abstract

The innermost secrets of the mysterious neutrino are being revealed in underground detectors. Recent data on atmospheric neutrinos, primarily from the Super-Kamiokande experiment, confirm the neutrino flavor mixing and non-zero masses. The high precision measurement of angular distribution allows to determine the value of \(\Delta m^2\) between \(0.0013\) eV\(^2\) and \(0.0054\) eV\(^2\) at 90% c.l. Studies of up–down asymmetries in different event samples indicate that \(\nu _{\mu }\leftrightarrow \nu _{\tau }\) oscillations are more likely explanation of the data than \(\nu _{\mu }\leftrightarrow \nu _{s}\). The deficit of the observed solar neutrino flux compared to the predictions of the standard solar model, often interpreted by neutrino oscillations, is further studied in the SuperKamiokande detector. The energy spectrum is measured above 5.5 MeV for the Sun’s positions above and below the horizon. A day–night effect is observed at a statistical significance of 2 \(\sigma \). The K2K (KEK to Kamioka) is the first long-baseline neutrino-oscillation experiment. During runs in 1999 the first 3 events were observed in the SuperKamiokande detector caused by neutrinos produced at the KEK accelerator at a distance of 250 km. The predicted number of events without any oscillations is 12.3\(^{+1.7}_{-1.9}\).

abstract

Tritium \(\beta \) decay experiments are currently running in Mainz and Troitsk. Both experiments are investigating the endpoint region of the tritium \(\beta \) decay spectrum with a MAC-E spectrometer to determine the mass of the electron antineutrino. By the recent upgrade at Mainz the former problem of dewetting \(T_2\) films has been solved and the signal-to-background-ratio was improved by a factor of 10, resulting in a sensitivity similar to Troitsk. The latest Mainz measurement in 1998 leads to \(m_\nu ^2 = -3.7 \pm 5.3_{\rm stat} \pm 2.1_{\rm sys}\) eV\(^2/c^4\), from which an upper limit of \(m_\nu \lt 2.8\) eV/\(c^2\) (95% C.L.) is derived. Some indication for the anomaly, reported by the Troitsk group, was found by the Mainz group, but the postulated half year period of the anomaly is contradicted by Mainz. To push the sensitivity on the neutrino mass below 1 eV/\(c^2\) a new larger MAC-E spectrometer is proposed. Besides its integrating mode it could run in a new non-integrating operation MAC-E-TOF mode.

abstract

We discuss models of neutrino masses that, in the context of the see-saw mechanism, could lead to a large mixing angle for the atmospheric neutrino oscillations without requiring too much fine-tuning between the Dirac and the Majorana sectors. These models are compatible with Abelian flavour symmetries and with the picture of flavour expected in grand unified theories.

abstract

The important role of neutrino induced reactions on nuclei at low and intermediate energies both in accelerator-based experiments in Neutrino Physics and in Neutrino Astrophysics is discussed. After a short description of the theoretical nuclear model we present selected applications to various neutrino experiments. We will focus on the sensitivity of neutral current neutrino scattering to the strangeness content of the nucleon and on the calculation of neutrino induced reactions on \(^{56}\)Fe and \(^{208}\)Pb, which have been discussed as target materials in future neutrino detectors.

abstract

We discuss neutrino thresholds in the light of the recent experimental data, which indicates the presence of neutrino oscillations. In particular, we analyse the effects of radiative corrections on the stability of neutrino textures and discuss how Yukawa unification is also affected. Renormalisation induced lepton flavour violating processes from non-zero neutrino masses are also studied.

abstract

We discuss a problem concerning the ultrahigh energy neutrino propagation through the Earth. We present calculation of the neutrino–nucleon cross section at high energies, based on the unified evolution equation at small \(x\). We also show the solution of the transport equation for different neutrino fluxes originating from active galactic nuclei, gamma ray bursts and top-down model.

abstract

A \(\nu _\mu \) disappearance effect has been seen in atmospheric neutrino experiments. This has led to the “evidence for neutrino oscillations”. The next problem in neutrino physics is to perform the right experiment(s) to elucidate in a comprehensive way the pattern of neutrino masses and mixings. The long baseline experiments will play a fundamental role at settling definitively the question of flavor oscillation and at measuring with good precision the oscillation parameters. The CERN–NGS beam coupled with the proposed ICANOE and OPERA detectors is the only programme capable of sensitive tau and electron appearance searches.

abstract

Neutrino oscillations provide an unique opportunity to probe physics beyond the Standard Model. Fermilab is constructing two new neutrino beams to provide a decisive test of two of the recent positive indications for neutrino oscillations: MiniBOONE experiment will settle the LSND controversy, MINOS will provide detailed studies of the region indicated by the SuperKamiokande results.

abstract

The enthusiasm for building a muon storage ring as an intense source of neutrinos is growing rapidly world-wide. This paper will describe how such a neutrino facility, called a neutrino factory, might be built and outline the physics that could be done with it. The status of related R&D projects in Europe and the US will also be discussed.

abstract

We propose a phenomenological model of lepton mixing and CP violation based on the flavor democracy of charge leptons and the mass degeneracy of neutrinos. A nearly bi-maximal flavor mixing pattern, which is favored by current data on atmospheric and solar neutrino oscillations, emerges naturally from this model after explicit symmetry breaking. The rephasing-invariant strength of CP or T violation can be as large as one percent, leading to significant probability asymmetries between \(\nu _\mu \rightarrow \nu _e\) and \(\bar {\nu }_\mu \rightarrow \bar {\nu }_e\) (or \(\nu _e \rightarrow \nu _\mu \)) transitions in the long-baseline neutrino experiments. The possible matter effects on CP- and T-violating asymmetries are also taken into account.

abstract

The neutrinoless double beta decay as well as any other laboratory experiment has not been able to answer the question of the neutrino’s nature. Hints on the answer are available when neutrino oscillations and \((\beta \beta )_ {0 \nu }\) are considered simultaneously. In this case phenomenologically interesting neutrino mass schemes can lead to non-vanishing and large values of \(\left \lt m_{\nu } \right \gt \). As a consequence, some schemes with Majorana neutrinos can be ruled out even now. If we assume that in addition neutrinos contribute to Hot Dark Matter then the window for Majorana neutrinos is even more restricted, e.g. GENIUS experiment will be sensitive to scenarios with three Majorana neutrinos.

abstract

After summarizing principles of solar model construction and presenting an updated prediction for the neutrino counting rates, I focus this review on the question of reliability of current models. Methods and results of seismic sounding of the solar interior are presented in some detail. The results confirm the standard scenario of the solar evolution. This conclusion, combined with the evidences for neutrino oscillations, means the end of astrophysical aspect of the solar neutrino problem. The models of the Sun interior remain important for interpretation of the data from the neutrino detectors but the data cannot be used to contradict the models, not even to constrain them.

abstract

The origin of extremely high energy cosmic rays is still unknown and is one of most important puzzles in astrophysics. Identifying neutrinos among cosmic rays at the extreme energies would be a major step toward explaining the origin of cosmic rays. In this paper, the cosmic ray research at highest energies is outlined and the role of neutrinos in cosmic ray astrophysics is discussed. The prospects to detect extremely high energy neutrinos in the Pierre Auger Observatory are presented.