Evidence For Neutrino Oscillations Research Articles

Neutrino masses and mixing: evidence and implications

Measurements of various features of the fluxes of atmospheric and solar neutrinos have provided evidence for neutrino oscillations and therefore for neutrino masses and mixing. We review the phenomenology of neutrino oscillations in vacuum and in matter. We present the existing evidence from solar and atmospheric neutrinos as well as the results from laboratory searches, including the final status of the LSND experiment. We describe the theoretical inputs that are used to interpret the experimental results in terms of neutrino oscillations. We derive the allowed ranges for the mass and mixing parameters in three frameworks: First, each set of observations is analyzed separately in a two-neutrino framework; Second, the data from solar and atmospheric neutrinos are analyzed in a three active neutrino framework; Third, the LSND results are added, and the status of accommodating all three signals in the framework of three active and one sterile light neutrinos is presented. We review the theoretical implications of these results: the existence of new physics, the estimate of the scale of this new physics and the lessons for grand unified theories, for supersymmetric models with R-parity violation, for models of extra dimensions and singlet fermions in the bulk, and for flavor models.

Determination of neutrino mixing parameters after SNO oscillation evidence

An updated analysis of all available neutrino oscillation evidence in solar experiments (SK day and night spectra, global rates from Homestake, SAGE, and GALLEX) including the latest SNO CC and NC data is presented. Assuming that the shape of the SNO CC energy spectrum is undistorted and using the information provided by SNO we obtain, for the fraction of electron neutrinos remaining in the solar beam at energies \ensuremath{\gtrsim}5 MeV, ${\ensuremath{\varphi}}_{\mathrm{CC}}/{\ensuremath{\varphi}}_{\mathrm{NC}}{=0.34}_{\ensuremath{-}0.04}^{+0.05},$ which is nominally many standard deviations away from the standard value. The fraction of oscillating neutrinos into active ones is computed to be $({\ensuremath{\Phi}}_{\mathrm{NC}}\ensuremath{-}{\ensuremath{\Phi}}_{\mathrm{CC}})/({\ensuremath{\Phi}}_{\mathrm{SSM}}\ensuremath{-}{\ensuremath{\Phi}}_{\mathrm{CC}}{)=0.92}_{\ensuremath{-}0.20}^{+0.39},$ nearly $5\ensuremath{\sigma}$ deviations from the pure sterile oscillation case. The data are still compatible with an important fraction of sterile component in the solar beam (up to 20%\char21{}30% of the total). In the framework of two active neutrino oscillations we determine individual neutrino mixing parameters and their errors in the region of no spectrum distortion $(\ensuremath{\Delta}〈{T}_{e}〉l1%);$ we obtain $\ensuremath{\Delta}{m}^{2}{=4.5}_{\ensuremath{-}1.4}^{+2.7}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}{\mathrm{eV}}^{2},{\mathrm{tan}}^{2}\ensuremath{\theta}{=0.40}_{\ensuremath{-}0.08}^{+0.10}.$ These parameters are in agreement with those ones of the best ${\ensuremath{\chi}}^{2}$ solution in the LMA region.

FERMION MASSES AND MIXINGS IN GAUGE THEORIES

The recent evidence for neutrino oscillations stimulate us to discuss again the problem of fermion masses and mixings in gauge theories. In the standard model, several forms for quark mass matrices are equivalent. They become ansatze within most extensions of the standard model, where also relations between quark and lepton sectors may hold. In a seesaw framework, these relations can constrain the scale of heavy neutrino mass, which is often related to the scale of intermediate or unification gauge symmetry. As a consequence, two main scenarios arise. Hierarchies of masses and mixings may be explained by broken horizontal symmetries.

Statistical analysis of differentν¯μ→ν¯esearches

A combined statistical analysis of the experimental results of the LSND and KARMEN ${\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}\ensuremath{\rightarrow}{\overline{\ensuremath{\nu}}}_{e}$ oscillation search is presented. LSND has evidence for neutrino oscillations that is not confirmed by the KARMEN experiment. This joint analysis is based on the final likelihood results for both data sets. A frequentist approach is applied to deduce confidence regions. At a combined confidence level of 36%, there is no area of oscillation parameters compatible with both experiments. For the complementary confidence of $1\ensuremath{-}0.36=64%,$ there are two well defined regions of oscillation parameters $({\mathrm{sin}}^{2}(2\ensuremath{\Theta}),\ensuremath{\Delta}{m}^{2})$ compatible with both experiments.

