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

However, only one-third of the expected neutrinos, based on the brightness of the Sun, were observed. Subsequent solar neutrino observation experiments confirmed this discrepancy. Man-made neutrino sources, whose production rates are well understood, are useful for solving a problem that remained unsolved for over 30 years.

Hida city, where KamLAND is constructed, is located about 180km on average from the world's most powerful Kashiwazaki Kariwa Nuclear Power Station and the nuclear reactor groups of Wakasa Bay. Whereas the Sun generates electron neutrinos through fusion reactions, nuclear reactors generate electron　anti-neutrinos, their antiparticles, through fission reactions. It is possible to calculate the production of electron anti-neutrinos precisely based on the operational history of each nuclear reactor, allowing us to study how electron anti-neutrinos propagate over the 180km distance.

As a matter of fact, there is a phenomenon called “neutrino oscillation,” in which neutrinos change their type as they propagate. The mixing of elementary particles is known for quarks, but there is much greater mixing in neutrinos. Neutrinos are very light but have mass. There are three different mass states of neutrinos, which　are mixed and make up the electron neutrino, muon neutrino, and tau neutrino. This does not mean that neutrinos are composite particles, but that they are a quantum-mechanical superposition of different states.

In quantum mechanics, a heavy neutrino is a wave of fast period, and a light neutrino is a wave of slow period. These different wave periods superpose, causing neutrinos to change their type in response to the undulation of the waves, a phenomenon called neutrino oscillation. If we focus on a specific neutrino, such as the electron neutrino, it will repeatedly annihilate and restore itself. This observation of neutrino oscillations led to the discovery that neutrinos have three different masses and that each flavor of neutrino is a mixture of these mass states.

By observing electron antineutrinos from nuclear reactors, KamLAND has successfully observed neutrino oscillation, a quantum-mechanical phenomenon that occurs over the long distance of 180 km. By applying the conversion of distance divided by energy, we can clearly see the repeated annihilation and restoration of electron antineutrinos over two periods. This clarified the solar neutrino problem and provided information on the neutrino mass difference (square difference of the mass) 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 for their research on neutrino oscillation. Prof. Kajita observed that the number of atmospheric muon neutrinos appeared to decrease over long-distance travel, indicating that they were changing into tau neutrinos. Prof. McDonald found that the total number of all types of neutrinos coming from the Sun matched the expected number of electron neutrinos, providing evidence that neutrinos change type during flight. The electron　anti-neutrino oscillation observed by KamLAND has special significance because the same type of neutrinos is produced in large numbers in the Earth's interior.

Now that the propagation of neutrinos has been discovered, the observation of invisible astronomical objects using neutrinos' elusiveness has become a reality. 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, formed by the accumulation of meteorites 4.6 billion years ago, develop into its present state? What is the cause of present-day Earth's dynamics, such as earthquakes and geomagnetism? Understanding the Earth's heat is very important to solving these questions. Radioactive isotopes in the Earth's interior, such as uranium and thorium, produce heat through their decay and emit electron anti-neutrinos and geo-neutrinos.

KamLAND successfully measured the world's first geo-neutrinos in 2005. Neutrino observations have provided a new tool for directly measuring the Earth's interior, leading to the creation of "Neutrino Geophysics." Geo-neutrino observations are also planned in different areas around the globe. Combining multi-site measurement results will result in a more detailed understanding of the Earth's interior.

KamLAND's geo-neutrino observation demonstrated for the first time that radiogenic heat production inside the Earth is less than half of 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 still cooling. Future precise measurements are expected to reveal the type of meteorites that created the Earth. We believe that geo-neutrino observation can perform “neutrino tomography.” For more detailed information, multi-site stereo measurements and directional measurements of neutrinos will be required. New detectors in Canada and China are under construction. We are developing new technology to measure the directional information of antineutrinos. Moreover, we are also promoting the Ocean Bottom Detector (OBD) project, in which a neutrino detector like KamLAND will be towed and deployed on 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 improve further. We believe that this research field will continue to grow in the future.

KamLAND can detect neutrinos coming from astronomical objects. We have already succeeded in detecting solar neutrinos, and we are exploring whether there are any other neutrinos related to spontaneous astronomical phenomena. We are looking for correlative neutrino events from solar flares, gamma-ray bursts, and gravitational waves.

Observing astronomical phenomena by various methods, including telescopes, is called “multi-messenger observation.” We can understand astronomical phenomena in more detail by combining the various methods. Each detector specializes in a different energy range for neutrino detection: low energy by KamLAND, middle energy by Super-Kamiokande, and high energy by IceCube. KamLAND can particularly contribute to the study of supernova explosions. It is the only detector capable of observing all types of neutrinos using proton recoil reactions. This method allows the simultaneous observation of temperature and brightness during a supernova explosion.

Betelgeuse, one of the nearby red giant stars, is a significant astronomical object because its explosion could happen at any moment, along with several other similar candidates. The neutrino and optical information obtained from the supernova explosion of Betelgeuse is expected to have an enormous impact on understanding the explosion process. Moreover, with the existence of gravitational wave telescopes, we will be able to obtain more information and perform perfect multi-messenger observations together with gravitational wave detection.

Telescopes cannot always monitor Betelgeuse continuously. Given that light arrives at a neutrino detector sometime after observing supernova explosion neutrinos, the neutrino detector can send a supernova alarm to the telescopes. In contrast, gravitational waves and neutrinos arrive simultaneously. Additionally, gravitational wave observation detectors have inactive periods for performance upgrades. If we know about the explosion in advance, we can prepare for the observation. In the case of a nearby supernova explosion, KamLAND is able to detect neutrinos when the astronomical object heats up during silicon burning before the explosion and can give a supernova alarm nearly a week in advance. The gravitational wave telescope always works in conjunction with KamLAND to get early warnings of nearby supernova explosions.

