Super-Kamiokande observes various neutrinos and other particles.
Here we introduce the research activities of Super-Kamiokande.

Solar neutrinos

Solar neutrinos

What are solar neutrinos?

The Sun is the most powerful neutrino sources in our circumstances. The energy source of the Sun shining is a fusion reaction in the center of the Sun. Four hydrogen nuclei (i.e. protons) form a helium-4 nuclei (two protons and two neutrons) via the fusion reaction, in which fusion energy, two positrons and two electron neutrinos are also released.(4p → He + 2e+ + 2νe + fusion energy)The electron neutrinos generated in this reaction are called as “solar neutrinos”. The solar neutrino flux, on the Earth, per one second, per one square centimeter, is about 66 billion.

It will take about 100,000 years to appear the heat, generated by the fusion reaction in the center of the Sun, on the solar surface. On the other hand, solar neutrinos, born in the center of the Sun, will arrive in approximately 8 minutes to Earth, since neutrinos are very hard to interact. In other words, we see 100,000 years ago solar activity in the light, but in the neutrino we are able to observe the current activities of the center of the Sun.

The Sun seen with neutrinos. The coordinate system in which the Sun is places at the center is used. The yellow part shows there are many events from that direction. It is firstly shown that neutrinos are really coming from the direction of the Sun in Kamiokande experiment. (This plot is made from the observation data of Super-Kamiokande from 1996 to 2018)

Lack of solar neutrinos

Observation of solar neutrinos was began with Homestake experiment by R. Davis and so on from the late 1960s in US. In the Homestake experiment, solar neutrino flux was measured via the production rate of argon atoms from the neutrino reaction of chlorine atoms. It was impossible to measure the neutrino direction. As a results of the experiment, the observed production rate was about 1/3 of the expected value from Standard Solar Model (SSM). There were several questions on this result: does it really capture the neutrinos coming from the Sun?, why is the neutrino flux small?, is SSM correct?, does neutrinos have finite masses and then neutrino oscillation occur?, and so on. This problem was called as “solar neutrino problem”. It annoyed the researchers for many years.

In 1988, the solar neutrino results other than the Homestake experiment was firstly reported from the Kamiokande II experiment group. Kamiokande, which was the former experiment of Super-Kamiokande, was able to measure neutrino coming direction in real time. It was firstly shown by Kamiokande that the observed neutrinos were coming from the direction of the Sun. However, the observed solar neutrinos flux was about half of the expected value from SSM. Therefore, the solar neutrino problem still remained.

Solving the solar neutrino problem, then unraveling further questions

In June 2000, Super-Kamiokande has reported the observation result of the solar neutrino flux with a highest accuracy than ever before. As a result, the observed solar neutrino flux was about 45% of the expected flux in SSM with more than 99.9% confidence level, suggesting the solar neutrino problem was caused by a neutrino oscillation. Furthermore, measuring energy distribution of solar neutrinos and time variation of the solar neutrinos in day time and night time with high accuracy, a large limit on the neutrino oscillation parameter (mass difference and mixing angle) area was obtained. It was also shown that the mixing between neutrinos are large. In June 2001, a combined analysis of solar neutrino observations between Super-Kamiokande and SNO experiment in Canada, showed a reliable evidence that a neutrino oscillation really occurred. In addition, it was confirmed at the same time that the neutrino flux calculated from SSM was correct.

Although the problem that observed solar neutrino flux looked smaller than SSM was solved by neutrino oscillation, there are still unsolved questions in nature of neutrinos or the burning mechanism of the Sun (SSM). For example, they are true values of the solar neutrino oscillation parameters (mass difference and mixing angle), confirmation of the Earth’s matter effect on solar neutrinos, elucidation of the chemical composition of the solar interior, and so on. Towards unraveling these questions, Super-Kamiokande continues to measure solar neutrinos with more precision and high statistical accuracy.

Atmospheric neutrinos

What is atmospheric neutrinos?

