Neutrinos,axions and conformal symmetry

We demonstrate that radiative breaking of conformal symmetry (and simultaneously electroweak symmetry) in the standard model with right-chiral neutrinos and a minimally enlarged scalar sector induces spontaneous breaking of lepton number symmetry, which naturally gives rise to an axion-like particle with some unusual features. The couplings of this ‘axion’ to standard model particles, in particular photons and gluons, are entirely determined (and computable) via the conformal anomaly, and their smallness turns out to be directly related to the smallness of the masses of the light neutrinos.     

Limits on Treh for thermal leptogenesis with hierarchical neutrino masses

We make a simple observation that if one of the right-chiral neutrinos is very heavy or its Yukawa couplings to the standard lepton doublets are negligible, so that it effectively decouples from the see-saw mechanism, the prediction for the baryon asymmetry of the universe resulting from leptogenesis depends only on the masses M₁ and M₂ of the remaining two right-chiral neutrinos and the element of the neutrino Yukawa coupling. For M₂5M₁ the lower bound on M₁ and also on T_reh, resulting from the requirement of ‘successful leptogenesis’ is then significantly increased compared to the one computed recently by Buchmüller et al. in the most general case. Within the framework of thermal leptogenesis, the only way to lower this limit is then to allow for sufficiently small mass difference M₂−M₁.     

Vacuum solutions of neutrino anomalies through a softly broken U(1) symmetry

We discuss an extended model which naturally leads to mass scales and mixing angles relevant for understanding both the solar and atmospheric neutrino anomalies in terms of the vacuum oscillations of the three known neutrinos. The model uses a softly broken –– symmetry and contains a heavy scale GeV. The –– symmetric neutrino masses solve the atmospheric neutrino anomaly while breaking of –– generates the highly suppressed radiative mass scale needed for the vacuum solution of the solar neutrino problem. All the neutrino masses in the model are inversely related to , thus providing seesaw-type of masses without invoking any heavy right-handed neutrinos. The possible embedding of the model into an SU(5) grand unified theory is discussed. Received: 5 August 1999 / Revised version: 18 November 1999 / Published online: 6 April 2000     

Neutrino masses in the supersymmetric SU(3)C⊗SU(3)L⊗U(1)X model with right-handed neutrinos

The R-symmetry formalism is applied for the supersymmetric SU(3)_C⊗SU(3)_L⊗U(1)_X (3-3-1) model with right-handed neutrinos. For this kind of models, we study the generalization of the MSSM relation among R-parity, spin and matter parity. Discrete symmetries for the proton stable in this model are imposed, and we show that in such a case it is able to give leptons masses at only the tree level contributions required. A simple mechanism for the mass generation of the neutrinos is explored. We show that at the low-energy effective theory, the neutrino spectrum contains three Dirac fermions, one massless and two degenerate in mass. At the energy level where the mixing among them with the neutralinos is turned on, neutrinos obtain Majorana masses and correct the low-energy effective result which naturally gives rise to an inverted hierarchy mass pattern. This mass spectrum can fit the current data with minor fine-tuning. Consistent values for masses of the charged leptons are also given. In this model, the MSSM neutralinos and charginos can be explicitly identified in terms of the new constraints on masses which is not as in a supersymmetric version of the minimal 3-3-1 model. PACS 11.30.Er; 14.60.Pq; 14.60.-z; 12.60.Jv     

Conformal symmetry and the Standard Model

We re-examine the question of radiative symmetry breaking in the Standard Model in the presence of right-chiral neutrinos and a minimally enlarged scalar sector. We demonstrate that, with these extra ingredients, the hypothesis of classically unbroken conformal symmetry, besides naturally introducing and stabilizing a hierarchy, is compatible with all available data; in particular, there exists a set of parameters for which the model may remain viable even up to the Planck scale. The decay modes of the extra scalar field provide a unique signature of this model which can be tested at LHC.     

