Neutrino radiative decay in N=1 supergravity theories

We show that the supersymmetrization of models where neutrinos acquire mass through the see-saw mechanism allows for tau neutrinos heavier than 15–20 MeV to decay radiatively in a cosmologically safe way (i.e. with a lifetime τ_v⩽5×10³s).     

Tau neutrino mass from semileptonic decays

The semileptonic three particle decays of the tau provide determinations of the tau neutrino mass. The shift of the maximal energy of the observable final state particles islinear in the neutrino mass. The endpoint energy of the pion and a partially integrated decay rate in τ → πωv _τ and τ → πρV _τ are sensitive to a neutrino mass smaller than 100 MeV. Thus, the present bound on \(m_{v_\tau } \) can significantly be improved.     

Mass of tau neutrino in SO(10) GUTs

We investigate the allowed ranges of masses for an unstable tau neutrino in the context of SO(10) GUTs. In light of the new nucleosynthesis results we obtain that there is a narrow window for m_ντ where the LEP, neutrino oscillation and nucleosynthesis data are compatible. This window, which depends on the effective number of neutrinos contributing to nucleosynthesis, has important cosmological consequences and will be tested by ongoing neutrino oscillation and LEP II experiments.     

Supernova explosions and constraints on mass and lifetime of heavy neutrinos

Constraints on mass and lifetime of heavy neutrinos imposed by supernova explosions are investigated. It is found that in the mass range of 10–70 MeV the constraint imposed by supernovae is more stringent than those given by cosmology. Any lifetime in this mass range is almost ruled out by the present constraint imposed by supernovae together with those imposed by high-energy experiments. It is suggested that if heavy neutrinos have mass and lifetime not ruled out by these constraints, the energy released by neutrino decay can induce supernova explosions even if the standard bounce-shock mechanism fails in explosions.     

An upper limit on the tau neutrino mass from leptonic tau decays

The partial width of the τ → e ^- v _e v _τ can be calculated from the tau mass, lifetime and branching ratio. The increasing precision on these measurements is making the partial width sensitive to the mass of the tau neutrino. We present the tau neutrino mass dependence on the partial width and show that this leads to a limit of m _r < 71 MeV at the 95% confidence level. We show that the limit is determined by the uncertainty in the tau lifetime and τ → e?v _e v _τ branching ratio. Additional measurements of these values are expected in the next few years and we show that the limit could become competitive with the direct measurements.     

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.     

Experimental limits on the decay of reactor neutrinos

We consider the possible decay of massive reactor neutrinos into a light neutrino and either photons or electron-positron pairs. In a detector placed at the power reactor in Gösgen, Switzerland, the difference of the counting rates for reactor on minus reactor off is consistent with zero. From the experimental bounds we deduce lifetime limits for dominantly coupled light neutrinos as well as restrictions on the mixing parameter |U_eH|² for heavy, subdominantly coupled neutrinos.     

Impact of massive tau-neutrinos on primordial nucleosynthesis. Exact calculations

The influence of a massive Majorana v_τ on primordial nucleosynthesis is rigorously calculated. The system of three integro-differential kinetic equations is solved numerically for m_vτ in the interval from 0 to 20 MeV It is found that the usual assumption of kinetic equilibrium is strongly violated and non-equilibrium corrections considerably amplify the effect. Even a very weak restriction from nucleosynthesis, allowing for one extra massless neutrino species, permits to conclude that m_vτ < 1 MeV For a stricter bound, e.g. for ΔN_v < 0.3, the limit is tm_vτ < 0.35MeV.     

Fitting Simpson's neutrino into the standard model

I show how to accomodate the 17 keV state recently reported by Simpson as one of the neutrinos of the standard model. Experimental constraints can only be satisfied if the μ and τ neutrino combine to a very good approximation to form a Dirac neutrino of 17 keV leaving a light ν_e. Neutrino oscillations will provide the most stringent test of the model. The cosmological bounds are also satisfied in a natural way in models with Goldstone bosons. Explicit examples are given in the framework of majoron-type models. Constraints on the lepton symmetry breaking scale which follow from astrophysics, cosmology and laboratory experiments are discussed.     

