Neutrino Oscillations and Their Origin

The following sections are included:

• Preface

• Program Committee

• Organizing Committee

• Contents

We study primordial nucleosynthesis in the presence of a net lepton asymmetry. We explore a previously unnoted region of the parameter space in which very large baryon densities 0.1 ≤ Ω_b ≤ 1 can be accommodated within the light-element constraints. This parameter space consists of large ν_μ and ν_τ degeneracies with a moderate ν_e degeneracy. Constraints on this parameter space from cosmic microwave background fluctuations are discussed ¹. We also study the r-process nucleosynthesis in neutrino-driven winds of gravitational core collapse SNeII. Appropriate physical conditions are found for successful r-process nucleosynthesis, which meet with several features of heavy elements discovered recently in metal-deficient halo stars.

Using 1258 days of solar neutrino-electron scattering data, the ⁸B neutrino flux, the spectrum of the recoiling electrons and time-variations of the flux have been measured between 5 MeV and 20 MeV. The measured flux is or of the flux predicted by the standard solar model. No significant spectral distortions from the standard solar model or anomalous time-variations have been found. This limits neutrino mixing and mass difference in a flux-independent way. Combining flux-measurement, spectrum and daily variations of the flux, two allowed areas remain for two-neutrino oscillations at 95% C.L.

The S-factor for the ³He(p,e⁺ν_e)⁴He reaction has been recently calculated using realistic interactions and currents. The present talk summarizes the main results of that calculation.

We made a series of experiments at RIKEN for the Coulomb dissociation of ⁸B, which is related to the solar-neutrino production reaction ⁷Be(p,γ)⁸B. This technique has been developed recently to investigate radiative capture processes of astrophysical interest. Its high experimental efficiency enables one to study the systems involving unstable nuclei, which are now available as beams but with relatively weak intensities. The results are compared with those of direct capture measurements and "ANC" determination by nucleon transfers.

With the constraints of the sound speed and the density profiles determined with helioseismology, together with the updated microphysics, we construct a seismic solar model, which has the advantages over the standard evolutionary solar models and is faithfully consistent with all observations except for the neutrino fluxes. We discuss the theoretically expected neutrino fluxes of this most faithful solar model.

We have measured absolute fluxes of primary protons, helium nuclei and atmospheric muons with the BESS spectrometer. Precise measurement of these cosmic-ray particles is indispensable for improving the accuracy in the atmospheric neutrino calculations.

An upgrade of the BESS detector is being carried out to improve the rigidity resolution of the spectrometer, and precise measurements of energy spectra of the primary protons and helium nuclei is to be extended up to around 1 TV, which should be crucial for further study of the oscillation of the atmospheric neutrino. New outer drift chambers (ODCs) will be installed at upper- and lower-most ends of the BESS detector to maximize the deflection angle resolution with longer track path of the incident particle. A complex of a jet-type drift chamber and two cell-type inner drift chambers, placed in the bore of the solenoid, has also been developed to get better spatial resolution and more sampling points compared to the current drift chambers. The new drift chamber system is now under construction and will be integrated into the BESS spectrometer in next year. The next flight of the BESS spectrometer is to be scheduled in September 2001 at Fort Sumner, New Mexico.

In June 1998 the Alpha Magnetic Spectrometer (AMS) was flown on the space shuttle Discovery. The cosmic ray spectra in the energy range 0.1 to 200 GeV for protons, 0.2 to 40 GeV for e^- , 0.2 to 3 GeV for e⁺ and 0.1 to 100 GeV/nucleon for helium nuclei were measured. Two distinct spectra were observed: a primary spectrum and a substantial second spectrum of particles with "forbidden trajectories". Tracing particles from the second spectrum shows that the positive ( p and e⁺) and negative (e^-) particles originate from two complementary geographic regions. Also observed helium second spectrum was found to be almost pure ³He.

