Experimental Nuclear and Particle Physics

Experimental Partcle Physics (Neutrino)

Staff

Professor : Kunio Inoue
Associate Professor : Fumihiko Suekane HP Masayuki Koga Tadao Mitsui HP Itaru Shimizu
Lecturer : Kengo Nakamura
Assistant Professor : Kyoko Tamae Yoshihito Gando Haruo Ikeda Kota Ueshima
Koji Ishidoshiro Hiroko Watanabe

Research

Non-Accelerator Experiments
 The Nature and the Universe provide us with a good laboratory for experiments on particle physics. A plenty of elementary particles are flying around us, some of which have the information of the birth and the evolution of the Universe, and others are continually emitted from the Galaxy, the Sun and the Earth and coming to the ground. The purpose of non-accelerator experiments is to detect these particles and study their characteristics and the Universe. Among the elementary particles, neutrinos interact only through weak interactions and they provide us with direct information of the beginning of the Universe, the interior of the Sun, the end of stars and deep interior of the Earth. Furthermore, the extremely smallness of the neutrino masses and the relation between them are considered to be originated from the grand unified theory of elementary particles and to explain why the Universe is made up by the matter only. The observational studies of neutrinos are therefore very important not only in particle physics but also in astrophysics and geo-physics.

KamLAND experiment
 Main purposes of the experiment are to clarify characteristics of neutrinos through detection of reactor antineutrinos, to study structure and formation of the Earth by the detection of the Earth antineutrinos, and to study evolution of stars by solar neutrino detection. The group constructed the world’s largest detector of 1000ton liquid scintillator in 2002, which is located in the 1000m-deep underground laboratory in Kamioka mine in Hida city in Gifu prefecture. To detect neutrinos the radioactivity surrounding the detector is made extremely low by a factor of several 10 million comparing to that of the ground level. The experiment is carried out 24 hours a day by a collaboration of researchers from the Research Center for Neutrino Science in Tohoku University and several institutes of US.

 Firstly the group made a search for periodical transformation of neutrinos (neutrino oscillation) caused by the masses of the neutrinos which are the most fundamental properties of the neutrinos. The group successfully detected the antineutrinos in 2002 coming from distant power reactors at 180km in average away from the detector. The result shows for the first time that the number of observed antineutrinos is less than expected. In 2004, the energy spectrum of the observed antineutrinos is found to be consistent with the neutrino oscillation. This resolves the long-standing solar neutrino problem lasting more than 30 years that the observed solar neutrino fluxes are less than the prediction of the standard solar model. The result is very important to make clear the mass structure and the relation of neutrinos.

 Once the fundamental properties of neutrinos became clear, neutrinos can be used to study various mechanisms in nature. The heat generation mechanism in the Earth is a key problem to study the formation and evolution of the Earth, the dynamics of the Earth such as the origin of the Earth magnetism, mantle convection etc. However, it is very hard to understand the mechanism because we cannot directly observe the Earth interior. The KamLAND has successfully detected for the first time antineutrinos which are generated together with the heat by the decays of radioactive elements in deep interior of the Earth. The direct information of the heat generation mechanism obtained by neutrinos means the opening of a new field of science called “Neutrino Geophysics”.

KamLAND-Zen experiment
The Big bang model predicts equal amounts of matter and antimatter in the early universe. But today, everything we see is made of matter. Why? Neutrinos have a possibility to generate the matter-antimatter asymmetry if neutrinos and anti-neutrinos are same particles. Now there are many experiments going and proposed to solve this mystery. KamALND-Zen is one of such experiments.

The KamLAND-Zen experiment searches for evidence of identity between neutrinos and anti-neutrinos by introducing 136Xe to a part of liquid scintillator. Its evidence is neutrino less double beta decay of 136Xe. KamLAND-Zen has already set one of the tightest upper limits on half period of neutrino less double beta decay. Several R&D for higher sensitivity by using novel ideas are ongoing in parallel.

DoubleChooz/Sterile Neutrino group 
This group is performing reactor neutrino oscillation experiment, Double Chooz (Figure below), at Chooz nuclear power plant in France and measuring the 3rd neutrino oscillation, following the SuperKamiokande and KamLAND experiments. The Double Chooz group announced the first indication of short-baseline reactor neutrino oscillation in November 2011 and have measured neutrino oscillation angle θ13 using two detectors as follows
sin213 = 0.119 ± 0.016 (preliminary)
The measurement of basic parameter θ13 deepens our understanding of neutrino transitions, which are not accounted for in the standard model of elementary particles. It also sheds light to possibility of measurement of CP violation in neutrino oscillations in the future.
This group is also developing a reactor monitor system which measures plutonium production in nuclear reactors by making use of reactor neutrino detection technology. Neutrinos may contribute to social benefit through this safeguard technology.
From the year 2013, the group is preparing a new experiment, JSNS2 which detects yet unknown 4th neutrino called ”sterile neutrino” at J-PARC accelerator complex in Tokai. The sterile neutrino is not included in the standard model and if its existence is confirmed, it gives a strong impact to physics.

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