Lepton mixing under the lepton charge nonconservation, neutrino masses and oscillations and the “forbidden” decay µ− → e − + γ

The lepton-charge (L _e , L _μ , L _τ ) nonconserving interaction leads to the mixing of the electron, muon, and tau neutrinos, which manifests itself in spatial oscillations of a neutrino beam, and also to the mixing of the electron, negative muon, and tau lepton, which, in particular, may be the cause of the “forbidden” radiative decay of the negative muon into the electron and γ quantum. Under the assumption that the nondiagonal elements of the mass matrices for neutrinos and ordinary leptons, connected with the lepton charge nonconservation, are the same, and by performing the joint analysis of the experimental data on neutrino oscillations and experimental restriction for the probability of the decay µ^? → e ^? + γ per unit time, the following estimate for the lower bound of neutrino mass has been obtained: m ^(ν) > 1.5 eV/c ².     

Neutrino physics at the turn of the millennium

I discuss the implications of the latest data on solar and atmospheric neutrinos which strongly indicate the need for physics beyond the Standard Model. I review the theoretical options for reconciling these data in terms of three-neutrino oscillations. Even though not implied by the data, bimaximal models of neutrino mixing emerge as an attractive possibility. Supersymmetry with broken R-parity provides a predictive way to incorporate it, opening the possibility of testing neutrino anomalies at high-energy collider experiments such as the LHC or at the upcoming long-baseline or neutrino factory experiments. Reconciling, in addition, the hint provided by the LSND experiment requires a fourth, light sterile, neutrino. The simplest theoretical scenarios are the most symmetric ones, in which two of the four neutrinos are maximally mixed and lie at the LSND scale, while the others are at the solar mass scale. The lightness of the sterile neutrino, the nearly maximal atmospheric neutrino mixing, and the generation of Δm _⊙ ² &; Δm _atm ² all follow naturally from the assumed lepton-number symmetry and its breaking. These two basic schemes can be distinguished at neutral-current-sensitive solar &; atmospheric neutrino experiments such as the Sudbury Neutrino Observatory. However, underground experiments have not yet proven neutrino masses, since there is a variety of alternative mechanisms. For example, flavor changing interactions can play an important role in the explanation of solar and of contained atmospheric data and could be tested through effects such as μ → e+γ, μ-e conversion in nuclei, unaccompanied by neutrino-less double beta decay. Conversely, the room is still open for heavy unstable neutrinos. A short-lived ν_μ might play a role in the explanation of the atmospheric data. Finally, in the presence of a sterile neutrino v_s, a long-lived ν_τ in the MeV range could delay the time at which the matter and radiation contributions to the energy density of the Universe become equal, reducing the density fluctuations on the smaller scales and rescuing the standard cold-dark-matter scenario for structure formation. In this case, the light v_e ν_μ, and v_s would account for the solar and atmospheric data.     

The ντ-mass and the τ-decay modes

The behaviour of the different decay modes of the charged heavy lepton τ versus the neutrino ν_τ-mass is analyzed in detail. The τ→ν_τ A ₁ and τ→ν_τ K ^* decay rates have been evaluated using finite energy sum rules. The τ→ν_τ+ “Hadron Continuum” decay rate has been estimated within the framework of Quantum Chromodynamics (QCD). We find that the branching ratios of the semi-leptonic processes: τ→ν_τρ, τ→ν_τπ and τ→ν_τ+ “Hadron Continuum” are very sensitive to the value of the ν_τ-mass. Thus a more precise measurement of these branching ratios could provide an improved upper bound for the neutrino ν_τ-mass.     

Primodal nucleosynthesis,additional neutrinos and neutral currents from the superstring

The constraint on the number of neutrino species N_v imposed by Big Bang nucleosynthesis is reviewed critically. It is argued that uncertainites in the neutron lifetime, systematic errors in the measurement of ⁴He abundance, and new astrophysical data on ³He which suggest extensive astration of D, permit N_v = 5 or even 6. Some phenomenological models based on the superstring expect three additional right-handed neutrinos of low mass which are weakly coupled to ordinary matter through a new heavy Z′ gauge boson. Their compatibility with the relaxed nucleosynthesis limits quoted above imposes an interesting lower limit on m_Z′. As a byproduct of our analysis, we present the corrections to low energy v_μe scattering due to Z′ exchange.     