The extremely low-radioactivity environment of KamLAND, ideal for effective observation of neutrinos, is also suitable for investigating rare phenomena. Taking advantage of the characteristics of this detector, the current major theme at KamLAND is "searching for neutrinoless double-beta decay." This search experiment is named “カムランド禅 (KamLAND-Zen).” The name incorporates multiple meanings: 'Zen' stands for 'Zero neutrino double beta decay,' and the philosophy of 'patiently waiting for a rare phenomenon' aligns with the concept of '禅 (Zen). Additionally, the Japanese words 'その後 (Sonogo)', meaning 'then', and 'キセノン (Kisenon)' - this is the noble gas used for our research, pronounced 'zenon', both tying it to the word 'Zen'. We infused 'KamLAND-Zen' with all these meanings. This search has captured attention for its potential to elucidate significant questions about space and elementary particles. Allow me to introduce it.

There is a possibility that neutrinos, which do not have an electric charge, do not differentiate between particles and antiparticles. In this theory, originating from Dr. Majorana in 1937, the neutrino is called the Majorana neutrino. Conversely, if neutrinos do distinguish between particles and antiparticles, they are called Dirac neutrinos. Experimentally, neutrinos and anti-neutrinos behave differently. According to the Majorana theory, neutrinos rotate to the left relative to their direction of travel, while anti-neutrinos rotate to the right. In fact, there was almost no need to distinguish between Majorana and Dirac neutrinos, but the discovery of neutrino oscillations, which proved that neutrinos have mass, has changed the situation dramatically. Neutrinos with mass travel slower than the speed of light and can change their handedness. For Majorana neutrinos, a neutrino rotating to the left can appear to rotate to the right when you overtake it, effectively becoming an antineutrino. For Dirac neutrinos, a left-handed neutrino would have a right-handed counterpart, so it remains a neutrino. Now, according to the Dirac equation, the great invention of the 20th century, which combines special relativity and quantum mechanics, particles with mass that compose matter have four states. In the case of Dirac neutrinos, the four states are left-handed neutrino, right-handed neutrino, left-handed anti-neutrino, and right-handed anti-neutrino. For Majorana neutrinos, only left-handed neutrinos and right-handed anti-neutrinos exist. The other two states are naturally assumed to be heavy right-handed neutrinos and heavy left-handed anti-neutrinos. It is a long story, but a Majorana neutrino would require the existence of a heavy neutrino.

Now, this heavy neutrino is awesome. By appearing, the heavy neutrino can explain 'the mystery of the extremely light neutrino mass' through the seesaw model. Another mystery is the matter-antimatter imbalance in the universe. According to Dirac's theory, matter and antimatter were 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 anti-particles, it has a special meaning for the Majorana neutrino, which does not discriminate between particles and antiparticles. The Leptogenesis theory explains that a slight asymmetry in the particles and anti-particles created by heavy neutrinos is the origin of the matter we know today. Additionally, there is a theory that the heavy neutrino is the origin of dark matter. Furthermore, the Grand Unified Theory, which unifies the constitution of complex elementary particles into a single expression, requires the heavy neutrino to create the SO(10) Grand Unified Theory. While the heavy neutrino is theoretically effective, it is thought to be impossible to produce it experimentally. Instead, we can research whether the neutrino is a Majorana neutrino to prove its existence.

Neutrinoless double-beta decay (0ν2β) would provide proof that a neutrino is a Majorana neutrino and also give the absolute value of the neutrino mass (effective mass), which cannot be determined by neutrino oscillation. Double-beta decay involves two simultaneous beta decays, resulting in a nucleus with two fewer protons. This process becomes observable when normal beta decay is energetically forbidden. In two-neutrino double-beta decay (2ν2β), two beta particles and two anti-neutrinos are emitted. This phenomenon occurs within the elementary particle standard model category and has been observed in several nuclei.

In the case of Majorana neutrinos, an anti-electron neutrino produced in one beta decay can be absorbed as an electron neutrino by the other beta decay, resulting in 0ν2β, where no neutrinos are emitted, only two electrons (2β). Because of its significance, extensive research has been conducted worldwide. Since 2ν2β itself is rare and 0ν2β is even rarer, it is a difficult process to observe and remains undiscovered. To conduct this research, a large amount of double-beta decay nuclei in a low-background environment is required. KamLAND has a very low-background environment, and its liquid scintillator can easily dissolve Xe-136, a double-beta decay nucleus. A mini-balloon filled with Xe-136 loaded liquid scintillator is placed at the center of KamLAND. The natural abundance of Xe-136 is only 8.9%, so we used xenon isotopically enriched to 91% by centrifugal separation.

KamLAND-Zen 400 started in 2011 and continued until the end of 2015 with 380 kg of xenon. Although 0ν2β was not discovered, it was found that the 0ν2β half-life of Xe-136 is more than ten to the twenty-sixth power years. This represents 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, inverted hierarchy with two heavy neutrinos, and normal hierarchy with one heavy neutrino. KamLAND-Zen almost ruled out the degenerate hierarchy.

We increased the xenon amount to 750 kg and continued our search from 2019 to 2024 as the KamLAND-Zen 800 experiment. In the published result in 2023, the most stringent constraint on the 0ν2β decay half-life was provided, marking the first search in the inverted mass hierarchy region. Furthermore, we are preparing to start the "KamLAND2" experiment, with improved detector performance, in order to discover 0ν2β decay. We anticipate making great discoveries, as 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 is not yet discovered.

KamLAND2 will improve the performance of neutrino observations, such as geoneutrinos, and will provide a very low radioactivity environment suitable for conducting searches for new and rare phenomena. Please support KamLAND2.

By Prof. Kunio Inoue