Remarks to the standard scheme (theory) of neutrino oscillations. Corrected scheme of neutrino oscillations

In the standard theory of neutrino oscillations, it is supposed that physically observed neutrino states ν _e , ν_μ, ν_τ, have no definite masses, that they are initially produced as a mixture of the ν₁, ν₂, ν₃ neutrino states (i.e., they are produced as a wave packet), and that neutrino oscillations are the real ones. Then, this wave packet must decompose at a definite distance into constituent parts and neutrino oscillations must disappear. It was shown that these suppositions lead to violation of the law of energy and momentum conservation. An alternative scheme of neutrino oscillations obtained within the framework of particle physics has been considered, where the above mentioned shortcomings are absent, the oscillations of neutrinos with equal masses are the real ones, and the oscillations of neutrinos with different masses are the virtual ones. Expressions for probabilities of neutrino transitions (oscillations) in the alternative (corrected) scheme are given. The text was submitted by the author in English.     

Lepton mixing matrix in standard model extended by one sterile neutrino

We consider the simplest extension of the standard electroweak model by one sterile neutrino that allows for neutrino masses and mixing. We find that its leptonic sector contains much less free physical parameters than previously realized. In addition to the two neutrino masses, the lepton mixing matrix in charged current interactions involves (n-1) free physical mixing angles for n generations. The mixing matrix in neutral current interactions of neutrinos is completely fixed by the two masses. Both interactions conserve CP. We illustrate the phenomenological implications of the model by vacuum neutrino oscillations, tritium β decay and neutrinoless double β decay. It turns out that, due to the revealed specific structure in its mixing matrix, the model with any n generations cannot accommodate simultaneously the data by KamLAND, K2K and CHOOZ. PACS 14.60.Pq; 14.60.St; 23.40.-s     

Neutrino masses and flavor mixing

We study a model for the mass matrices of the leptons, based on texture zero elements. We are able to relate the mass eigenvalues of the charged leptons and of the neutrinos to the mixing angles, and can predict the masses of the neutrinos. We find a normal hierarchy—the masses are 0.005 eV, 0.01 eV and 0.05 eV. Predictions for the double beta decay and the reactor neutrino experiments are made.     

Majorana neutrino masses in the three-flavor Pauli model

A special Majorana model for three neutrino flavors is developed on the basis of the Pauli transformation group. In this model, the neutrinos possess a partially conserved generalized lepton (Pauli) charge that makes it possible to discriminate between neutrinos of different type. It is shown that, within the model in question, a transition from the basic “mass” representation, where the average value of this charge is zero, to the representation associated with physical neutrinos characterized by specific Pauli “flavor” charges establishes a relation between the neutrino mixing angles θ _{mix, 12}, θ _{mix, 23}, and θ _{mix, 13} and an additional relation between the Majorana neutrino masses. The Lagrangian mass part, which includes a term invariant under Pauli transformations and a representation-dependent term, concurrently assumes a “quasi-Dirac” form. With allowance for these relations, the existing set of experimental data on the features of neutrino oscillations makes it possible to obtain quantitative estimates for the absolute values of the neutrino masses and the 2β-decay mass parameter m _ββ and a number of additional constraints on the neutrino mixing angles.     

Neutrino mixing and masses in a left–right model with mirror fermions

In the framework of a left–right model containing mirror fermions with gauge group SU(3) _C ⊗SU(2) _L ⊗SU(2) _R ⊗U(1) _Y′, we estimate the neutrino masses, which are found to be consistent with their experimental bounds and hierarchy. We evaluate the decay rates of the Lepton Flavor Violation (LFV) processes μ→eγ, τ→μγ and τ→eγ. We obtain upper limits for the flavor-changing branching ratios in agreement with their present experimental bounds. We also estimate the decay rates of heavy Majorana neutrinos in the channels N→W ^± l ^∓, N→Zν _l and N→Hν _l , which are roughly equal for large values of the heavy neutrino mass. Starting from the most general Majorana neutrino mass matrix, the smallness of active neutrino masses turns out from the interplay of the hierarchy of the involved scales and the double application of seesaw mechanism. An appropriate parameterization on the structure of the neutrino mass matrix imposing a symmetric mixing of electron neutrino with muon and tau neutrinos leads to tri-bimaximal mixing matrix for light neutrinos.     