We have carried out balloon observations of atmospheric gamma-rays in the GeV region to calibrate our simulation calculation for the neutrino flux. The detector is an imaging calorimeter composed of scintillating fibers and lead plates with an anti-trigger system to reject the backgrounds of charged particles. The balloon flights were successfully done in 1999 and 2000 at an altitude from 15 km to 25 km. It is confirmed that new simulation code of nuclear interactions. Lund Fritiof V7.02. might give fairly consistent results with the observed energy spectra of gamma-rays and the altitude variation.

We have been observing the primary electron spectrum from 30 GeV to 3 TeV and the atmospheric gamma-ray spectrum from 30 GeV to 8 TeV for many years with the emulsion chamber at balloon altitude. Atmospheric gamma rays observed at high altitude of several g/cm² are produced by almost a single interaction of primary cosmic rays. We can reliably estimate the flux of muons without referring to the primary flux or hadronic interaction model, assuming that charged pions are produced almost twice neutral pions. Also we can estimate the primary flux of cosmic rays, if we refer to an appropriate hadronic model. Proton spectrum estimated by our observations covers from 400 GeV to 15 TeV, filling a gap in the currently observed proton spectrum.

Recently accurate measurements of primary cosmic ray, especially that of BESS and AMS, becomes available. Also, the atmospheric muon flux is measured by BESS group in a good accuracy at several altitudes. Using both accurately measured spectrums, We examine the calculation of the atmospheric neutrino flux, and present a strategy to improve it.

This contribution is a critical discussion of the systematic uncertainties in the calculation of the atmospheric neutrino event rates, and on their effect in the interpretation of the data. The conclusion that the observed "disappearance" of ν_μ and events requires the existence of some form of "new physics" beyond the standard model appears to be robust.

Data from a 79 kiloton-yr (1289 live-day) exposure of Super-Kamiokande to the atmospheric neutrino flux are discussed. The data are consistent with two flavor oscillations with sin² 2θ0.88 and with 1.6×10^-3 eV²<Δm²<4 × 10^-3 eV² Two flavor ν_μ↔ν_τ oscillation scenario is favored over ν_μ↔ν_s. We find that a sample selected to be consistent with ν_τ CC interaction topologies is consistent with the expected ν_τ appearance from ν_μ↔ν_τ oscillations.

The ICARUS T600 is a self-contained experimental programme, with significant physics potentialities, though with a strong connotation of technological development in view of the operability of a large mass liquid Argon "electronic bubble chamber" inside the Gran Sasso underground laboratory. Main physics issues are observations, with unique experimental features, of neutrinos from various sources and possibly of proton decay.

As a major achievement, the completion of the detector construction has been reached at the end of the year 2000.

A full test of the T600 experimental set-up, from the cryogenics and electronics performance up to detection of cosmic ray tracks, is now foreseen to take place in an external site, during the first months of the year 2001.

A second important achievement of the ICARUS activity is represented by the successful long-term test run of a large scale prototype (14 t of LAr), operated for the first time at the Gran Sasso Laboratory, during the year 2000: this test aimed to the detection and reconstruction of cosmic ray events and concluded the R&D programme of the Collaboration, opening the way for the T600 installation at the Gran Sasso Laboratory, expected by the end of the year 2001.

Various solutions of the atmospheric neutrino data are reviewed. Apart from orthodox two flavor ν_μ ↔ ν_τ oscillations and three flavor oscillations, there are still possibilities, such as four flavor oscillations with the (2 + 2)- and (3+1)- schemes, a neutrino decay scenario and decoherence, which give a good fit to the data.

We study the optimal setup for observation of the CP asymmetry in neutrino factory experiments — the baseline length, the muon energy and the analysis method. First, we point out that the statistical quantity which has been used in previous works doesn't represent the CP asymmetry. Then we propose the more suitable quantity, , which is sensitive to the CP asymmetry. We investigate the behavior of with ambiguities of the theoretical parameters. The fake CP asymmetry due to the matter effect increases with the baseline length and hence the error in the estimation of the fake CP asymmetry grows with the baseline length due to the ambiguities of the theoretical parameters. Namely, we lose the sensitivity to the genuine CP-violation effect in longer baseline.

