Neutrinos and Implications for Physics Beyond the Standard Model

CONTENTS

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I will summarize in four slides the 40 years of development of the standard solar model that is used to predict solar neutrino fluxes and then describe the current uncertainties in the predictions. I will dispel the misconception that the p-p neutrino flux is determined by the solar luminosity and present a related formula that gives, in terms of the p-p and ⁷Be neutrino fluxes, the ratio of the rates of the two primary ways of terminating the p-p fusion chain. I will also attempt to explain why it took so long, about three and a half decades, to reach a consensus view that new physics is being learned from solar neutrino experiments. Finally, I close with a personal confession and some personal remarks.

The solar neutrino results from 1496 days of measurement from Super-Kamiokande-I (SK-I) are reported. The accident at SK-I, rebuilding of SK-II detector, and some prospects for SK-II are also reported briefly.

The Sudbury Neutrino Observatory is a 1000 T D₂O Cerenkov detector that is sensitive to ⁸B and hep solar neutrinos. Both Charged Current and Neutral Current interaction rates on deuterons as well as the Elastic Scattering interaction rate on electrons can be measured simultaneously. Assuming an undistorted ⁸B neutrino spectrum, the total flux measured with the NC reaction is , which is consistent with solar models. The ν_e component of the ⁸B solar flux is for a kinetic energy threshold of 5 MeV. The non-ν_e component is , which is 5.3σ greater than zero, giving strong evidence for solar ν_e flavor transformation. The Day-Night Asymmetry for the Charged Current interaction is . If the total flux of active neutrinos is additionally constrained to have no asymmetry, the ν_e asymmetry is found to be . Combined with other solar neutrino data, a global MSW oscillation analysis strongly favors the Large Mixing Angle (LMA) solution.

Latest atmospheric neutrino results from 1489 days exposure of the Super-Kamiokande detector are presented. Our data are well explained by two flavor ν_μ ↔ ν_τ oscillations. In the three flavor oscillation case, the result shows no evidence for non zero θ₁₃. Other models, ν_τ – ν_s admixture and neutrino decay, are strongly constrained. Moreover, the appearance searches for tau neutrino interactions are performed. Our data are consistent with ν_τ appearance. For the nucleon decay, p → e⁺π⁰ and are searched for, but no evidence for nucleon decays has been found. Experimental lower limits for nucleon partial lifetime are obtained to be 5.7 × 10³³ years for p → e⁺π⁰ and 2.0 × 10³³ years for .

The 5.90 fiducial kiloton-year exposure of the Soudan-2 iron tracking calorimeter is analyzed for effects of neutrino oscillations. Using fully contained single track, single shower and multiprong events, we determine the atmospheric ν_μ/ν_e ratio-of-ratios in the sub-GeV E_ν regime to be R = 0.68±0.12±0.06. Assuming this anomalously low R-value to be the result of ν_μ flavor disappearance via ν_μ → ν_τ oscillation, we distribute fully contained and partially contained events into bins of log(L/E_ν) and fit to oscillation hypotheses using extended maximum likelihood. The region in the (sin² 2θ, Δm²) plane allowed at 90% CL is obtained using the Feldman-Cousins procedure: 0.52 < sin² 2θ ≤ 1.0 and 3 × 10⁻⁴ < Δm² < 2 × 10⁻²eV².

A different oscillation process which could occur in our contained event samples is baryon instability in the form of neutron-antineutron oscillations. Our search requires candidate occurrences to have ≥ 4 prongs (tracks and showers) and to have kinematics compatible with annihilation within a nucleus. We observe five candidate events, with estimated atmospheric neutrino plus cosmic ray backgrounds totalling 4.5 ± 1.2 events. Previous experiments with smaller exposures observed no candidates, with estimated background rates similar to this experiment. We set a lifetime lower limit at 90% CL for the oscillation time in iron: T_A(Fe) > 7.2 × 10³¹ years. The corresponding lower limit for oscillation of free neutrons is seconds.