Effective potential for highly relativistic neutrinos in a weakly magnetized medium and their oscillation

We have calculated the effective potential experienced by highly relativistic neutrinos in a weakly magnetized electron–positron plasma, where a momentum-dependent finite-width correction to the propagator of W is considered to account for the threshold effect. Magnetars are believed to be sources of TeV–PeV neutrinos which are produced due to photomeson and proton–proton interactions in their atmosphere. We have studied the resonant-oscillation process ν _e ↔ ν _μ,τ of the highly relativistic neutrinos in the atmosphere of SGR 1806-20, which is a magnetar. It is shown that, for high-energy neutrinos propagating within the magnetar atmosphere, the resonance condition can never be satisfied. On the other hand, if GeV neutrinos are produced deep inside the magnetar atmosphere, where the temperature is about 50 keV or more, then these neutrinos can undergo resonant oscillation.     

Neutrinos: precursors of new physics

Since neutrinos are the only elementary particles that interact only weakly, the study of their properties, albeit experimentally difficult, reflects the true nature of the Weak Interactions. We begin with a historical review, emphasizing the central role of neutrinos in the formulation of the Standard Model. We review the generalizations of the Standard Model needed to accommodate both Dirac and Majorana neutrino masses. The recent experimental findings which demonstrate that neutrinos have tiny masses are discussed. We argue that small neutrino masses as well as the unexpected mixing patterns between the three neutrino flavors give us a glimpse, through the Seesaw mechanism, of physics at or near the Planck scale. To cite this article: P. Ramond, C. R. Physique 6 (2005).     

Heavy sterile neutrinos and supernova explosions

We consider sterile neutrinos with rest masses 0.2 GeV and with vacuum flavor mixing angles θ²>10⁻⁸ for mixing with τ-neutrinos, or 10⁻⁸<θ²<10⁻⁷ for mixing with muon neutrinos. Such sterile neutrinos could augment core collapse supernova shock energies by enhancing energy transport from the core to the vicinity of the shock front. The decay of these neutrinos could produce a flux of very energetic active neutrinos, detectable by future neutrino observations from galactic supernova. The relevant range of sterile neutrino masses and mixing angles can be probed in future laboratory experiments.     

New answer to the solar neutrino problem

We suggest a new answer to the problem of the solar neutrinos: a neutrino-photon interaction that would cause the neutrinos to disappear before they leave the sun or make them lose energy towards detection thresholds. We calculate the available energy in the system of the centre of mass, and show that the photons may be endowed with a pseudo-cross-section in the system of the sun. Under the assumption of an absorption, made to simplify the neutrino transport calculation, the chlorine experiment yields:σ _a =1.8( _−1.0 ^+0.7 )*10⁻⁹ barn, which is close tog _β/(ℏc)=4·49^*10⁻⁹ barn. The escape probability is substantially larger for the gallium neutrinos than for the chlorine neutrinos. Thermal radiation in the core of a supernova is suppressed by electrical conductivity, therefore the neutrinos from SN1987A could escape; they interacted with the photon piston in the outer layers of the supernova and the interaction has to be a scattering. The cosmological implications of a neutrino-photon interaction are discussed; Hubble’s constant may have to be modified. The case of an elastic scattering between neutrino and photon is discussed in more detail. An erratum to this article is available at .     

Nuclear responses for neutrinos and neutrino studies by double beta decays and inverse beta decays

This is a brief report on recent studies of nuclear responses for neutrinos (v) by charge exchange reactions, v masses by double beta (ββ) decays and of solar and supernova v’s by inverse β decays. Subjects discussed include (1) v studies in nuclear micro-laboratories, (2) v masses studied by ββ decays of ¹⁰⁰Mo and nuclear responses for ββ — v, (3) solar and supernova v’s by inverse β decays and v responses for ⁷¹Ga and ¹⁰⁰Mo, and (4) MOON (molybdenum observatory of neutrinos) for spectroscopic studies of Majorana v masses with sensitivity of m _v ∼ 0.03 eV by ββ decays of ¹⁰⁰Mo and real-time studies of low energy solar and supernova v’s by inverse β decays of ¹⁰⁰Mo.     