The K2K experiment is the first accelerator-based long baseline neutrino experiment aiming to confirm the evidence of neutrino oscillation found in the atmospheric neutrino observation. The ν_μ beam of ~1 GeV is produced at KEK and observed by Super-Kamiokande (SK) at 250 km distance. So far 2.3% of proposed number of protons on target is accumulated. We observed 28 neutrino events in SK which originate at KEK. Expected number of events are . As a next generation experiment, we plan a neutrino oscillation experiment from 50 GeV PS in Japan Hadron Facility, which is approved last year, to SK at 295 km distance. Using the ~MW PS, we expect about 2 orders of magnitude higher neutrino flux than K2K. Current design of the neutrino facility and its physics potential are introduced.

Neutrino oscillations provide an unique opportunity to probe physics beyond the Standard Model. Recent experimental results, especially the observation of the disappearance of ν_μ neutrinos as a function of the zenith angle in the SuperK experiment, provide a very strong indications that neutrinos do have a mass and undergo oscillations. Fermilab is constructing a new neutrino beam and detectors to provide study possible neutrino oscillations in the range of Δm² indicated by the SuperK results.

A sequence of hadroproduction experiments, mainly but not exclusively motivated by the needs of neutrino physics, is in progress at CERN. The results of the SPY experiment, now completed, are reviewed, along with their impact on our knowledge of neutrino fluxes produced by the CERN SPS. The prospects of the HARP experiment, in preparation, are then summarized. Its data are expected to have a major impact on the design of Neutrino Factories and on our knowledge of atmospheric neutrino fluxes. Some of its data will also be of interest for the K2K and for the FNAL Booster neutrino beams. Possible extensions of the program to higher proton momenta are also briefly mentioned.

We have advocated a new approach to build models of fermion masses and mixings, namely anarchy. The approach relies only on the approximate flavor symmetries, and scan the O(1) coefficients randomly. The randomness in O(1) coupling constants is indeed what one expects in models which are sufficiently complicated or which have a large number of fields mixed with each other. Assuming there is no physical distinction among three generations of neutrinos, the near-maximal mixings, as observed in the atmospheric neutrino data and as required in the LMA solution to the solar neutrino problem, are highly probable.

Two-loop radiative neutrino mechanism combined with L_e - L_μ - L_τ (≡ L′)-conservation provides dynamical generation of maximal atmospheric neutrino mixing and yields , which explains because of m_e/m_τ ≪ 1, where ε measures the breaking of the L′-conservation. This estimate yields for , which covers the LOW, QVO and VO solution to the solar neutrino problem for ε ≲ 0.1.

The search for possible mixing patterns of charged leptons and neutrinos is important to get clues of the origin of nearly maximal mixings, since there are some preferred bases of the lepton mass matrices given by underlying theories. We systematically examine the mixing patterns which could lead to large lepton mixing angles. We find out 37 mixing patterns are consistent with experimental data if taking into account phase factors in the mixing matrices. Only 6 patterns of them can explain the observed data without any tuning of parameters, while the others need particular choices for phase values.

[1] Introduction ; [2] Why is U(1)_X interesting? ; [3] Why is GUT so attractive? ; [4] Family Quantum Number Assignment ; [5] Mystery of Neutrino Masses and Mixings ; [6] Neutrino Mass Matrix and Family Structure

Double beta decay is indispensable to solve the question of the neutrino mass matrix together with ν oscillation experiments. The most sensitive experiment since eight years — the HEIDELBERG-MOSCOW experiment in Gran-Sasso — already now, with the experimental limit of 〈m_ν〉 < 0.26 eV excludes degenerate ν mass scenarios allowing neutrinos as hot dark matter in the universe for the small angle MSW solution of the solar neutrino problem. It probes cosmological models including hot dark matter already now on the level of future satellite experiments MAP and PLANCK. It further probes many topics of beyond Standard Model physics at the TeV scale. Future experiments should give access to the multi-TeV range and complement on many ways the search for new physics at future colliders like LHC and NLC. For neutrino physics some of them (GENIUS) will allow to test almost all neutrino mass scenarios allowed by the present neutrino oscillation experiments. A GENIUS Test Facility has just been funded and will come into operation by end of 2001.

