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Introduction of KamLAND
(The Kamioka Liquid-scintillator Anti-Neutrino Detector)

There are 4 major research themes being studied in KamLAND.

•  Investigation of neutrino properties by reactor neutrinos
•  Leading neutrino geophysics
•  Promotion of neutrino astronomy
•  Search for neutrinoless double-beta decay

Through these researches, we elucidate the“origin of material", then, we will be able to explore the beginning of the universe and the future by deciphering the“unification of forces”such as the gravitational and electromagnetic forces.

The elementary particles constituting a matter are classified in a quark and a lepton. The quark is an elementary particle constituting a proton and a neutron, and it is known that there are 6 kinds in all. The quark receives all 4 forces known in nature (electromagnetic force, weak force, strong force, gravitational force).

Against the quark, the lepton is the elementary particle which does not receive strong force, and it has 6 kinds as well. In the six kinds of leptons, three are the charged leptons such as electron, muon and tauon, and the other three are electron neutrino, muon neutrino and tau neutrino. Neutrinos are free from both positive and negative charge, so, do not receive electromagnetic force. Nneutrinos are very light, less than hundred-thousandth the mass of electron, which is the lightest element except neutrinos.

In the meantime, when the existing number of elementary particles is averaged out in the whole space, the neutrinos exist about 300/1cm3, but, other elementary particles exist only one hundred millionth in the same size area. The understanding of the properties of neutrinos, which are incommensurably abundant in the universe, has great importance for constructing of the Great Unified Theory and investigating the origin of the universe.

Also, the astronomical objects such as the sun and the earth emit a large quantity of neutrinos, and as for the solar neutrino, the 66 billion neutrinos /sec/cm2 arrive at the earth. Since neutrinos are not affected by strong or electromagnetic forces, most of all neutrinos pass through without leaving a trace. The elusiveness of neutrinos makes it difficult to observe, on the other hand, it brings us useful information on the inside of the astronomical objects which we can not search directory.

In order to detect neutrinos, a huge and extremely low-radioactivity detector, KamLAND, was constructed at deep underground where the cosmic rays are shielded. KamLAND stores 1000-ton of liquid scintillator and catches the imperceptible scintillation light occurring from elementary particle reaction by using the 1879 pcs of the high sensitivity solar sensors which are put around inside of in the spherical tank. KamLAND liquid scintillator includes the radioactive impurity only about the billionth comparing with the normal materials. KamLAND provides an extremely low radioactivity environment suitable for the study of rear phenomena.

Precise measurement of neutrino oscillation by reactor neutrino

The sun shines with energy from nuclear fusion reactions at its center. Since it is impossible to see into the center of the sun, an experiment was conducted in the 1960s to investigate the status of nuclear fusion reactions by observing neutrinos being emitted simultaneously.

However, just only one third of the neutrinos expected from the brightness of the sun were observed, then, the subsequent solar neutrino observation experiments had confirmed. Man-made neutrino sources, whose production rate is well understood, are useful to solve the problem which has been unsolved for over 30 years.

Hida city where is constructed KamLAND is located about 180km on average away from the world's most powerful Kashiwazaki Kariwa Nuclear Power Station and nuclear reactor groups of Wakasa Bay. Whereas the sun generates the electron neutrinos through fission reactions, the nuclear reactor generates the electron anti-neutrinos, their antiparticles, through fission reactions. It is possible to calculate the production of the anti-electron neutrinos exactly by the operation history of each nuclear reactors, so, we can study how an electron anti-neutrinos propagates over the distance 180km.

As a matter of fact,there is the phenomenon called“neutrino oscillation", in which neutrinos propagate repeatedly by changing their type. The mixing of the elementary particles is known for the quark, but, there is much greater mixing in neutrinos. Neutrino is very light, but, it has mass. It is known there are three different mass states of neutrinos, these are mixed, then, the electron neutrino, muon neutrino and tau neutrino with three flavors are constituted. This is not the meaning that the neutrino is composite particle, but, it means that it is the quantum‐mechanical superposition of different state.