High energy particles, such as protons and helium nuclei, are continuously raining down on the earth from space. When these so-called “cosmic rays” collide with atoms in the atmosphere they produce a “shower” of many kinds of particles. These showers include pions, kaons, and muons, which produce neutrinos, known as “atmospheric neutrinos,” when they decay (Figure 1). Atmospheric neutrinos come in two types: electron neutrinos and muon neutrinos.

While most of a shower’s particles are either absorbed by the ground or atmosphere, neutrinos can travel all the way though the earth since they very rarely interact. Super-Kamiokande was made as large as possible (50 kilotons) to observe the few that do interact. In fact, though there are roughly 100 atmospheric neutrinos passing through each square meter of the planet’s surface every second, Super-Kamiokande observes only eight per day on average.

Since atmospheric neutrinos are produced all over the planet and neutrinos can easily pass through it, Super-Kamiokande observes neutrinos from all directions. Neutrinos that are produced above the detector (downward-going) travel a short distance, about 10 km, while those that are produced below it (upward-going) on the other side of the earth travel more than 13,000 km (Figure 2) to the detector.



If neutrinos have no mass, as was once thought, one would expect to observe about the same number of upward-going and downward-going atmospheric neutrinos. While this is precisely the case for Super-Kamiokande’s electron neutrino data (Figure 3, left) there is a large deficit of upward-going

events observed in the muon neutrino data (Figure 3, right). At the same time the downward-going events agree with the expectation. Muon neutrinos that travel long distances before reaching the detector seem to be disappearing!


We now know that this disappearance is due to “neutrino oscillations,” a phenomenon in which massive neutrinos change their type mid-flight. In this case the upward-going muon neutrinos have oscillated into a third neutrino type, the tau neutrino. This result indicates that neutrinos have mass. Downward-going muon neutrinos, on the other hand, interacted inside of Super-K before they had traveled far enough to change types.

So how do we know that muon neutrinos have oscillated into tau neutrinos? Actually its possible to search for signs of tau neutrino interactions inside of Super-K. Since there are no tau neutrinos to start with, finding evidence of tau neutrino interactions would indicate they came from oscillations.

However, tau neutrino interactions are even more rare than those of the other neutrinos and they often produce many particles, which makes them hard to distinguish from those other neutrinos. For this reason Super-K searches for tau neutrino-like interactions in the data (Figure 4). The colored part of the histogram shows the number of events that are attributed to tau neutrino interactions. Notice that the upward-going data, shown as circles, would not agree with the prediction without these events. Also, notice that there is no sign of tau interactions in the downward-going data. Tau neutrinos are appearing just where the muon neutrinos disappeared!


There are still many things about neutrinos which are currently unknown but that can learned by studying using neutrino oscillations. For instance, though it is known that there are three massive neutrinos, it is not known which of the three is the heaviest. Neutrino oscillations depend on the difference of the neutrino masses (more precisely the difference of the squared masses), and the sign of that difference changes the number of events expected at Super-Kamiokande. So it is possible to determine which neutrino is heaviest by studying oscillations with more atmospheric neutrino data.

Atmospheric neutrinos can also be used to study the effects of matter on neutrino oscillations. It is thought that electron neutrinos should undergo additional interactions with matter that are not felt by the other neutrinos as they travel through it. Since the earth’s core is made of very dense material ( 13 g/cm3 ) with a lot of electrons the oscillations of neutrinos that pass through it are expected to be enhanced when those neutrinos have certain energies. This enhancement has not yet been seen, but is of great interest since it is closely related to the neutrino mass ordering.

In addition, atmospheric neutrinos can provide insight into the behavior of neutrinos and their antiparticles, anti-neutrinos, both of which are observed at Super-Kamiokande. If the behavior of neutrinos and anti-neutrinos is found to be different, it would have profound impact on our understanding of the universe, its origins, and evolution.


Precise measurements of the atmospheric neutrino spectrum itself is another research topic with particular importance, not only in the understanding cosmic rays, but also for the characterization of backgrounds to searches for rare processes at Super-Kamiokande, like the decay of protons.

Figure 5 shows the atmospheric neutrino spectrum measured by Super-Kamiokande and other experiments. Solid lines show the prediction from detailed computations of the expected spectrum, which is in good agreement with the data.

