Seasonal and energy dependence of solar neutrino vacuum oscillations

We make a global vacuum neutrino oscillation analysis of solar neutrino data, including the seasonal and energy dependence of the recent Super-Kamiokande 708-day results. The best fit parameters for ν_e oscillations to an active neutrino are δm²=4.42×10⁻¹⁰ eV², sin²2θ=0.93. The allowed mixing angle region is consistent with bi-maximal mixing of three neutrinos. Oscillations to a sterile neutrino are disfavored. Allowing an enhanced hep neutrino flux does not significantly alter the oscillation parameters.     

Status of the Chorus experiment

The CHORUS experiment at CERN searches for ν_μ ? ν_τ oscillations by looking for τ decays from charged-current ν_τ interactions. The emulsion target of the detector, having a resolution of about a micron, enables the detection of the decay topology of the τ. After having analyzed a sample of 126000 events containing an identified muon and 7500 purely hadronic events, no ν_τ candidate has been found. This result translates in a limit on the mixing angle sin²2θ_μτ < 8 × 10^?4 at 90% C.L. for large Δm _μτ ² .     

The NOνA experiment

NOνA is a long-baseline neutrino experiment designed to study ν _μ →ν _e and $\overline \nu_{\mu} \to \overline \nu_e $ oscillations. It will measure the neutrino mixing angles θ ₁₃ and θ ₂₃ with high precision, probe the neutrino mass hierarchy, and search for CP violation in neutrino oscillations. The experiment consists of two detectors. The Near Detector will be located at Fermilab close to the source of the neutrino beam. The Far Detector is being built at Ash River in northern Minnesota. It is positioned 14 mrad off the neutrino beam axis where the neutrinos have an energy distribution with a narrow peak around 2 GeV, and where the transition probability of ν _μ →ν _e is close to its maximum.     

The OPERA experiment

The OPERA (oscillation project with emulsion-tracking apparatus) experiment aims to observe an unambiguous ν_π?ν_τ oscillation in the parameter region allowed by previous experiments. The OPERA detector will be installed in the underground Gran Sasso Laboratory, 732 km away from CERN, where the CNGS (CERN neutrino beam to Gran Sasso) ν_π beam will be produced. The signature of the presence of ν_τ's in the ν_π beam will be the detection of τ leptons produced by ν_τ interactions. Nuclear emulsions will be used for precise event reconstruction, while electronic detectors will be used for triggering.     

New MACRO results on atmospheric neutrino oscillations

The final results of the MACRO experiment on atmospheric neutrino oscillations are presented and discussed. The data concern different event topologies with average neutrino energies of ～3 and ～50 GeV. Multiple Coulomb scattering of the high-energy muons in absorbers was used to estimate the neutrino energy of each event. The angular distributions, the L/E_ν distribution, the particle ratios, and the absolute fluxes all favor ν_π?ν_τ oscillations with maximal mixing and Δm²=0.0023 eV². A discussion is made on the Monte Carlos used for the atmospheric neutrino flux. Some results on neutrino astrophysics are also briefly discussed.     

Results from the T2K experiment

The T2K (Tokai-to-Kamioka) experiment is a long baseline neutrino oscillation experiment designed to probe the θ ₁₃ neutrino mixing parameter by looking for the appearance of ν _e in an almost pure ν _μ beam. The concurrent measurement of the ν _μ disappearance allows refined measurements of the atmospheric Δm ² and θ ₂₃ mixing parameters.     

Limits on neutrino oscillations from a study of νe charged current interactions

We have fully analysed all events with a single electron obtained in an exposure of BEBC to the 350 GeV wide-band neutrino beam at CERN. The data agrees well with expectations based on the calculated ν_e flux and assuming μ?e universality. Using both the rate and the kinematic distributions, improved limits are set on ν_e → ν_X, ν_μ → ν_e, ν_μ → ν_τ mixing.     

Matter effects for solar neutrino oscillations

Possible solar neutrino oscillations are reviewed in the two-neutrino case taking into account the effect of coherent forward scattering when neutrinos travel through the sun and earth. As recently pointed out by Mikheyev and Smirnov this effect can induce a large suppression of the solar ν_e flux for values of Δm ² around 10^?4–10^?8 eV² even for small values of the mixing angle. It also may cause substantial modifications of the solar neutrino spectrum shape. All this may be used for determining Δm ² and sin² 2θ in a large domain from the experimental results of the chlorine, gallium, indium and heavy water detectors.     

A simple modification of the maximal mixing scenario for three light neutrinos

We suggest a simple modification of the maximal mixing scenario (withS ₃ permutation symmetry) for three light neutrinos. Our neutrino mass matrix has smaller permutation symmetryS ₂(ν _μ ?ν _e ), and is consistent with all neutrino experiments except the³⁷Cl experiment. The resulting mass eigenvalues for three neutrinos arem ₁≈(2.55?1.27)×10^?3eV,m _2,3≈(0.71?1.43)eV for Δm _LSND ² =0.5?2.0eV². Then these light neutrinos can account for ～(2.4?4.8)% (6.2?12.4%) of the dark matter forh=0.8 (0.5). Our model predicts theν _μ →ν _τ oscillation probability in the range sensitive to the future experiments such as CHORUS and NOMAD.     

Upper limit to the cross-section for the process νμ + e− → νμ + e−

An upper limit of 3 × 10^?42E_ν cm² per electron, at 90% confidence level, has been obtained for the cross section of the process ν_μ + e^? → ν_μ + e^?, interpreted in the usual V-A framework, for an electron recoil energy larger than 0.3 GeV. This estimate is based on the observation of one candidate event in exposures of Gargamelle to the CERN PS neutrino beam.     

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.     

