[en] Double Chooz is a reactor neutrino experiment which investigates the last neutrino mixing angle; θ₁₃. It is necessary to measure reactor neutrino disappearance with precision 1% or better to detect a finite value of θ₁₃. This requirement is the most strict compared to other reactor neutrino experiments performed so far. The Double Chooz experiment makes use of a number of techniques to reduce the possible errors to achieve the sensitivity. The detector is now under construction and it is expected to take first neutrino data in 2009 and to measure sin²2θ₁₃ with a sensitivity of 0.03 (90%C.L.) In these proceedings, the technical concepts of the Double Chooz detector are explained stressing on how it copes with the systematic errors.

[en] Despite its extremely small reaction probability and difficulty of its detection, the reactor neutrinos have been playing a significant roles in elementary particle physics since its early days. Recently, new reactor neutrino oscillation experiments, which aim to discover the last neutrino mixing angle θ₁₃, are being planned worldwide. These experiments are complementary to accelerator based neutrino oscillation experiments and by combining both experiments, precious measurements will become possible. In this article, significance of these new-generation reactor-neutrino experiments, as well as recent world activities are described. (author)

[en] Positron annihilation processes in a liquid scintillator to be used for the detection of electron antineutrinos in the KamLAND project were studied. The liquid scintillator was a mixture of isoparaffin, pseudocumene and 2,5-diphenyloxazole. We measured positron annihilation lifetime (PAL) spectra which contained important timing information on identification of the electron antineutrinos. The PAL spectra of a nitrogen-saturated liquid scintillator consisted of three lifetime components. Two fast components with the lifetime of 0.19±0.05 ns and 0.48±0.05 ns correspond to the mixture of para-positronium annihilation and free positron annihilation. The delayed component (3.41±0.02 ns) with the intensity of 48.9±0.3% is ascribed to pick-off annihilation of ortho-positronium (o-Ps) annihilation. To understand the 2γ annihilation processes of o-Ps in the liquid, we examined the effects of dissolved gases (nitrogen, oxygen, helium and argon) and mixing ratio of isoparaffin and pseudocumene which were the main components of the liquid scintillator. The lifetime of o-Ps was drastically shortened in the presence of oxygen, but not for other gases. The reaction rate constant for quenching of o-Ps by oxygen was obtained from the O₂ concentration dependence of o-Ps lifetime component. The pick-off annihilation of o-Ps with the liquid scintillator is explained based on the bubble model. We discuss a possible microscopic picture of the surroundings of Ps in the liquid scintillator. (author)

[en] Neutrinos play an exceptional role in particle or nuclear physics as well as in astrophysics. Postulated by Pauli in 1930, they were named by Fermi in 1933 and experimentally discovered by Reines and Cowan in 1956. A second family of neutrinos was discovered in 1962 and a third in 1975. The CERN collider LEP proved in 1989 that 3 types of interacting neutrinos are enough in the standard model of particle physics. They are named electron-neutrino (ν_e), muon-neutrino (ν_μ) and tau-neutrino (ν_τ), associated to the charged leptons electron, muon and tau. They have no charge, a very tiny mass and interact only weakly, so these elusive particles can cross large quantity of matter (like the Sun or the Earth) without interacting. Emitted in huge numbers (about 1020 per second) in nuclear reactors, they are also artificially produced in man-made accelerators which deliver intense neutrino beams. But the main source of neutrinos is the Universe itself: the relic neutrinos from the Big Bang have been wandering for more than 13.6 billion years, with a density of 330 per cm³ everywhere. Starting with the fusion of two protons, nuclear reactions in the core of the Sun produce about 2 10³⁸ electron-neutrinos per second, which means 65 billions of neutrinos per second per cm² on Earth. Supernova explosions emit about 10⁵⁸ neutrinos in a few seconds and the central engines of active galactic nuclei produce them abundantly. On Earth, many neutrinos are produced by the interaction of high energy cosmic rays in the upper atmosphere and are also emitted by radioactive elements in the crust and the mantle of the Earth. We are bathed in neutrinos which cross us continually and abundantly. Witnesses of the core of the Sun, solar electron-neutrinos have been observed since 1968, but their number is significantly less than what is predicted by solar models built by astrophysicists. It took more than 30 years to solve the problem of the deficit, when the SNO experiment showed in 2001 that part of the solar electron-neutrinos had been transformed into mu-neutrinos or tau-neutrinos. This was explained by the fact that neutrinos were oscillating between the three families, a mechanism invented by Pontecorvo in 1958 and authorized by quantum mechanics (mechanism completed by the MSW effect for solar neutrinos). In fact the oscillation mechanism was first observed in 1998 by the SuperKamiokande experiment via the study of atmospheric neutrinos: muon-neutrinos produced in the atmosphere at the antipodes were oscillating into tau-neutrinos during their travel through the Earth. Neutrino oscillation is possible only if neutrinos have a mass (which is not necessary in the minimal standard model of particle physics) and its discovery opens the door towards, at least, the completion of the standard model. Since 1970, neutrino beams have been used also to study neutrino properties but also to penetrate deep inside the nucleons and unveil their fine structure. This document is the compilation of all presentations given at the conference