The mixing angles in matter for three generations of neutrinos and the MSW mechanism

Mikheyev and Smirnov have recently shown that oscillations between two species of neutrinos may be amplified in matter. We give analytic expressions for the energy eigenvalues, all the mixing angles and the CP-violating phase in matter for three generations of neutrinos using the Fritzsch parametrization for the flavor mixing matrix. For clearly separated neutrino masses Δm ₃₁ ² ?Δm ₂₁ ² we find two MSW resonance effects—one forv _e ?v _µ and one forv _e ?v _τ conversions —which can each be approximated by a separate two neutrino treatment as has been recently shown by Kuo and Pantaleone. Nearly degenerate neutrino masses Δm ₃₁ ² ～Δm ₂₁ ² on the other hand lead to only one resonance region withs _{1 2} ^{m 2} no longer necessarily approaching 1 for very high densities.     

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.     

Global analysis of solar and atmospheric neutrino data

A global analysis of solar (including the recent SNO result), atmospheric, and reactor neutrino data is presented in terms of three-and four-neutrino oscillations. We first present the allowed regions of solar and atmospheric oscillation parameters assuming three-neutrino families, showing that in this framework it is possible to reconcile the two anomalies and providing an unified fit of all the observables at a time. Then, we consider scenarios where a sterile neutrino is added to the three standard ones and the mass spectra present two separate doublets. We evaluate the allowed active-sterile admixture in both solar and atmospheric oscillations, showing that, although the Super-Kamiokande data disfavor both the pure ν_μ→ν _s atmospheric channel and, in combination with SNO, the pure ν _e →ν _s solar channel, the result from the combined analysis still favors close-to-pure active and sterile oscillations and disfavors oscillations into a near-maximal active-sterile admixture.     

LSND, SN1987A, and CPT violation

We point out that neutrino events observed at Kamiokande and IMB from SN1987A disfavor the neutrino oscillation parameters preferred by the LSND experiment. For Δm²>0 (the light side), the electron neutrinos from the neutronization burst would be lost, while the first event at Kamiokande is quite likely to be due to an electron neutrino. For Δm²<0 (the dark side), the average energy of the dominantly events is already lower than the theoretical expectations, which would get aggravated by a complete conversion from to  . If taken seriously, the LSND data are disfavored independent of the existence of a sterile neutrino. A possible remedy is CPT violation, which allows different mass spectra for neutrinos and anti-neutrinos and hence can accommodate atmospheric, solar and LSND data without a sterile neutrino. If this is the case, Mini-BooNE must run in rather than the planned ν mode to test the LSND signal. We speculate on a possible origin of CPT violation.     

Are there sterile neutrinos in the flux of solar neutrinos on the earth?

It is shown that the future SNO and Super-Kamiokande experiments, in which high energy⁸B neutrinos will be detected through the observation of CC, NC and –e elastic scattering processes, could allow to reveal in a model independent way the presence of sterile neutrinos in the flux of solar neutrinos on the earth. Lower bounds for different averaged values of the probability of transition of solar v^e'S into sterile states and for the total flux of⁸B neutrinos are derived in terms of measurable quantities. The possibilities to reveal the presence of v and/or v in the solar neutrino flux on the earth are also considered and the case of transitions of solar v^e'S only into sterile states is discussed. Some numerical results for a simple model with v–v^s mixing are given.     

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.     

Futurev τ oscillation experiments and present data

Our goal in this paper is to examine the discovery covery potential of laboratory experiments searching for the oscillationv _μ(ν _e)→v _τ, in the light of recent data on solar and atmospheric neutrino experiments, which we analyse together with the most restrictive results from laboratory experiments on neutrino oscillations. In order to explain simultaneouslyall present results we use a four-neutrino framework, with an additional sterile neutrino. Our predictions are rather pessimistic for the upcoming experiments NOMAD and CHORUS, which, we find, are able to explore only a small area of the oscillation parameter space. On the other hand, the discovery potential of future experiments is much larger. We consider three examples. E803, which is approved to operate in the future Fermilab main injector beam line, MINOS, a proposed long-baseline experiment also using the Fermilab beam, and NAUSICAA, an improved detector which improves by an order of magnitude the performance of CHORUS/NOMAD and can be operated either at CERN or at Fermilab beams. We find that those experiments can cover a very substantial fraction of the oscillation parameter space, having thus a very good chance of discoveringboth v _μ→v _τ andν _e→v _τ oscillation modes.     

Futurev �� oscillation experiments and present data

Our goal in this paper is to examine the discovery covery potential of laboratory experiments searching for the oscillationv _??(?? _e)??v _??, in the light of recent data on solar and atmospheric neutrino experiments, which we analyse together with the most restrictive results from laboratory experiments on neutrino oscillations. In order to explain simultaneouslyall present results we use a four-neutrino framework, with an additional sterile neutrino. Our predictions are rather pessimistic for the upcoming experiments NOMAD and CHORUS, which, we find, are able to explore only a small area of the oscillation parameter space. On the other hand, the discovery potential of future experiments is much larger. We consider three examples. E803, which is approved to operate in the future Fermilab main injector beam line, MINOS, a proposed long-baseline experiment also using the Fermilab beam, and NAUSICAA, an improved detector which improves by an order of magnitude the performance of CHORUS/NOMAD and can be operated either at CERN or at Fermilab beams. We find that those experiments can cover a very substantial fraction of the oscillation parameter space, having thus a very good chance of discoveringboth v _????v _?? and?? _e??v _?? oscillation modes.     

