Detection of heavy Majorana neutrinos and right-handed bosons

The SU _c (3) ⊗ SU _L (2) ⊗ SU _R (2) ⊗ U(1) left-right (LR) symmetric model explains the origin of the parity violation in weak interactions and predicts the existence of additional gauge bosons W _R and Z′. In addition, heavy right-handed Majorana neutrino states N arise naturally within the LR symmetric model. The states N could be partners of light neutrino states, related to their nonzero masses through the seesaw mechanism. This makes the searches for W _R , Z′, and N interesting and important. In the framework of the minimal LR model, we study the possibility to observe signals from N and W _R production in pp collisions after three years of running at low LHC luminosity. We show that their decay signals can be identified with a small background, especially in the case of same-sign leptons in the final state. For the integral LHC luminosity of L _t = 30 fb⁻¹, the 5σ discovery of W _R boson and heavy Majorana neutrinos N _e with masses up to 4 TeV and up to 2.4 TeV, respectively, is found to be possible. The text was submitted by the authors in English.     

Constraints on the masses and couplings of heavy Majorana right-handed neutrinos

The masses and couplings of heavy unstable right-handed Majorana neutrinos can be constrained using existing and expected future results from both accelerator and astrophysics experiments. In particular we examine limits on rare decay modes of particles containing s, c, and b quarks as well as the τ lepton and interpret these in terms of a hypothetical massive neutrino. In addition, cosmological limits result from a consideration of the nucleosynthesis epoch in the early universe.     

On the Majorana equations and the rest mass of neutrinos

As is well known, the law of parity conservation does not hold in weak interactions. This type of asymmetry created a number of theoretical problems which were solved, first of all, by a new understanding of the features of neutrinos and their role in weak interactions. These solutions were based, however, essentially on the handedness (chirality) of neutrinos which is closely related to their vanishing rest mass. The thesis of neutrinos with nonvanishing rest mass, newly considered in the literature, therefore requires a rediscussion of the early arguments concerning the relation between the neutrino theory and some weak interaction essentials. When one does this, as in the present paper, it is noted that neutrinos with rest mass lead to some difficulties, in particular to a violation of T invariance.     

Probing new physics scale with high-energy neutrinos

Motivated by the new physics beyond the Standard Model through the scale at which neutrino mass has origin, we investigate its possible manifestation at low energy within the corresponding propagation of the energetic neutrino. Then, we consider the SN1987A and base on the recorded neutrino data to explore the scale range of the new physics.     

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.     

Dirac and Majorana neutrinos in matter

We consider the matter effects on neutrinos moving in background on the basis of the corresponding quantum wave equations. Both Dirac and Majorana neutrino cases are discussed. The effects for Dirac neutrino reflection and trapping as well as neutrino—antineutrino annihilation and ν pair creation in matter at the interface between two media with different densities are considered. The spin light of neutrinos in matter is also discussed. The text was submitted by the authors in English.     

Majorana neutrinos in rare meson decays

The rare meson decays K ⁺ → π ^??⁺?′⁺ and D ⁺ → K ^??⁺?′⁺ (?, ?′ = e, μ), which are induced by Majorana neutrino exchange and which are accompanied by lepton-number nonconservation, are considered. The effects of the meson structure are taken into account on the basis of the Gaussian model for the respective Bethe-Salpeter amplitudes. It is shown that existing direct experimental constraints on the decay branching ratios are overly lenient and therefore give no way to set realistic limits on effective Majorana masses. On the basis of the constraints on the lepton-mixing parameters and neutrino masses from precision measurements of electroweak processes, neutrino-oscillation experiments, searches for neutrinoless double-beta decay of nuclei, and cosmological data, indirect constraints on the branching ratios for the decays in question are obtained and found to be much more stringent than the above direct constraints.     

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.     

Magnetic moments of Dirac and Majorana neutrinos

We present two models where the magnetic moment of the electron neutrino can be as high as the experimental bound of 10⁻¹⁰ Bohr magnetons. The models, one in which the neutrinos are Dirac particles and the other where they are Majorana particles possessing transition moments, are consistent with known experiments and are realizations of a recent suggestion by Voloshin, Vysotsky and Okun to explain the solar neutrino experiment.     

The effects of Majorana phases in three-generation neutrinos

Neutrino-oscillation solutions for the atmospheric neutrino anomaly and the solar neutrino deficit can determine the texture of the neutrino mass matrix according to three types of neutrino mass hierarchy: Type A: , Type B: , and Type C: , where is the absolute mass of the ith generation neutrino. The relative sign assignments of the neutrino masses in each type of mass hierarchy play crucial roles in the stability against quantum corrections. Actually, two physical Majorana phases in the lepton flavor mixing matrix connect the relative sign assignments of the neutrino masses. Therefore, in this paper we analyze the stability of the mixing angles against quantum corrections according to three types of neutrino mass hierarchy (Type A, B, C) and two Majorana phases. The two phases play crucial roles in the stability of the mixing angles against quantum corrections. Received: 9 May 2000 / Revised version: 23 May 2000 / Published online: 8 September 2000     

Signatures for right-handed neutrinos at the large hadron collider

We explore possible signatures for right-handed neutrinos in a TeV scale B-L extension of the standard model at the Large Hadron Collider. The studied four lepton signal has a tiny standard model background. We find the signal experimentally accessible at the LHC for the considered parameter regions.     

Majorana neutrinos from inverse seesaw in warped extra dimension

We propose the inverse seesaw mechanism as a way to understand small Majorana masses for neutrinos in warped extra dimension models with seesaw scale in the TeV range. The ultra-small lepton number violation needed in implementing inverse seesaw mechanism in 4D models is explained in this model as a consequence of lepton number breaking occurring on the Planck brane. We construct realistic models based on this idea that fit observed neutrino oscillation data for both normal and inverted mass patterns. We compute the corrections to light neutrino masses from the Kaluza-Klein modes and show that they are small in the parameter range of interest. Another feature of the model is that the absence of global parity anomaly implies the existence of at least one light sterile neutrino with sterile and active neutrino mixing in the range suggested by the LSND and MiniBooNE observations.     

Distinguishing between Dirac and Majorana neutrinos in neutral-current reactions

We calculate cross sections for neutral-current reactions initiated by massive Dirac and Majorana neutrinos and analyze such reactions as a possible new method of distinguishing between these types of neutrinos.     

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.