.     

Angular distribution of electrons in neutrinoless double-beta decay and new physics

For neutrinoless double-beta decay caused by the exchange of light Majorana neutrinos, an expression for the differential width with respect to the angle between the final-electron momenta is obtained on the basis of a Lorentz-invariant effective Lagrangian of the general form. The shape of this angular distribution is analyzed within various extensions of the Standard Model that allow this process—in particular, within theories that involve Majorana super partners and (or) right-handed currents. The angular correlation coefficient for electrons as a function of the mass of the right-handed W boson and the effective Majorana neutrino mass in the decay of the ⁷⁶Ge nucleus is considered within the model involving left—right symmetry.     

Connecting dark energy to neutrinos with an observable Higgs triplet

To connect the scalar field (acceleron) responsible for dark energy to neutrinos, the usual strategy is to add unnaturally light neutral singlet fermions (right-handed neutrinos) to the Standard Model. A better choice is actually a Higgs triplet, through the coupling of the acceleron to the trilinear Higgs triplet–double–doublet interaction. This hypothesis predicts an easily observable doubly-charged Higgs boson at the forthcoming Large Hadron Collider (LHC).     

Testing the mechanism for the LSP stability at the LHC

The lightest supersymmetric particle (LSP) is a natural candidate for the cold dark matter of the universe. In this Letter we discuss how to test the mechanism responsible for the LSP stability at the LHC. We note that if R-parity is conserved dynamically one should expect a Higgs boson which decays mainly into two right-handed neutrinos (a “leptonic” Higgs) or into two sfermions. The first case could exhibit spectacular lepton number violating signals with four secondary vertices due to the long-lived nature of right-handed neutrinos. These signals, together with the standard channels for the discovery of SUSY, could help to establish the underlying theory at the TeV scale.     

Electroweak vacuum stability in classically conformal $$$$ extension of the standard model

We consider the minimal U(1)\(_{B-L}\) extension of the standard model (SM) with the classically conformal invariance, where an anomaly-free U(1)\(_{B-L}\) gauge symmetry is introduced along with three generations of right-handed neutrinos and a U(1)\(_{B-L}\) Higgs field. Because of the classically conformal symmetry, all dimensional parameters are forbidden. The \(B-L\) gauge symmetry is radiatively broken through the Coleman–Weinberg mechanism, generating the mass for the \(U(1)_{B-L}\) gauge boson (\(Z^\prime \) boson) and the right-handed neutrinos. Through a small negative coupling between the SM Higgs doublet and the \(B-L\) Higgs field, the negative mass term for the SM Higgs doublet is generated and the electroweak symmetry is broken. In this model context, we investigate the electroweak vacuum instability problem in the SM. It is well known that in the classically conformal U(1)\(_{B-L}\) extension of the SM, the electroweak vacuum remains unstable in the renormalization group analysis at the one-loop level. In this paper, we extend the analysis to the two-loop level, and perform parameter scans. We identify a parameter region which not only solve the vacuum instability problem, but also satisfy the recent ATLAS and CMS bounds from search for \(Z^\prime \) boson resonance at the LHC Run-2. Considering self-energy corrections to the SM Higgs doublet through the right-handed neutrinos and the \(Z^\prime \) boson, we derive the naturalness bound on the model parameters to realize the electroweak scale without fine-tunings.     

Magnetic dipole moment and keV neutrino dark matter

We study magnetic dipole moments of right-handed neutrinos in a keV neutrino dark matter model. This model is a simple extension of the standard model with only right-handed neutrinos and a pair of charged particles added. One of the right-handed neutrinos is the candidate of dark matter with a keV mass. Some bounds on the dark matter magnetic dipole moment and model parameters are obtained from cosmological observations.     

Baryogenesis from mixing of lepton doublets

It is shown that the mixing of lepton doublets of the Standard Model can yield sizable contributions to the lepton asymmetry, that is generated through the decays of right-handed neutrinos at finite temperature in the early Universe. When calculating the flavour-mixing correlations, we account for the effects of Yukawa as well as of gauge interactions. We compare the freeze-out asymmetry from lepton-doublet mixing to the standard contributions from the mixing and direct decays of right-handed neutrinos. The asymmetry from lepton mixing is considerably large when the mass ratio between the right-handed neutrinos is of order of a few, while it becomes Maxwell-suppressed for larger hierarchies. For an intermediate range between the case of degenerate right-handed neutrinos (resonant leptogenesis) and the hierarchical case, lepton mixing can yield the main contribution to the lepton asymmetry.     

Baryogenesis and lepton number violation

The cosmological baryon asymmetry can be explained by the nonperturbative electroweak reprocessing of a lepton asymmetry generated in the out-of-equilibrium decay of heavy right-handed Majorana neutrinos. We analyze this mechanism in detail in the framework of a SO(10)-subgroup. We take three right-handed neutrinos into account and discuss physical neutrino mass matrices.     