Constraints on a muon - neutrino mass around 100 keV

In the context of left-right symmetric models, we examine the possibility of a neutrino ν₂ with mass around 100 keV that is essentially the muon neutrino. To meet cosmological constraints, ν₂ must decay into three lighter neutrinos; within the SU(2)_L×SU(2)_R×U(1) model we discuss the relation of this decay to the decay μ→3e.     

Search for finite-mass neutrinos in the decay π+→μ+vμ

If neutrinos possess non-zero mass, pion decay might have small decay branches to neutrino states with large masses. We have searched for such branches in the decay of pions produced at the Indiana University Cyclotron. The energy spectrum of decay muons shows no evidence for such neutrino branches and if these decays do exist, their branching ratios must be less than 10^?2 to 10^?3 for neutrino masses in the ranFge 7–33 MeV.     

Majorana neutrinos, <Emphasis Type="Italic">CP</Emphasis> violation,neutrinoless double beta and tritium beta decays

If the present or upcoming searches for neutrinoless double beta ((ββ)_0ν) decay give a positive result, the Majorana nature of massive neutrinos will be established. From the determination of the value of the (ββ)_0ν-decay effective Majorana mass parameter (|〈m〉|), it would be possible to obtain information on the type of neutrino mass spectrum. Assuming 3-ν mixing and massive Majorana neutrinos, we discuss the information that a measurement of, or an upper bound on, |〈m〉| can provide on the value of the lightest neutrino mass m₁. With additional data on the neutrino masses obtained in ³H β-decay experiments, it might be possible to establish whether the CP symmetry is violated in the lepton sector. This would require very high precision measurements. If CP invariance holds, the allowed patterns of the relative CP parities of the massive Majorana neutrinos would be determined.     

Neutrino masses and family symmetry

Neutrino masses in the 100 eV?1 MeV range are permitted if there is a spontaneously broken global family symmetry that allows the heavy neutrinos to decay by Goldstone boson emission with a cosmologically acceptable lifetime. The family symmetry may be either abelian or nonabelian; we present models illustrating both possibilities. If the family symmetry is nonabelian, then the decay τ → μ + Goidstone boson or τ → e + oldstone boson may have an observable rate.     

Radiative neutrino decay and the unusual X-ray flux from Sn 1987A

The radiative neutrino decay in the mass and a lifetime range, m_v=10–100 keV and τ_v=10³–10⁵ s, is often argued to be forbid by γ-ray data from past supernovas. We demonstrate that this argument is not always correct, and accordingly attempt to explain via radiative neutrino decay the unusual soft component of the X-ray spectrum from SN 1987A, observed by the satellite Ginga. Our fit to the data suggests that τ_v=(1–3)×10⁴s·(m_v/50 keV).     

Statistical theory of nuclear neutrino capture: (II). Inclusion of first-forbidden transitions

The statistical theory of nuclear neutrino capture is extended to include first-forbidden transitions. A comparison with the theory of Bahcall and Frautschi is made. It is found that the present theory predicts neutrino capture cross sections which are smaller than those of Bahcall and Frautschi by a factor 2–3 for neutrino energies less than 50 MeV when first-forbidden transitions are dominant. Calculation of the cross section is made for the process in which ³⁷Cl nuclei capture electron neutrinos that are emitted in muon decay. The present calculation gives a cross section which is around one half of that of Donnelly and Haxton. Finally the contributions of the highly excited states in ³⁷Ar to the neutrino capture cross section are evaluated. It is shown that the contributions from the highly excited states (E > 6.02 MeV) to the neutrino capture cross section amount to 60% for E_v = 50 MeV.     

Constraints on neutrino mixing

We explore the implications of imposing the constraint that two neutrino flavors (which for definiteness we take to be ν_μ and ν_τ) are similarly coupled to the mass basis in addition to the unitarity constraints. We allow three active and an arbitrary number of sterile neutrinos. We show that in this scheme one of the mass eigenstates decouples from the problem, reducing the dimension of the flavor space by one. This result allows significant simplification in the treatment of matter-enhanced neutrino transformation where multiple flavors and level crossings are involved.