Neutrino Oscillations with Three Active and Three Sterile Neutrinos

This is an extension of estimates of the probability of μ to e neutrino oscillation with one sterile neutrino to three sterile neutrinos, using a 6x6 matrix. Since the mixing angle for only one sterile neutrino has been experimentally determined, we estimate the μ to e neutrino oscillation probability with different mixing angles for two of the sterile neutrinos.     

Neutrino Oscillations With Two Sterile Neutrinos

This work estimates the probability of μ to e neutrino oscillation with two sterile neutrinos using a 5×5 U-matrix, an extension of the previous estimate with one sterile neutrino and a 4×4 U-matrix. The sterile neutrino-active neutrino mass differences and the mixing angles of the two sterile neutrinos with the three active neutrinos are taken from recent publications, and the oscillation probability for one sterile neutrino is compared to the previous estimate.     

Mass properties of active and sterile neutrinos in a phenomenological (3 + 1 + 2) model

A generalized model involving three active neutrinos and three sterile neutrinos of different mass, one being relatively heavy [(3 + 1 + 2) model], is considered on the basis of experimental data, which admit the existence of anomalies beyond the minimally extended standard model featuring three active neutrinos of different mass. Basic properties used to describe massive active and sterile neutrinos are studied along with methods for determining the absolute scale of neutrino masses and for estimating neutrino masses on the basis of available experimental data. In the approximation of CP conservation, admissible values of the elements of the neutrino mass matrix are found from numerical calculations versus the possible values of the mass of one of the sterile neutrinos. The dependences of the mass properties of the neutrinos on the sterile-neutrino mass are constructed with allowance for possible sterile-neutrino contributions. The respective results can be used to interpret and predict results of various neutrino experiments.     

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.     

Status of the Sudbury Neutrino Observatory

Solar neutrinos from the decay of ⁸B have been detected at the Sudbury Neutrino Observatory via the charged-current (CC) and neutral-current (NC) reactions on deuterium and by the elastic scattering (ES) of electrons. The CCr eaction is sensitive exclusively to electron neutrinos, the NCr eaction is sensitive to all neutrino species, and the ES reaction also has a small sensitivity to muon and tau neutrinos. These measurements provided strong evidence that neutrinos change flavor as they propagate from the center of the Sun to the Earth at the 5.3σ level. It will also be shown that a global solar neutrino analysis of matter-enhanced neutrino oscillations of two active flavors strongly favors the large mixing angle solution.     

Allowance for <Emphasis Type="Italic">CP</Emphasis> violation in the exponential parametrization of neutrino mixing

An exponential form of the Pontecorvo–Maki–Nakagawa–Sakatamixingmatrix for neutrinos is discussed. This form makes it possible to separate the contributions of purely rotational components of the mixing matrix and elements that are responsible for CP violation. The logarithm of the mixing matrix and exact values for each parameter of the exponential neutrino mixing matrix are found on the basis of the most recent experimental data on neutrino mixing. The parameters of the real rotational part in the exponential representation of the mixing matrix and the parameters of its imaginary part, which is responsible for CP violation, are evaluated. The hypothesis of complementarity of quark and neutrino mixing is confirmed. Factorization in the form of the product of rotations about the real and imaginary axes is demonstrated.     

Neutrino mass spectrum with vμ → vs oscillations of atmospheric neutrinos

We consider the “standard” spectrum of the active neutrinos (characterized by strong mass hierarchy and small mixing) with additional sterile neutrino, v_s. The sterile neutrino mixes strongly with the muon neutrino, so that v_μ ↔ v_s oscillations solve the atmospheric neutrino problem. We show that the parametric enhancement of the v_μ ↔ v_s oscillations occurs for the high energy atmospheric neutrinos which cross the core of the Earth. This can be relevant for the anomaly observed by the MACRO experiment. Solar neutrinos are converted both to v_μand v_s. The heaviest neutrino (≈ v_τ) may compose the hot dark matter of the Universe. The phenomenology of this scenario is elaborated and crucial experimental signatures are identified. We also discuss properties of the underlying neutrino mass matrix. 1998 Published by Elsevier Science B.V.     

Stability of the lepton-flavor mixing matrix against quantum corrections

Recent neutrino experiments suggest strong evidence of tiny neutrino masses and the lepton-flavor mixing. Neutrino-oscillation solutions for the atmospheric neutrino anomaly and the solar neutrino deficit can determine the texture of the neutrino mass matrix according to the neutrino mass hierarchies as Type A: , Type B: , and Type C: , where is the i-th generation neutrino mass. In this paper we study the stability of the lepton-flavor mixing matrix against quantum corrections for all three types of mass hierarchy in the minimal supersymmetric Standard Model with an effective dimension-five operator which gives the Majorana masses of neutrinos. The relative sign assignments of neutrino masses in each type play crucial role for the stability against quantum corrections. We find that the lepton-flavor mixing matrix of Type A is stable against quantum corrections, and that of Type B with the same (opposite) signs of and are unstable (stable). For Type C, the lepton-flavor-mixing matrix approaches the definite unitary matrix according to the relative sign assignments of the neutrino mass eigenvalues as the effects of quantum corrections become large enough to neglect the squared mass differences of neutrinos. Received: 24 June 1999 / Revised version: 23 December 1999 / Published online: 17 March 2000     

<Emphasis Type="Italic">O</Emphasis>(1) eV sterile neutrino in <Emphasis Type="Italic">f</Emphasis>(<Emphasis Type="Italic">R</Emphasis>) gravity