Global analysis of Solar neutrino oscillation evidence including SNO and implications for Borexino

An updated analysis of all available neutrino oscillation evidence in Solar experiments including the latest SNO data is presented. Predictions for total rates and day–night asymmetry in Borexino are calculated. Our analysis features the use of exhaustive computation of the neutrino oscillation probabilities and of an improved statistical χ 2 treatment. In the framework of two neutrino oscillations we conclude that the best fit to the data is obtained in the LMA region with parameters (Δm 2, tan 2θ)=(5.4×10 −5 eV 2,0.38) , ( χ 2 min/ n=0.81, n=38 degrees of freedom). Although less favored, solutions in the LOW and VAC regions are still possible with a reasonable statistical significance. The best possible solution in the SMA region gets a maximum statistical significance as low as ∼3%. We study the implications of these results for the prospects of Borexino and the possibility of discriminating between the different solutions. The expected normalized Borexino signal is 0.63 at the best fit LMA solution, where the DN asymmetry is negligible (approximately 10 −3). In the LOW region the signal is in the range ∼0.6–0.7 at 90% confidence level while the asymmetry is ≃1–20%. As a consequence, the combined Borexino measurements of the total event rate with an error below ±5–10% and day–night total rate asymmetry with a precision comparable to the one of SuperKamiokande will have a strong chance of selecting or at least strongly favoring one of the Solar neutrino solutions provided by present data.

Neutrinos that violate CPT, and the experiments that love them

Recently we proposed a framework for explaining the observed evidence for neutrino oscillations without enlarging the neutrino sector, by introducing CPT-violating Dirac masses for the neutrinos. In this Letter we continue the exploration of the phenomenology of CPT violation in the neutrino sector. We show that our CPT-violating model fits the existing SuperKamiokande data at least as well as the standard atmospheric neutrino oscillation models. We discuss the challenge of measuring CP violation in a neutrino sector that also violates CPT. We point out that the proposed off-axis extension of MINOS looks especially promising in this regard. Finally, we describe a method to compute CPT-violating neutrino effects by mocking them up with analog matter effects.

Neutrinos, Helioseismology, and Solar Structure.

There has been a long-standing discrepancy between the number of neutrinos expected from the sun and the number we actually detect. One possible way to account for this is that our theoretical solar model is inaccurate. However, recent progress in helioseismology has shown that the real sun is very close to the latest solar models. On the other hand, very recent experiments of neutrino detection provided us evidence for neutrino oscillation. I discuss what we should do and what we can do in this situation for the neutrino physics from the astrophysical side.

The MACRO detector at Gran Sasso

MACRO was an experiment that ran in the Laboratori Nazionali del Gran Sasso from 1988 to 2000. Its principal goal was to observe magnetic monopoles or set significantly lower experimental flux limits than had been previously available in the velocity range from about β=10 −4 to unity. In addition it made a variety of other observations. Examples are: setting flux limits on other so far unobserved particles such as nuclearites and lightly ionizing particles, searching for WIMP annihilations in the Earth and the Sun and for neutrino bursts from stellar collapses in or near our Galaxy, and making measurements relevant to high energy muon and neutrino astronomy and of the flux of up-going muons as a function of nadir angle showing evidence for neutrino oscillations. The apparatus consisted of three principal types of detectors: liquid scintillator counters, limited streamer tubes, and nuclear track etch detectors. In addition, over part of its area it contained a transition radiation detector. The general design philosophy emphasized redundancy and complementarity. This paper describes the technical aspects of the complete MACRO detector, its operational performance, and the techniques used to calibrate it and verify its proper operation. It supplements a previously published paper which described the first portion of the detector that was built and operated.