The NuTeV collaboration has performed precision measurements of the ratio of neutral current to charged current cross-sections in high rate, high energy neutrino and anti-neutrino beams on a dense, primarily steel, target. The separate neutrino and anti-neutrino beams, high statistics, and improved control of other experimental systematics, allow the determination of electroweak parameters with significantly greater precision than past νN scattering experiments. Our null hypothesis test of the standard model prediction measures , a value which is 3.0σ above the prodiction. We discuss possible explanations for and implications of this discrepancy.

The MINOS experiment will study muon neutrino oscillations in the range of parameter space implied by the measurements with atmospheric neutrinos. The two magnetized toroidal tracking calorimeters, 1 kton at Fermilab and 5.4 kton in the Soudan Mine 735-km away, are constructed out of extruded scintillator strips and use wavelength-shifting and clear fibers read out by multi-anode photomultipliers. The far detector is now more than 3/4 completed and commenced collecting cosmic ray data. The neutrino beamline and the near detector will be finished by the end of 2004. In addition, a calibration detector was also constructed and exposed to the test beams. Over the last two years the entire project has progressed on schedule and budget and with most of the civil construction out of the way the road to this interesting physics program is finally cleared.

This paper reviews the current status and the physics program of the OPERA experiment with the future CNGS long baseline neutrino beam from CERN to the Gran Sasso Laboratory. The expected background, the expected number of signal events at the atmospheric Δm² scale and the sensitivity for both ν_μ ↔ ν_τ and ν_μ ↔ ν_e oscillations are given.

Borexino is a real-time solar neutrino detector under commissioning at the Gran Sasso underground laboratories (Italy). The main goal of the experiment is the spectroscopy of sub-MeV solar neutrinos focusing on the measurement of the ⁷Be neutrino flux on Earth. In order to achieve such a goal Borexino has developed a new technology for large-scale low-count rate experiments. This paper reviews the status of the experiment and the strategy adopted to reach a high level of radiopurity. Besides its primary task Borexino will also be able to search for SuperNova neutrinos, non-standard neutrinos properties and neutrino from the Earth.

Current constraints on neutrino mass and mixing parameters are briefly reviewed, and the prospects for future measurements in long-baseline neutrino experiments are discussed. Parameter degeneracies are a generic problem in the three-neutrino analysis of long-baseline neutrino appearance measurements, and can lead to different inferred values for the neutrino mixing angle θ₁₃ and often mix CP violating and CP conserving solutions. Possible experimental strategies for reducing or eliminating such degeneracies and/or the CP confusion are discussed.

Future long baseline neutrino oscillation (LBL) setups are discussed and the remarkable potential for very precise measurements of mass splittings, mixing angles, MSW effects, the sign of Δm² and leptonic CP violation is shown. Furthermore we discuss the sensitivity improvements which can be obtained by combining the planned JHF-Superkamiokande and the proposed NuMI off-axis experiment.

In this talk I discuss the problem of accounting for light neutrino masses in theories with dynamical electroweak symmetry breaking. I will first describe this problem generally in a class of extended technicolor (ETC) models, describing the full set of Dirac and Majorana masses that arise in such theories. I will then present an explicit model exhibiting a combination of suppressed Dirac masses and a seesaw involving dynamically generated condensates of standard-model singlet, ETC-nonsinglet fermions. Because of the suppression of the Dirac neutrino mass terms, a seesaw yielding realistic neutrino masses does not require superheavy Majorana masses; indeed, the Majorana masses are typically much smaller than the largest ETC scale.

Some features of SO(10) GUT models are reviewed, and a number of such models in the literature are compared. While some have been eliminated by recent neutrino data, others are presently successful in explaining the quark and lepton mass and mixing data. A short description of one very predictive model is given which illustrates some of the features discussed. Future tests of the models are pointed out including one which contrasts sharply with those models based on an L_e – L_µ – L_τ type symmetry.