We refer [1] to the role of an additional O(1) eV sterile neutrino in modified gravity models. We find parameter constraints in particular f(R) gravity model using following up-to-dated cosmological data: measurements of the cosmic microwave background (CMB) anisotropy, the CMB lensing potential, the baryon acoustic oscillations (BAO), the cluster mass function and the Hubble constant. It was obtained for the sterile neutrino mass 0.47 eV < m _ν,sterile < 1 eV (2σ) assuming that the sterile neutrinos are thermalized and the active neutrinos are massless, not significantly larger than in the standard cosmology model within the same data set: 0.45 eV < m _ν,sterile < 0.92 eV (2σ). But, if the mass of sterile neutrino is fixed and equals ≈ 1.5 eV according to various anomalies in neutrino oscillation experiments, f(R) gravity is much more consistent with observation data than the CDM model.     

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.     

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.     

Total cross sections νμ and ḡnμcharged-current interactions between 20 and 200 GeV

Exposures of the Ne/H₂ filled Big European Bubble Chamber (BEBC) to a dichromatic neutrino (antineutrino) beam produced by 400 GeV protons of the CERN SPS yielded ～ 3100 events with a negative, and ～ 1100 with a positive, muon. The neutrino flux is determined from the muon flux in the shielding. Assuming a linear energy dependence of the cross section, the values $\text{σ}\text{E}$ between 20 and 200 GeV are found to be 0.657 ± 0.012 (stat.) ± 0.027 (syst.) and 0.309 ± 0.009 (stat.) ± 0.013 (syst.) cm² (GeV nucleon)^?1, for neutrinos and antineutrinos, respectively. The scaling variable $\text{q}{}^2\text{E}$ decreases significantly with increasing energy both for neutrinos and antineutrinos.     

Neutrino mass and mixing implied by underground deficit of low energy muon-neutrino events

Recent observations of a deficit of cosmic ray muon-neutrino interactions in underground detectors suggest that the muon neutrinos may have oscillated to another state. We examine possible neutrino mass and mixing patterns, and their implications for vacuum and matter effects on solar neutrinos, on neutrinos passing through the earth, and on terrastrial neutrino beams. By invoking the see-saw mechanism of neutrino mass generation, we draw inferences on closure of the universe with neutrino masses, on the number of generations, on t-quark and fourth generation masses, and on the Peccei-Quinn symmetry breaking scale. Testable predictions are suggested.     

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     

Experimental study of opposite-sign dimuons produced in neutrino and antineutrino interactions

A large sample of opposite-sign dimuons, produced by the interaction of neutrinos and antineutrinos in iron, is analysed. The data agree very well with the hypothesis that the extra muon is the product of charm decay. They yield information on the strength and space-time structure of the charmproducing weak current. The strange-sea structure functionxs(x) is determined. The difference between neutrino and antineutrino dimuon production is analysed to provide a value of the Kobayashi-Maskawa weak mixing angleθ ₂.     

Total cross sections for ve and ve interactions and search for neutrino oscillations and decay

About 200 and 60 candidates for electron neutrino and antineutrino interactions, respectively, have been analyzed in the heavy liquid bubble chamber Gargamelle exposed to the CERN PS neutrino beam. Evidence for scaling has been found for these interactions, with slopes of the cross sections in good agreement with those obtained for muon neutrino and antineutrino events in the same chamber. No evidence appears for oscillations of neutrinos or antineutrinos, which would induce in the present experiment an excess of electron or positron events. The corresponding limits are given as functions of the mixing parameter, for the finite mass Majorana neutrinos. The possibility of a multiplicative law for the lepton number has also been investigated. A search for isolated electron-positron pairs revealed no excess in the forward direction, in contradiction to the expectation for muonic neutrino and antineutrino decays. The corresponding limits on the c.m. half lifetimes are given.     

Probing the seesaw mechanism with neutrino data and leptogenesis

We use the current low-energy neutrino data to understand the structure of the neutrino mass matrix. Considering this information and assuming hierarchical neutrino Yukawa couplings, we use the seesaw formula to study the properties of the heavy right-handed neutrinos N_i. We find that successful baryogenesis via leptogenesis requires mass degeneracy and maximal mixing of N₁ and N₂.     

Neutrino masses and oscillations: triumphs and challenges

The recent progress in establishing the existence of finite neutrino masses and mixing between generations of neutrinos has been remarkable, if not astounding. The combined results from studies of atmospheric neutrinos, solar neutrinos, and reactor antineutrinos paint an intriguing picture for theorists and provide clear motivation for future experimental studies. In this review, we summarize the status of experimental and theoretical work in this field and explore the future opportunities that emerge in light of recent discoveries.     

Phenomenological relations for quark and neutrino mixing angles

The most recent experimental data on quark and neutrino mixing angles are discussed. It is indicated that the results of the latest kaon-decay experiments are consistent with the unitarity condition for the first row of the Cabibbo-Kobayashi-Maskawa matrix if the currently available world-average value of the neutron lifetime is used to determine the element V _ud of this matrix. The quark mixing angles are calculated within the Fritzsch-Scadron-Delbourgo-Rupp phenomenological approach on the basis of values of the masses of light and heavy constituent quarks. The neutrino mixing angles are calculated to a high precision with the aid of the hypothesis that the quark and neutrino mixing angles are complementary. The results are compatible with experimental data.