The neutrino factory: machine and detectors

The main features of a neutrino factory using storedmuon beams are discussed. The accumulating evidence forneutrino oscillations mean that there is a strongmotivation for further studies of neutrino mixing withmore intense, accelerator-generated beams. In particular,the probability is increasing that the large mixing angleMSW solution is the explanation of the solar neutrinodeficit observed over many years. This indicates that CP-or T-violation in the lepton sector may be experimentallyaccessible if sufficiently intense beams can beconstructed, in order to serve detectors with baselinesof several thousand kilometres. The neutrino factory, withits intense neutrino beams of well known energy spectrum,flux and flavour composition, satisfies the requirements.

Recent Results from the Palo Verde Neutrino Oscillation Experiment

The [Formula: see text] flux and spectrum have been measured at a distance of about 800 m from the reactors of the Palo Verde Nuclear Generating Station using a segmented Gd-loaded liquid scintillator detector. Results from the experiment show no evidence for neutrino oscillation. [Formula: see text] oscillation were excluded at 90% CL for Δm2>1.12×10-3 eV 2 for full mixing, and sin 2 2θ>0.21 for large Δm2. These results support the conclusion that the observed atmospheric neutrino oscillation does not involve νe.

Neutrino-lepton masses, Zee scalars, and muon g-2.

Evidence for neutrino oscillations points to the existence of tiny but finite neutrino masses. Such masses may be naturally generated via radiative corrections in models, such as the Zee model, where a singlet Zee scalar plays a key role. We minimally extend the Zee model by including a right-handed singlet neutrino nu(R). The radiative Zee mechanism can be protected by a simple U(1)(X) symmetry involving only the nu(R) and a Zee scalar. We further construct a class of models with a single horizontal U(1)(FN) (à la Froggatt-Nielsen) such that the mass patterns of the neutrinos and leptons are naturally explained. We then analyze the muon anomalous magnetic moment (g(mu)-2) and the flavor changing mu-->egamma decay. The nu(R) interaction in our minimal extension is found to induce the BNL g(mu)-2 anomaly, with a light charged Zee scalar of mass 100-300 GeV.

The Super-Kamiokande experiment

Super-Kamiokande has accumulated data on atmospheric neutrinos and solar neutrinos for 1117–1289 live days. We present updated results on the flavor ratio, the zenith angle distribution of atmospheric neutrinos and the zenith angle distribution of upward going muons. We also present results on the energy spectrum and the day–night variation of solar neutrinos. The data on atmospheric neutrinos show clear evidence of neutrino oscillation.

Neutrino mass and mixing measurements at Super-Kamiokande

The latest atmospheric and solar neutrino measurements from over 1100 live days of the Super-Kamiokande detector are presented. Atmospheric results continue to strongly support the hypothesis of ν μ oscillations via the measured deficit of μ - like interactions and the strong dependence of the deficit on neutrino flight distance. Analysis of the data gives best fit 2-component oscillation parameters (sin 2 2θ, δ m 2) = (1.00, 3.2 × 10 −3 ev 2) with χ 2/ DOF = 124.7/135. In contrast, the no-oscillation hypothesis gives χ 2/ DOF = 296.5/139. For solar neutrinos, no evidence of neutrino oscillations is seen in the spectral shape or expected day/night enhancement effect. The Small Mixing Angle (SMA) and Vacuum Oscillation (VO) allowed regions from flux deficit alone are excluded at the 95% c.l.

Neutrino masses and mixing from the astrophysical perspective

The role of neutrino masses and mixing in the astrophysical and cosmological context is reviewed. If neutrino mass differences are indeed as small as suggested by the current evidence for neutrino oscillations (Δ m ν ⪡ 1 eV), the overall neutrino mass scale in scenarios of degenerate neutrino masses is the most challenging open issue. Cosmological structure-formation arguments will continue to provide the most restrictive limits unless neutrinoless ββ experiments find positive evidence for a sub-eV mass scale. Neutrino Majorana masses in the range suggested by current oscillation experiments are fundamentally important in leptogenesis scenarios for creating the matter-antimatter asymmetry of the universe. The existence of a sterile neutrino, as suggested by the current evidence for neutrino oscillations, could affect big-bang nucleosynthesis as well as r-process nucleosynthesis in the neutrino-driven wind of type II supernovae. If the solar neutrino problem is explained by large-angle oscillations, a significant modification of the detectable signal characteristics of a future galactic SN is inevitable. The oscillation interpretation of the atmospheric neutrino anomaly suggests that the high-energy neutrino fluxes expected from cosmic beam dumps will have a large ν τ component.