Experimental consequences of νe−νμ mixing in neutrino beams

The experimental consequences of neutrino mixing and decay are analyzed. Existing neutrino beam experiments are consistent with a finite but small mixing angle unless |m_νμ²?m_νe²| ? (3.0 eV)². A finite ν_μ lifetime in the range τ/m_νμc² ? 7 × 10³ sec/MeV is shown to be consistent with experimental data.     

Limits on the τ neutrino electromagnetic properties from single photon searches ate + e − colliders

We set limits on the magnetic moment and charge radius of the τ neutrino by examining the contributions to the processe ⁺ e ^?→v \(\bar \nu \) γ due to such interactions. We find thatK(ν_τ)<4×10^?6 (i.e.μ(ν_τ)<4×10^?6μ _B , μ _B =e/2m _e ) and 〈r ²〉<2×10^?31 cm² using the combined data of the MAC, ASP, CELLO, and Mark J collaborations for this process. We briefly discuss whether these bounds can be improved in any futuree ⁺ e ^? experiments.     

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.     

Overview on neutrinos and the Daya Bay experiment

Neutrinos are elementary particles in the Standard Model. Neutrino oscillation is a quantum mechanical phenomenon beyond the Standard Model. Neutrino oscillation can be described by two independent mass-squared differences Δm ₂₁ ² , Δm ₃₁ ² (or Δm ₃₂ ² ) and a 3 × 3 unitary matrix, containing three mixing angles θ ₁₂, θ ₂₃, θ ₁₃, and one charge-parity (CP) phase. θ ₁₂ is about 34° and determined by solar neutrino experiments and the reactor neutrino experiment KamLAND. θ ₂₃ is about 45° and determined by atmospheric neutrino experiments and accelerator neutrino experiments. θ ₁₃ can be measured by either accelerator or reactor neutrino experiments. On Mar. 8, 2012, the Daya Bay Reactor Neutrino Experiment reported the first observation of non-zero θ ₁₃ with 5.2 standard deviations. In June, with 2.5× previous data, Daya Bay improved the measurement of sin²2θ ₁₃ = 0.089 ± 0.010(stat) ± 0.005(syst).     

ν<Subscript>μ</Subscript> → ν<Subscript> <Emphasis Type="Italic">e</Emphasis> </Subscript> oscillations search in the OPERA experiment

The tracking capabilities of the OPERA detector allow to reconstruct τ-leptons and electrons. It gives a possibility to observe ν_μ → ν_τ oscillations in the appearance mode and to study ν_μ → ν_e oscillations in the ν_μ CNGS beam. Current results on ν_μ → ν_e channel in the three-flavour mixing model are presented. The same data allow to constrain the presence of additional sterile neutrino states. The analysis of the full 2008–2012 OPERA data set and work on its improvement are going on. Details of the achievements are presented.     

Observation of possible production and decay of short-living heavy neutral particles in the SKAT neutrino experiment

In the SKAT bubble chamber neutrino experiment production of a short-living neutral particle with mass (1.4?m?2.5) GeV/c² and lifetime τ≈6×10^?12 s has been detected. The event may be interpreted as production and decay of the heavy lepton M⁰→μ^?+e⁺+ν_e with mass (1.4?m_M⁰?2.1) GeV/c². It might also be the production of a charmed particle D⁰→e⁺+τ^?+ν_e (D⁰→e⁺+π^?+ν_e) with mass 1.4?m_D⁰?2.5) GeV/c² in a non-diagonal neutral current. The probability to interpret the event as other possible processes is very low.     

New results from the T2K neutrino oscillation experiment

The T2K experiment searches for the appearance of electron neutrinos in a muon neutrino beam. The rate of this process is sensitive to the neutrino mixing parameter θ ₁₃. Recent measurements that $\theta_{13} \ne 0$ imply that ν _μ → ν _e oscillations should be observable. Using all data through May 15, 2012 the T2K experiment has detected 10 candidate ν _e events, with an expected background for θ ₁₃?=?0 of 2.73±0.37 events. This 3.2σ excess of ν _e events is the strongest indication to date for appearance of electron neutrinos in a neutrino oscillation experiment, and for normal mass hierarchy and δ _CP ?=?0 yields $0.059 < \sin^2 2\theta_{13} < 0.164$ at the 68 % C.L.     

Measurements of tau decay modes and a precise determination of the mass

The cross sections for e⁺e^? → e⁺(μ⁺ + non showering track + any photons have been measured for cm energies between 3.1 GeV and 5.2 GeV. We observe τ-pair production below the thresholdfor charm production and determine the τ mass to be 1.807 ± 0.020 GeV from a fit to the energy dependence of the cross section. The ration of the leptonic branching ratios B_μ/B_e = 0.92 ± 0.32 is consistent with eμ-universality. The following branching ratios are determined for a V-A coupling: $\text{B(τ → ν}{}_{\mathrm{\tau }}\text{e}\text{ν}\text{) = B(τ → ν}{}_{\mathrm{\tau }}\text{μ}\text{ν}\text{) = 0.182 + 0.028}$. B(τ → ν_τ + charged hadron + any photons) = 0.29 ± 0.11, B(τ → ν_τ + three or more charged hadrons + any photons) = 0.35 ± 0.11.     

Estimation ofv μ→v τ oscillation parameters in the E-564 hybrid experiment

The limits on the parameters of thev _μ→v _τ oscillations are obtained in the E-564 hybrid experiment: Δm ²≦4.5 eV² and sin²(2θ)≦6.0·10^?2 (90% CL). A possibility of direct observation of τ-lepton decays in the nuclear emulsion vertex detector allowed the sensitivity of this experiment to the parameters of the oscillations to be increased.