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 electron anti-neutrinos exactly by the operation history of each nuclear reactors, so, we can study how 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 mechanics, 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 different masses, and that each flavor of neutrino is composed by a mixture of these neutrinos.

By observing the electron anti-neutrino from the nuclear reactors, 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 electron anti-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 by 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 has a special meaning that same type of neutrinos is produced in large numbers in the earth's interior.

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 perform “neutrino tomography”, then, for getting more detailed information, the multi-site stereo measurements and the directional measurement of neutrinos will be required. 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.

KamLAND can detect the neutrino coming from the astronomical objects. We have already succeeded in detecting the solar neutrinos, and we are exploring if there are any other neutrinos which are related to the spontaneous astronomical phenomena. We are looking for the correlative neutrino events from solar flare, gamma-ray burst and gravitational wave.

It is called “multi-messenger observation” to observe some astronomical phenomena by various methods as well as by telescopes. We are able to understand astronomical phenomena in more detail by combining the various methods. Each detector has a specialized energy range for neutrino detection; low energy by KamLAND, middle energy by Super-Kamiokande, and high energy by IceCube. KamLAND can contribute to the supernova explosion characteristically. KamLAND is the only capable detector to observe all types of neutrinos by using proton recoil reaction. This method is able to observe the temperature and the brightness at the same time when the supernova explosion occurs. Also, the Betelgeuse, one of neighborhood red giant stars, is a remarkable astronomical object because its 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 have enormous influence on understanding the process of explosion. Moreover, since gravitational wave telescopes are existed now, we will be able to get more information and perform perfect multi messenger observation together with the gravitational wave observation.

The telescopes cannot keep observing the Betelgeuse always. Given the light arrives at a neutrino detector some time after observing supernova explosion neutrinos, the neutrino detector sends an supernova alarm to the telescopes. In contrast, the gravitational waves come over at the same time as neutrinos. Additionally, the gravitational wave observation detectors have inactive time because of its performance upgrade. If we can know the explosion in advance, we can get ready for the observation. In case of a nearby supernova explosion, KamLAND is able to detect neutrinos at the time when the astronomical object becomes hot during silicon burning before the explosion, and can give a supernova alarm nearly a week in advance. The gravitational wave telescope is always working with KamLAND whether it gets any preages of neighborhood supernova explosions.

The extremely low radioactivity environment of KamLAND for effective observation of neutrinos is also suitable to investigate the rare phenomena. Taking advantage of the characteristics of this detector, "searching for neutrinoless double-beta decay" is the current major theme at KamLAND. 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 is leading to elucidating the big question of space & elementary particles, 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 originates from Dr. Majorana in 1937, it is called Majorana neutrino. Additionally, if it has discrimination, it is called Dirac neutrino. Experimentally, neutrinos and anti-neutrinos behave differently. The Majorana theory distinguishes between neutrinos if they rotate to the left relative to the direction of travel, and anti-neutrinos if they rotate to the right. In fact, there was almost no need to distinguish between Majorana and Dirac neutrinos but the discovery of neutrino oscillations has changed the situation dramatically. The neutrino oscillation is the proof that neutrino has a mass. Neutrino with a mass travel 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 anti-neutrino. In the case of Dirac neutrino, the neutrino is a right-handed neutrino when it overtakes a left-handed neutrino. Now, according to the Dirac equation, the great invention of the 20th century, a combination of special 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 anti-neutrino and right-handed anti-neutrino. 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. It is a long story, 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 be explained 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, the fact that we, made of matter, exist in the universe is one of the major problems of the universe and elementary particles, known as the "mystery of the universe's matter dominance. Because matter consists of particles and 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. Furthermore, the Grand Unified Theory is built by unifying the constitution of complex 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 effectively, but it is thought that it is impossible to produce it experimentally. Instead, we can just research whether it is the Majorana neutrino in order to prove the existence.

0ν2β gives proof that a 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 in which two beta decays occur at once, decaying to a nucleus with two neighboring atomic numbers. 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. In Majorana neutrino, it is possible 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. Because of its importance, lots of researches have been done all over the world. Since 2ν2β itself is a rare phenomenon and 0ν2β is even rarer, it is difficult to observe and 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 low background level. KamLAND has a very low-background environment and its liquid scintillator can easily dissolve Xe-136, (double beta decay nucleus). A mini-balloon is introduced in the center of KamLAND and filled with Xe-136 loaded liquid scintillator. The natural abundance of Xe-136 is only 8.9%, so we used xenon isotopically enriched to 91% by centrifugal separation. KamLAND-Zen 400 was started in 2011 and continued by the end of 2015 with 380 kg xenon. In this experiment, the 0ν2β was undiscovered, and it was found that the 0ν2β half-life of Xe-136 is more than 10 to the 26th power of years. We have achieved the most sensitive search in the world. There are three types of neutrino mass hierarchy that explain neutrino oscillation: degenerate hierarchy with three heavy neutrinos, hierarchy with two heavy neutrinos, normal hierarchy with one heavy neutrino. KamLAND-Zen almost negated the degenerate hierarchy. We increased the xenon up to 750kg and continued our search from 2019 to 2024 as KamLAND-800 experiment. In the published result in 2023, the most stringent constraint on the 0ν2β decay half-life was provided and was the first search in the inverted mass hierarchy region. Furthermore, we are preparing for starting "KamLAND2" experiment with improvement of the detector performance in order to discover the 0ν2β decay. We can expect to make great discoveries because there are multiple theoretical predictions in this mass region. The sensitivity of KamLAND2 will be able to determine the mass hierarchy by elimination even if a 0ν2β decay event will not be discovered yet.

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

By Prof. Kunio Inoue