I discuss some possible influence of neutrino oscillations for supernova neutrinos mainly focusing on the observations of signal, and present some analysis of SN1987A data in the light of three neutrino mixing scheme.

A new μ → eγ experiment, which is currently being built at the Paul Scherrer Institute (PSI) in Switzerland, is described The goal of this experiment is to measure the lepton flavor violating μ → eγ decay with a sensitivity of 10^-14. A new detector will be installed at PSI using a stopped muon beam with a stopping rate of 10⁸/s. The detector will consist of a liquid xenon photon detector and a set of drift chambers contained in a 1.2T superconducting solenoidal magnet. After a short introduction to the physics motivation, details of the detector and electronics will be described

Possible experiments to search for muon lepton flavor violation at the high intensity proton machine of the KEK/JAERI Joint Project are presented. In particular, the PRISM project which is to construct a high intensity muon source with narrow beam energy spread and less beam contamination is shown. A possible experiment to search for μ - e conversion in a muonic atom with PRISM is also discussed.

We calculate the μ → eγ branching fraction in the Randall-Sundrum model with the Grossman-Neubert bulk neutrinos. The bulk neutrinos are introduced in order to produce the tiny Dirac masses for the ordinary neutrinos. In the presence of such bulk neutrinos, the Kaluza-Klein modes mediate the μ → eγ decay. We show that the experimental bound gives a stringent constraint on the KK mode masses, i.e. m_KK > 25 TeV.

We present a GUT-model-independent calculation of hadron matrix elements for all dimension-six operators associated with baryon number violating processes using lattice QCD. Our results cover all the matrix elements required to estimate the partial lifetimes of decay modes. We point out the necessity of disentangling two form factors that contribute to the matrix elements; previous calculations did not make the separation, which led to an underestimate of the physical matrix elements. With a correct separation, we find that the matrix elements have values 3 – 5 times larger than the smallest estimates employed in phenomenological analyses of the nucleon decays, which gives stronger constraints on GUT models. We also find that the values of the matrix elements are comparable with the tree-level predictions of chiral Lagrangian.

Water Cherenkov detectors with total mass of ~ 1 Mton or more are candidates for next generation nucleon decay detectors. For the p → e⁺π⁰ decay, they have sensitivities to the partial lifetime of > 10³⁵ yr. They may also be sensitive to the decay up to the partial lifetime of > 10³⁴ yr, depending on photocathode coverage. As a specific example, Hyper-Kamiokande, which is proposed as a successor of Super-Kamiokande at the Kamioka Observatory, is described in some detail, where not only nucleon decay search but also other physics possibilities are discussed. Some other ideas for the next generation water Cherenkov detectors are also mentioned. The sensitivities of a large water Cherenkov detector having structure similar to Super-Kamiokande are studied for the p → e⁺π⁰ decay mode, assuming 40%, 10%, and 4.4% photocathode coverages, with the same or similar selection criteria as the Super-Kamiokande analysis and with a tighter total momentum cut.

The following sections are included:

• Introduction

• Current Status of the Proton Decay Life Time Measurements

• Sensitivity of the Future Proton Decay Experiment

• Underground vs Underwater

• The Multi-Megaton Water Cherenkov Detector—TITAND

• Construction of the Detector

• Other Physics Opportunities

• References

The following sections are included:

• List of Participants

• Scientific Programme