Encouraged by the phenomenological success of the tri-bimaximal hypothesis, we postulate that the neutrino mass matrix in the lepton flavour basis is an S3 group matrix in the natural representation of S3. This immediately requires one neutrino to be trimaximally mixed, as suggested by the solar neutrino data. We go on to postulate that the charged-lepton mass matrix in the neutrino mass-basis is an S3 class matrix in the natural representation of the S3 class-algebra, leading to exact tri-bimaximal mixing which is compatible with data overall. The above two postulates are mutually consistent, and imply that the neutrino mass matrix in the flavour basis is an S3 ⊃ S2 class operator, in the natural representation of the S3 group (the S2 being associated with mu-tau interchange). Thus the tri-bimaximal mixing matrix is seen to be closely related to the S3 group characters, and may be properly regarded as simply the table of induction coefficients for the [2] × [1] = [3] + [21] induced representation of S3.

A brief review is given of some ideas for explaining neutrino masses and mixings within the context of supersymmetric grand unification. Emphasis is put on so-called lopsided models.

The present atmospheric neutrino oscillation data viewed within the context of see-saw mechanism for small neutrino masses, requires right handed neutrino masses (M_R) around the conventional GUT scale i.e. 10¹⁵ to 10¹⁶ GeV. This raises a new hierarchy puzzle i.e. why M_R ≪ M_Pl? This can be interpreted as an indication in favor of new local symmetries around M_R. In this paper, we argue that depending on the nature of neutrino mass patterns, this symmetry could either be: (i) the local B-L symmetry arising from a larger symmetry group such as in the left-right symmetric model of weak interactions, grand unified SO(10) models etc. or (ii) a local horizontal symmetry group SU(2)_H operating on the fermions of the first two generations which is often invoked to understand the generation puzzles. We discuss some simple consequences of these symmetries and propose experimental tests for these ideas.

We discuss some implications of models with large extra dimensions, including effects on scattering cross sections, fermion masses and mixings, oscillations, and ideas on “half-strings”.

Alternatives to the traditional grand unified theory seesaw for neutrino masses are briefly described. These include the possibility of large extra dimensions and various possibilities for models involving an extra U(1)′ gauge symmetry. The difficulty of observing Majorana phases in neutrinoless double beta decay is also briefly commented on.

The field of neutrino oscillation physics is changing dramatically from one of exploration of an unexpected phenomenon to real measurements of the leptonic mixing and mass matrices. As the purposes of oscillation experiments change the detector and beamline requirements also change, such that future experiments are not simple extensions of the current round of experiments. This report discusses what the challenges in the next step are, what beamline techniques have been suggested to meet these challenges, and finally, what the detector possibilities are for precision measurments.

We analyze the prospects of a feasible, very long baseline neutrino oscillation experiment consisting of a conventional horn produced low energy wide band beam and a detector of 500 kT fiducial mass with modest requirements on event recognition and resolution. Such an experiment is intended primarily to measure CP invariance parameters in the neutrino sector. We analyze the sensitivity of such an experiment. The conclusion is that it will allow determination of the CP parameter δ_CP, if the currently unknown mixing parameter sin² 2θ₁₃ ≥ 0.01, a value about 10 times lower than the present experimental upper limit. Such an experiment has great potential for precise measurements of all parameters in the neutrino mixing matrix including , sin² 2θ₂₃, , and the mass ordering of neutrinos through the observation of the matter effect in the ν_μ → ν_e appearance channel.

A next generation water Cherenkov detector at Kamioka with a total mass of ~1 Mton is called Hyper-Kamiokande. If the beam intensity of a 50-GeV proton synchrotron now under construction at Tokai is upgraded to 4 MW in future, a long baseline neutrino oscillation experiment with Hyper-Kamiokande as a far detector will open the possibility of measuring CP violation in the neutrino sector. Also, Hyper-Kamiokande will allow to extend nucleon decay search to τ_p/B(p → e⁺π⁰) > 10³⁵ yr and yr. Aiming at the realization of Hyper-Kamiokande, various R&D efforts are in progress.