A four-neutrino mixing scheme for observed neutrino data

It has been observed that simultaneous explanation of the solar and atmospheric neutrino deficits and the reported evidence for oscillation from the Los Alamos Liquid Scintillator Detector (LSND) requires at least one extra neutrino species in addition to the three known ones. The extra neutrino must be sterile with respect to the known weak interactions. We present a new mass matrix for these four neutrinos in which the LSND effect and the atmospheric neutrino deficit are governed by only one parameter. We investigate the phenomenological implications of such a mass matrix ansatz and suggest possible ways to understand it in gauge theories.     

MiniBooNE overview and status

Recent discoveries in the neutrino sector have opened a new frontier in highenergy physics and cosmology. Evidence from neutrino oscillation experiments from around the world indicate that neutrinos oscillate between their different flavours and therefore may have mass. In addition, results from solar and atmospheric neutrino experiments as well as the accelerator neutrino experiment, LSND, cannot all be explained with the three standard model neutrinos. Is this new physics or is there some other explanation? The MiniBooNE experiment presently taking data at Fermilab is designed to address the LSND signal and answer this question. Progress on the MiniBooNE experiment will be presented and prospects for the future will be discussed.     

Sterile neutrino in a minimal three-generation see-saw model

We investigate symmetries in Dirac and Majorana mass matrices of neutrinos in a three-generation scenario. We show that if we invokeL _e +L _μ-L _τ x S _2R symmetry, one combination of right-handed neutrino states remains massless which can be interpreted as a sterile neutrino. Next we consider a SU2_L x U(1)_y x U(l)_R gauge model and show how higher-dimensional operators can induce mixing between left- and right-handed states which explains solar, atmospheric and LSND experimental results.     

The effect of random matter density perturbations on the MSW solution to the solar neutrino problem

We consider the implications of solar matter density random noise upon resonant neutrino conversion. The evolution equation describing MSW-like conversion is derived in the framework of the Schrödinger approach. We study quantitatively the effect of such matter perturbations upon both large and small mixing angle MSW solutions to the solar neutrino problem. This is carried out both for the active-active v_e → v_μ,τ as well as active-sterile v_e → v_s conversion channels. We find that the small mixing MSW solution is much more stable (especially in Δm² than the large mixing solution. The possible existence of solar matter density noise at the few percent level could be tested at future solar neutrino experiments, especially Borexino.     

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.     

Testing the vacuum oscillation and the MSW solutions of the solar neutrino problem

Solar model independent tests of the vacuum oscillation and MSW solutions of the solar neutrino problem are considered. Detailed predictions for time (seasonal) variations of the signals due to neutrino oscillations in vacuum are given for the future solar neutrino detectors (SNO, Super-Kamiokande, BOREXINO, HELLAZ). Results on the distortions of the spectra of ⁸B neutrinos, and of e⁻ from the reaction ν + e⁻ → ν + e⁻ induced by ⁸B neutrinos, are presented in the cases of vacuum oscillations or MSW transitions for a large number of values of the relevant parameters. The possibilities to distinguish between the vacuum oscillation, the MSW adiabatic, and the MSW nonadiabatic transitions in the future solar neutrino experiments are discussed.     

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).     

Results from the NOMAD experiment

The NOMAD experiment has sought ν_μ ? ν_τ oscillations by looking for the emergence of τ^? in events from the CERN SPS neutrino beam. With some improvements in the techniques of analysis in relation to the results published previously and with the inclusion of data from the 1998 run, no evidence for the oscillations has been found, which results in an updated limit on the oscillation probability [P(ν_μ → ν_τ) < 0.5 × 10^?3 at a 90% C.L.]. The corresponding limit on the oscillation mixing angle is given by sin²2θ_μτ < 1.0 × 10^?3 for large Δm ². By using a 1% contamination of ν_e in the neutrino beam, we can also rule out ν _e ? ν_τ oscillations and constrain the probability of the relevant transition as P(ν _e → ν_τ) < 3 × 10^?2 at a 90% C.L. (sin²2θ _eτ < 6 × 10^?2 at large Δm ²).     

Contribution of gallium experiments to the understanding of solar physics and neutrino physics

The results of gallium measurements of solar neutrino and measurements with artificial sources of neutrinos are presented. Conclusions are drawn from these results, and the potential of the SAGE experiment for studying transitions of active neutrinos to sterile states for Δm ² > 0.5 eV² and a sensitivity of a few percent to the disappearance of electron neutrinos is examined.     

Search for heavy sterile neutrinos in trileptons at the LHC

We present a search strategy for both Dirac and Majorana sterile neutrinos from the purely leptonic decays of W~±→e~±e~±μ~?ν and μ~±μ~± e~?ν at the 14 TeV LHC. The discovery and exclusion limits for sterile neutrinos are shown using both the Cut-and-Count(CC) and Multi-Variate Analysis(MVA) methods. We also discriminate between Dirac and Majorana sterile neutrinos by exploiting a set of kinematic observables which differ between the Dirac and Majorana cases. We find that the MVA method, compared to the more common CC method, can greatly enhance the discovery and discrimination limits. Two benchmark points with sterile neutrino mass m N =20 GeV and 50 GeV are tested. For an integrated luminosity of 3000 fb~(-1), sterile neutrinos can be found with 5σ significance if heavy-to-light neutrino mixings |U_(Ne)|~2~|U_(Nμ)|~2~10~(-6), while Majorana vs. Dirac discrimination can be reached if at least one of the mixings is of order 10~(-5).     

Reactor neutrinos—present and future

The main characteristics of experimental searches for neutrino oscillations at nuclear reactors are reviewed. We then describe the Chooz and Palo Verde experiments and report their results to date. We also describe the KamLAND experiment, presently under construction in the Kamioka laboratory in Japan, which is designed to extend the sensitivity to ν_e ? ν_X oscillations by two orders of magnitude in Δm ². In particular, this experiment expects to be able to carry out a terrestial test of the large-angle MSW solution to the solar neutrino problem.