In quantum-mechanical, the heavy neutrino is a wave of the fast period and the light neutrino is a wave of the slow period. By these different periods of waves are superposed, it is called neutrino oscillation because the neutrinos change their type in response to the undulation of the waves. If we focus on a specific neutrino such as the electron neutrino, it will repeatedly annihilate and restore itself. In fact, this observation of neutrino oscillations led to the discovery that neutrinos have three types of masses, these are mixed, and the neutrino with each flavor is constituted.

By observing the electron antineutrino from the nuclear power plant, KamLAND has successfully observed the neutrino oscillation, a quantum-mechanical phenomenon that occurs over long distance of 180 km. By applying the conversion of distance divided by energy, we can clearly see the repeated annihilation and restoration of the phe-electron neutrino over the two periods. From this, the solar neutrino problem was clarified, and information on the mass of neutrinos (square difference of the mass) was successfully determined with a high accuracy of 2.5 %.

Prof. Kajita of Super-Kamiokande and Prof. McDonald of the SNO experiment received the Nobel Prize in Physics on the neutrino oscillation research. What Prof. Kajita observed was that the number of atmospheric muon neutrinos appear to decrease at long-distance flight like the flying from the other side of the earth. Now, it is known that the muon neutrinos have changed to the tauon neutrinos. Prof. McDonald also found that the total number of all types of neutrinos coming from the Sun matched the number of electron neutrinos that scientists had expected to find. This is the evidence that neutrinos have changed to their type during the flight. The electron anti-neutrino oscillation that KamLAND observed, have a special meaning that same type of neutrinos is produced in large numbers in the earth interior.

Leading neutrino geophysics

Now that the propagation of the neutrinos has been discovered, the observation of invisible astronomical objects has been turned into the reality using neutrino's elusiveness. There are still many unsolved mysteries relating to the interior of the Earth and the Sun, which are familiar to us.

How did the Earth which was created by the accumulation of the meteorites 4.6 billion years ago develop into the present Earth? How are present Earth’s dynamics such as earthquakes and geomagnetism generation caused? Understanding the Earth's heat is very important to solve these questions. Radioactive isotopes in the Earth's interior such as uranium and thorium produce heat through their decays, and also emit electron anti-neutrinos, geo-neutrinos.

KamLAND successfully made the world's first measurement of geo-neutrinos in 2005. Neutrino observations have provided a new tool of directly measuring the Earth's interior, and have led to the creation of "Neutrino Geophysics". Geo-neutrino observations are also planned at different areas on the Earth. Combining the multi-site measurement results will lead to a more detailed understanding of the Earth's interior.

KamLAND geo-neutrino observation has demonstrated for the first time that radiogenic heat production inside the Earth is less than half the surface heat flow (47 TW). This result indicated that the Earth's primordial heat supply has not yet been exhausted and the Earth is cooling. It is expected to reveal the type of the meteorite which created the Earth by future precise measurement. We think geo-neutrino observation can be said “neutrino tomography”, then, for getting more detailed information, the multi-site stereo measurements and the directional measurement of neutrinos will be required. Construction of new detectors in Canada and China are under construction. We are developing new technology for measuring directional information of anti-neutrinos. Moreover, we are also promoting the Ocean Bottom Detector (OBD) project, in which the neutrino detector such as KamLAND will be towed and deployed into the deep ocean floor. If we can measure geo-neutrinos originating from the mantle at multiple points, our understanding of the Earth's interior will be further improved. We believe that this is the research field that will continue to grow in the future.

Promotion of Neutrino Astronomy

KamLAND can detect the neutrino coming from the astral body. We have already succeeded in detecting the solar neutrino, and we are exploring if there are any other neutrinos which are related to the sudden astral body phenomena. We are looking for the mutual relation of solar flare, gamma-ray burst and gravity wave. It is called “multi-messenger observation” to observe some astral body phenomena by various methods as well as a telescope. We are able to understand the astral body phenomenon in more details by combining the various methods.

With regard to neutrinos, KamLAND excels at low energy, Super-Kamiokande excels at medium energy, IceCUBE in south pole excels at high energy, cover a wide energy range. I think that KamLAND can contribute to the supernova explosion characteristically. It is only the KamLAND which is capable of observing all types of neutrino by the method called proton recoil. This method is able to observe the temperature and the brightness at the same time when the supernova explosion occurs.