Neutrinos offer a particularly promising eye on the extreme Universe. Neutrinos are not attenuated by intervening radiation fields such as the Cosmic Microwave Background, and so they are messengers from the very distant and very young phase of the universe. Also, neutrinos are not deflected by cosmic magnetic fields, and so they should point to their sources. In addition, there are particle physics aspects of neutrinos which can be tested only with cosmic neutrino beams. After a brief overview of highest-energy cosmic ray data, and the present and proposed experiments which will perform neutrino astronomy, we discuss two particle physics aspects of neutrinos. They are possible long-lifetime decay of the neutrino, and a measurement of the neutrino-nucleon cross-section at a CMS energy orders of magnitude beyond what can be achieved with terrestrial accelerators. Measurement of an anomalously large neutrino cross-section would indicate new physics (e.g. low string-scale, extra dimensions, precocious unification), while a smaller than expected cross-section would reveal an aspect of QCD evolution. We then discuss aspects of neutrino-primary models for the extreme-energy (EE) cosmic ray data. Primary neutrinos in extant data are motivated by the directional clustering at EE reported by the AGASA experiment. We discuss the impact of the strongly-interacting neutrino hypothesis on lower-energy physics via dispersion relations, the statistical significance of AGASA directional clustering, and the possible relevance of the Z-burst mechanism for existing EE cosmic ray data.

The ultrahigh energy neutrino cross section is well understood in the standard model for neutrino energies up to 10¹² GeV. Tests of neutrino oscillations (v_µ ↔ ν_τ) from extragalactic sources of neutrinos are possible with large underground detectors. Measurements of horizontal air shower event rates at neutrino energies above 10¹⁰ GeV will be able to constrain nonstandard model contributions to the neutrino-nucleon cross section, e.g., from mini-black hole production.

The recent neutrino oscillation experimental results indicate that at least one neutrino has a mass greater than 50 meV. The next generation of double-beta decay experiments will very likely have a sensitivity to an effective Majorana neutrino mass below this target. Therefore this is a very exciting time for this field of research as even null results from these experiments have the potential to elucidate the nature of the neutrino.

Double beta decay is indispensable to solve the question of the neutrino mass matrix together with ν oscillation experiments. Recent analysis of the most sensitive experiment since nine years - the HEIDELBERG-MOSCOW experiment in Gran-Sasso - yields a first indication for the neutrinoless decay mode. This result is the first evidence for lepton number violation and proves the neutrino to be a Majorana particle. We give the present status of the analysis in this report. It excludes several of the neutrino mass scenarios allowed from present neutrino oscillation experiments - only degenerate scenarios and those with inverse mass hierarchy survive. This result allows neutrinos to still play an important role as dark matter in the Universe. To improve the accuracy of the present result, considerably enlarged experiments are required, such as GENIUS. A GENIUS Test Facility has been funded and will come into operation by early 2003.

Dramatic progress has been made in the last several years in our understanding of the properties of neutrinos with evidence for neutrino flavor transformation coming from measurements of atmospheric neutrinos by SuperKamiokande, of solar neutrinos by the Sudbury Neutrino Observatory (SNO), and of reactor neutrinos by KamLAND. These results are a step in the ongoing program of science that is carried out in underground laboratories. The potential for additional significant discoveries with new capabilities in underground laboratories exists and should be exploited. Discoveries are likely to be made not only in nuclear and particle physics, but also in astrophysics, geophysics, and geobiology. A concerted effort is now underway in the United States to create a National Underground Science and Engineering Laboratory (NUSEL) that would provide the facilities and infrastructure necessary to capitalize on the opportunities presented by underground science.

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