MiniBooNE: Status of the booster neutrino experiment

MiniBooNE is preparing to search for v μ → v e oscillations at Fermilab. The experiment is designed to make a conclusive statement about LSND's neutrino oscillation evidence. We give a status report on the preparation of the experiment and outline the experimental prospects.

Results from the Palo Verde neutrino oscillation experiment

The ${\overline{\ensuremath{\nu}}}_{e}$ flux and spectrum have been measured at a distance of about 800 m from the reactors of the Palo Verde Nuclear Generating Station using a segmented Gd-loaded liquid scintillator detector. Correlated positron-neutron events from the reaction ${\overline{\ensuremath{\nu}}}_{e}\stackrel{\ensuremath{\rightarrow}}{p}{e}^{+}n$ were recorded for a period of 200 d including 55 d with one of the three reactors off for refueling. Backgrounds were accounted for by making use of the reactor-on and reactor-off cycles, and also with a novel technique based on the difference between signal and background under reversal of the ${e}^{+}$ and n portions of the events. A detailed description of the detector calibration, background subtraction, and data analysis is presented here. Results from the experiment show no evidence for neutrino oscillations. ${\overline{\ensuremath{\nu}}}_{e}\ensuremath{\rightarrow}{\overline{\ensuremath{\nu}}}_{x}$ oscillations were excluded at 90% C.L. for $\ensuremath{\Delta}{m}^{2}>1.12\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ ${\mathrm{eV}}^{2}$ for full mixing and ${\mathrm{sin}}^{2}2\ensuremath{\theta}>0.21$ for large $\ensuremath{\Delta}{m}^{2}.$ These results support the conclusion that the observed atmospheric neutrino oscillations do not involve ${\ensuremath{\nu}}_{e}.$

Neutrino physics during the next decade: Accelerator and atmospheric neutrino experiments

The current situation on experimental evidence for neutrino oscillations is briefly summarized. The planned experimental program to investigate both the LSND effect and atmospheric anomaly is described. A projection is made as to what we shall learn in the next ten years and what will be the most important issues at that time in neutrino oscillation physics.

Update on neutrino oscillation search with KARMEN

KARMEN, the Karlsruhe- Rutherford Medium Energy Neutrinoexperiment at the pulsed spallation neutron facility ISIS uses beam stop neutrinos ν μ, ν e and ν ̄ ν from π + and π + decay at rest to search for neutrino oscillations in the appearance channels ν μ → ν e and ν μ → ν e and in the disappearance channel ν e → ν x. The signature for the oscillations is based on charged current neutrino nuclear interaction spectroscopy in a high resolution 56t liquid scintillator calorimeter. This update on KARMEN oscillation results covers the new period of data acquisition (KARMEN 2) from February 1997 to February 1999 with the additional veto system installed in 1996 in operation. KARMEN 2 shows no evidence for neutrino oscillations and excludes an even larger region of the LSND ν μ → ν e oscillation parameter space than KARMEN 1.

Atmospheric neutrino flux: a review of calculations

Interpretation and understanding of the evidence for neutrino oscillations depends on knowledge of the atmospheric neutrino beam. In this talk I assess how well various features are known. The goal is to determine to what extent uncertainties in the neutrino beam may limit the conclusions about neutrino properties and which features of the evidence for neutrino oscillations are most robust.

Matter-affected neutrino oscillations in ordinary and mirror stars and their implications for gamma-ray bursts

It has been proposed that the annihilation process ν ν→e −e +→γγ may be responsible for the generation of gamma-ray bursts (GRBs). The incipient neutrino–antineutrino pairs carry virtually all of the gravitational binding energy available from the central engine. However, gamma-ray bursters proposed to date are inevitably surrounded by an excess of baryons, leading to the “baryon-loading problem”. In the light of growing evidence for neutrino oscillations, we discuss the implications of matter-affected oscillations for GRB energetics, and on the viability of “mirror” stars as GRB progenitors.