Also, the Betelgeuse which is one of neighborhood red giant stars is a notable astral body because an explosion could happen at any moment. There are several other similar candidates. The neutrino and optical information obtained from the supernova explosion of Betelgeuse is thought to be immense. Moreover, the present when a gravitational wave telescope really exist, if we can observe the gravitational wave, we will be able to get more information and a perfect multi messenger observation will become reality.

By the way, the telescope cannot keep observing the Betelgeuse always, but, the light arrives at a neutrino detector some time after observing supernova explosion neutrinos, the detector sends alarm to the telescope. However, the gravitational wave comes over with neutrino. Then, the gravitational wave observation takes more time for performance increase and it does not work during a long period. If we can know the explosion in advance, we stand by for observation. Believe it or not, if it is a neighborhood supernova explosion, KamLAND can grab the neutrino when astral body becomes high temperature at the time of the silicon combustion before the explosion, and it is possible for KamLAND to generate alarm near one week in advance. The gravitational wave telescope is always watching for KamLAND whether it gets any preages of neighborhood supernova explosion.

Search for the double beta decay without neutrino

The extremely low radioactivity environment of KamLAND in order to fulfill the observation of neutrinos is most suitable to observe the rare phenomena. The main theme of current KamLAND utilizing this characteristic is “ The search for the double beta decay (0ν2β) without neutrino”. This search experiment was named “カムランド禅=KamLAND-Zen”. The name has meaning of that “Zen=Zero neutrino double beta decay”, the style ‘patiently wait for the rare phenomenon’ is similar to “禅(Zen)=Zen”, “その後(Sonogo)=then”, “キセノン(Kisenon)=xenon” which uses for our search is pronounced as ‘zenon’ too, we infused “KamLAND-Zen” with these all meanings. The search has got attention as it will lead to elucidate the big question of space & elementary particle, please let me introduce it.

There is a possibility that the neutrino without electric charge does not have discrimination between particle and antiparticle. Because this theory’s history is old and it originates from Dr.Majorana in 1937, it is called Majorana neutrino. Additionally, if it has discrimination, it is called Dirac neutrino. Experimentally, neutrino and antineutrino behave differently. In Majorana theory, if it is rotating left for travelling direction, it is neutrino, then, if it is rotating right for travelling direction, it is antineutrino. In truth, it was not almost necessary to discriminate between Majorana neutrino and Dirac neutrino, but, the discovery of neutrino oscillations has changed the situation dramatically. The neutrino oscillation is the proof of that neutrino has a mass. Neutrino with a mass travels always slower than the speed of light, and can be passed off as a trial test.

In the case of Majorana neutrino, if you pass a neutrino rotating to the left, it will appear to rotate to the right, i.e., it will be an antineutrino. In the case of Dirac neutrino, the neutrino is a right-handed neutrino when it overtakes a left-handed neutrino. Now, the breakthrough of 20th century, as the Dirac equation which is made by the combination of special theory of relativity and quantum mechanics, it is known that the particle with a mass which composes a matter have 4 states. In Dirac neutrino, the 4 states are left-handed neutrino, right-handed neutrino, left-handed antineutrino and right-handed antineutrino. In Majorana neutrino, there are only left-handed neutrino and right-handed antineutrino. The other two are naturally assumed to be a heavy right-handed neutrino and a heavy left-handed antineutrino. I have talked too long so far, but, a Majorana neutrino would require a heavy neutrino.

Now, this heavy neutrino is awesome. By appearing heavy neutrino, “The mystery that neutrino has extremely light mass” can explain by the theory seesaw model. Also, we can know from the Dirac theory that matter and antimatter are created in equal numbers in a universe that arose from nothing. When they meet, they disappear and return to nothing. Nevertheless, we who made by matter exist in the space is one of the major problems of the universe and elementary particle, that is said as “the mystery of the cosmic material superiority”. Because the matter consists of particles and the antimatter consists of antiparticles, it has special meaning for the Majorana neutrino which does not discrimination between particles and antiparticles.