Single <Emphasis Type="Italic">Z</Emphasis>′ production at compact linear collider based on <Emphasis Type="Italic">e</Emphasis>-γ collisions

We analyze the potential of the compact linear collider (CLIC) based on e-γ collisions to search for the new Z′ gauge boson. Single Z′ production on e-γ colliders in two SU(3)_C?SU(3)_L ? U(1)_N models, the minimal model and the model with right-handed neutrinos is studied in detail. The results show that new Z′ gauge bosons can be observed on the CLIC and that the cross sections in the model with right-handed neutrinos are bigger than those in the minimal one.     

Neutrino masses as a second-order effect in Higgs coupling

It is suggested that in the usual type of gauge theory all fermions, including neutrinos, have right-handed components. The smallness or vanishing of the observed neutrino masses is explained by the fact that the appropriate neutral Higgs boson does not develop a non-zero vacuum expectation value. In the case when the neutrino masses do not vanish they are finite, of order G_Fm³, where m is the mass of the charged lepton. Non-conservation of lepton flavor gives rise to an instability of all neutrinos except v_e and to μ→e+γ decay, but at a very low level.     

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.     

Baryogenesis via leptogenesis in multi-field inflation

In multi-field reheating after modular j-inflation we investigate the conditions under which baryogenesis via non-thermal leptogenesis can be successfully realized. We introduce three heavy right-handed neutrinos to the non-supersymmetric Standard Model of particle physics, assuming hierarchical neutrino masses. Considering a typical mass for the first right-handed neutrino of the order of \(10^{11}\) GeV, suggested from the seesaw mechanism and also from concrete SO(10) grand unification models, we obtain the allowed parameter space for viable baryogenesis. An upper bound for the inflaton mass as well as a lower bound for its branching ratio to the pair of lightest right-handed neutrinos are found and reported.     

Massive and massless neutrinos on unbalanced seesaws

The observation of neutrino oscillations requires new physics beyond the standard model (SM).A SM-like gauge theory with p lepton families can be extended by introducing q heavy right-handed Majorana neutrinos but preserving its SU(2)L x U(1)y gauge symmetry.The overall neutrino mass matrix M turns out to be a symmetric (p+q) x (p+q) matrix.Given p＞q,the rank of M is in general equal to 2q,corresponding to 2q non-zero mass eigenvalues.The existence of (p-q) massless left-handed Majorana neutrinos is an exact consequence of the model,independent of the usual approximation made in deriving the Type-I seesaw relation between the effective p x p light Majorana neutrino mass matrix M,and the q x q heavy Majorana neutrino mass matrix MR.In other words,the numbers of massive left- and right-handed neutrinos are fairly matched.A good example to illustrate this "seesaw fair play rule"is the minimal seesaw model with p ＝ 3 and q ＝ 2,in which one masslese neutrino sits on the unbalanced seesaw.     

keV right-handed neutrinos from type II seesaw mechanism in a 3-3-1 model

We adapt the type II seesaw mechanism to the framework of the 3-3-1 model with right-handed neutrinos. We emphasize that the mechanism is capable of generating small masses for the left-handed and right-handed neutrinos and the structure of the model allows that both masses arise from the same Yukawa coupling. For typical values of the free parameters of the model we may obtain at least one right-handed neutrino with mass in the keV range. Right-handed neutrino with mass in this range is a viable candidate for the warm component of the dark matter existent in the universe.     

Dirac Neutrino Masses in NCG

Several models in nonocmmutative geometry(NCG) with mild changes to the standard model are introduced to discuss the neutrino mass problem.we use two constraints,Poincare duality and gauge anomaly free,to discuss the possibility of containing right-handed neutrinos in them.Our work shows that no model in this paper,with each generation containing a right-handed neutrino,can satisfy these two constraints at the same time.So,to consist with neutrino oscillation experiment results,maybe fundamental changes to the present version of NCG are usually needed to include Dirac massive neutrinos.     

Neutrinoless universe

We consider the consequences for the relic neutrino abundance if extra neutrino interactions are allowed, e.g., the coupling of neutrinos to a light (compared to m(nu)) boson. For a wide range of couplings not excluded by other considerations, the relic neutrinos would annihilate to bosons at late times and thus make a negligible contribution to the matter density today. This mechanism evades the neutrino mass limits arising from large scale structure.     

Some new symmetric relations and prediction of left-and right-handed neutrino masses using Koide's relation

The masses of the three generations of charged leptons are known to completely satisfy Koide's mass relation,but the question remains of whether such a relation exists for neutrinos.In this paper,by considering the seesaw mechanism as the mechanism generating tiny neutrino masses,we show how neutrinos satisfy Koide's mass relation,on the basis of which we systematically give exact values of both left-and right-handed neutrino masses.