Bi-large neutrino mixings by radiative magnification

Starting with the unification hypothesis of mixings of quarks and leptons and small quark-like mixings at the see-saw scale, we find that two large mixings for ν_e —νx03BC; andv _μ—v _τ at the weak scale are obtained as a result of renormalization group evolution and radiative magnification if the three neutrinos are quasi degenerate in masses and possess the same CP parity. We also find thatU _e3 remains small and well within the CHOOZ-Palo Verde bound since the correspondingV _ub for CKM mixing is very small. Several testable pedictions are pointed out.     

Neutrino masses in the left–right supersymmetric model

We show that in a left–right supersymmetric model with a Higgs structure that supports the see-saw mechanism, the neutrinos get additional contributions to their masses at one loop level. The mechanism responsible is analogous to the Grossman–Haber see-saw mechanism, but the additional mass terms are proportional to the mass difference of the right-handed sneutrinos. We show that the data on both the solar and the atmospheric neutrinos can be accommodated by either two almost degenerate right-handed sneutrinos, or two heavy sneutrino with different, but still relatively small, mass splittings. We discuss the implications of this result for the masses and mixings of the heavy sneutrinos, and the soft-breaking parameters of the left–right supersymmetric model.     

Leptonic flavor-changing Z decays in SU(2)U(1) theories with right-handed neutrinos

We analyze possible lepton-flavor-violating decays of the Z⁰ particle in a minimal extension of the standard model, in which one right-handed neutral field for each family has been introduced. Such rare leptonic decays are induced by Majorana neutrinos at the first electroweak loop level and are generally not suppressed by the ordinary “see-saw” mechanism. In particular, we find that experimental bounds on branching ratios of the order of 10⁻⁵–10⁻⁶ attainable at LEP may impose constraints on lepton-flavor-mixing parameters and the masses of the heavy Majorana neutrinos.     

Supersymmetric model with an extra U(1) gauge symmetry forbidding proton decay

In the standard model the proton is protected from decay naturally by gauge symmetries, whereas in the ordinary minimal supersymmetric standard model an ad hoc discrete symmetry is imposed for the proton stability. We present a new supersymmetric model in which the proton decay is forbidden by an extra U(1) gauge symmetry. Particle contents are necessarily increased to be free from anomalies, incorporating right-handed neutrinos. Both Dirac and Majorana masses are generated for neutrinos, yielding nonvanishing but small masses. The superpotential consists only of trilinear couplings and the mass parameter &mgr; of the minimal model is induced by spontaneous breaking of the U(1) symmetry.     

Parametrization of seesaw models and light sterile neutrinos

The recent recomputation of the neutrino fluxes from nuclear reactors relaxes the tension between the LSND and MiniBooNE anomalies and disappearance data when interpreted in terms of sterile neutrino oscillations. The simplest extension of the Standard Model with such fermion singlets is the addition of right-handed sterile neutrinos with small Majorana masses. Even when introducing three right-handed neutrinos, this scenario has less free parameters than the 3 + 2 scenarios studied in the literature. This begs the question whether the best fit regions obtained can be reproduced by this simplest extension of the Standard Model. In order to address this question, we devise an exact parametrization of Standard Model extensions with right-handed neutrinos. Apart from the usual 3×3 neutrino mixing matrix and the 3 masses of the lightest neutrinos, the extra degrees of freedom are encoded in another 3×3 unitary matrix and 3 additional mixing angles. The parametrization includes all the correlations among masses and mixings and is valid beyond the usual seesaw approximation. Through this parametrization we find that the best fit regions for the LSND and MiniBooNE anomalies in a 3 + 2 scenario can indeed be reproduced despite the smaller number of degrees of freedom.