The Leptogenesis theory explains that a slight asymmetry in the particles and antiparticles created by the heavy neutrinos is the origin of the matter we know today. Also, there is a theory that the heavy neutrino is the origin of dark matter. What is more, the Grand Unified Theory is built by unifying the constitution of complicated elementary particles into a single expression, but, the heavy neutrino is necessary to create the SO(10) Grand Unified Theory. The heavy neutrino does perform brilliantly, but, it is thought that it is impossible to produce it experimentally. Instead of it, we can just research whether it is the Majorana neutrino in order to prove the existence.

0ν2β gives proof that neutrino is Majorana neutrino, in parallel, it gives the absolute value of neutrino mass (effective mass) which is unable to settle by neutrino oscillation. The double-beta decay is the process that two beta decays occur at once, decaying to a nucleus with two neighboring atomic numbers, then, it becomes observable when normal beta decay is not allowed as an energy level. In doing so, 2ν2β which emits two beta rays and two anti-electronic neutrinos is a phenomenon that occurs within the category of the elementary particle standard model, and it has been observed in several nuclei.

n Majorana neutrino, it is possible phenomenon for an anti-electron neutrino produced in one beta decay to become an electron neutrino and be absorbed by the other. This phenomenon, in which no neutrinos are emitted but two electrons (β) are emitted, is 0ν2β. From the importance, lots of researches have been done all over the world. The 2ν2β itself is quite rare phenomenon, so, the 0ν2β is even rarer and the observation is difficult, and so, it remains undiscovered. For this research, we need to prepare large amount of double beta decay nuclei and to observe them under the environment with a few backgrounds.

KamLAND is under the environment with a few backgrounds, and there is liquid scintillator that can dissolve Xe-136 (which is double beta decay nucleus) easily. There is KamLAND detector - put in a mini-balloon in the center of the detector, dissolve Xe-136 and fill up the balloon with liquid scintillator. The natural abundance of Xe-136 is only 8.9%, so, we used the isotope xenon whose natural abundance becomes 91% by centrifugal separation.

The experiment began in 2011 and continued by the end of 2015, we used 380 kg xenon. In this searching, the 0ν2β is undiscovered, and it was found that the 0ν2β half-life of Xe-136 is more than 10 raised to the power of 26 years. We have achieved most sensitive search in the world, albeit undiscovered. There are three types of neutrino mass structures that explain neutrino oscillation: 3 neutrinos are heavy with degenerate structure, 2 neutrinos are heavy with inverted hierarchical structure ,1 neutrino is heavy with hierarchical structure.

The search for KamLAND-Zen almost negated the degenerate structure. We have increased the xenon up to 750kg and been continuing our search from 2019. In this search, it is possible to approach the inverted hierarchical structure and since there are multiple theoretical predictions, a big discovery can be expected. Furthermore, in KamLAND2-Zen project which will make KamLAND more high performance, by conducting the search covering the inverse hierarchical structure, we can hope for a big discovery and also, even if it is not discovered yet, we can aim to determine the structure by a process of elimination.

The KamLAND2 will improve the performance of neutrino observation such as geoneutrino, and will provide very low radioactivity environment which conducts new and rare phenomenon searches. Please support for the realization of KamLAND2.

By Prof. Kunio Inoue

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DoubleChooz / JSNS2

-  DoubleChooz / JSNS2  -

Q: Please tell us about your group DoubleChooz/JSNS2, Prof. Suekane.

I used to participate in electron-positron collider experiments and performed top quark search and measurement of the Weinberg angle ΘW. Since 1996, I have been involved in neutrino oscillation experiments. In 2002, I measured neutrino oscillation parameters (Θ12, Δm212) in KamLAND experiment, Tohoku Univ., by detecting neutrinos coming from reactors 180km away. It was an important discovery of reactor neutrino oscillation.

After that, I joined Double Chooz experiment in France and measured small oscillation parameter Θ13. This measurement also contributes to the CP violation measurement in neutrino oscillation.

Now I joined JSNS2 experiment at J-PARC and looking for 4th neutrino, called sterile neutrino and then hoping to measure the oscillation parameter (Θ14, Δm214). If the sterile neutrino is discovered, it gives huge impact on the elementary particle physics.

In addition, I am interested in teaching elementary particle physics intuitively and have been writing some text books.

By Prof. Fumihiko Suekane

(Click here, the "KASKA/DoubleChooz/JSNS2 official Home page" will be opened.)