To go even further, Figure 6 shows the horizontal directional distribution of atmospheric neutrino events together with the expectation. Due to the presence of the earth’s magnetic field the number of neutrinos from the west is expected to differ from the number from the east. Though neutrinos themselves are neutral particles and have no electric charge, their parent cosmic rays (mostly protons) are charged and feel the effects of the magnetic field. That the data and prediction agree so well suggests that our modeling of the processes that create atmospheric neutrinos is correct. Viewed another way, we can be sure that the neutrinos we are observing are in fact atmospheric neutrinos.


Proton decay

All materials in this space are made of atoms, which consist of nucleus and electrons. Furthermore, nucleus is a composite of protons and neutrons. Neutron is slightly heavier than proton and neutron can decay into proton, electron, and neutrino (beta decay), but it has been thought that proton is eternally stable because it is the lights it really true? Grand Unified Theory, which unifies strong, weak, and electromagnetic interactions, predicts proton will decay into lighter particle like mesons and leptons. The dominant decay mode is that proton decays into a neutral pion and electron. The neutral pion immediately decays into two gamma rays, thus we can observe three electron-like rings in Super-Kamiokande (Fig.1). If protons decay, all materials in the world will be broken in future. But, don’t worry! The predicted proton lifetime is much longer than age of the universe.

Then, how can we measure such long lifetime? Lifetime of a particle is defined as the time which number of the particle decrease to 1/2.7 from start time of measurement. If we can collect many protons and some of them decay, we can estimate proton lifetime unless waiting for so long time. Super-Kamiokande uses 50,000 tons of pure water and it contains 7×1033 protons. We are measuring proton lifetime with huge number of protons.

Super-Kamiokande has started measurement since 1996 and is running more than 20 years, however, we have not observed any evidence of proton decay yet. From this result, proton lifetime is estimated to be longer than 2×1034 years (age of the universe ~1010 years). If we find proton decay, it will be key of a door for Grand Unified Theory beyond the Standard Theory. Super-Kamiokande will keep running towards a new horizon of the world of particle physics.

Fig.1: One of the predicted proton decay mode: A proton decays to a positron and a neutral pion and a neutral pion immediately decays into two gamma rays.

Illustration of Proton Decay

Supernova burst neutrinos

A supernova explosion happens when a star at least 8 times more massive than the sun collapses. An enormous amount of energy (more than 99% of energy emitted from the Sun for 4.5 billion years) is released primarily in the form of neutrinos in just 10 seconds.

On February 23, 1987, a supernova explosion occurred in the Large Magellanic Cloud. From this explosion, the supernova neutrinos were detected for the first time. Kamiokande detected 11 events of these neutrinos. This observation confirmed that the theory of supernova explosion was correct and was the dawn of a new era in neutrino astronomy.

Supernova 1987A, which occurred on February 23, 1987. The right figure shows before explosion. (Anglo-Australian Observatory/David Malin)

The 11 points at 0 seconds show the observed events of the supernova1987a neutrinos.

A supernova explosion is expected to occur inside our galaxy once every 10 to 50 years. If such an explosion occurs, Super-Kamiokande is expected to detect about 8,000 neutrinos. We will be able to reveal the mechanism of supernova explosions by analyzing the information of the energy and the arrival time of these neutrinos.

Super-Kamiokande has a real-time supernova alert system which is running all the time in order not to miss a great discovery of the century. If a supernova occurs in the Galaxy, for example, the alert system will immediately get started to analyze data, so that within 1 hour we can announce that the Super-Kamiokande detects a supernova and send information about the detection time, the number of detected neutrinos, and the direction of the supernova etc.. Since the photons are emitted from the collapsed star after neutrinos, optical observatories will detect a supernova after Super-Kamiokande. Therefore, an announcement from Super-Kamiokande will help astronomers in the word to watch the moment of the explosion.

T2K experimentTokai to Kamioka long baseline neutrino oscillation experiment

From K2K experiment to T2K experiment

In 1998, the Super-Kamiokande discovered the phenomenon called “neutrino oscillation,” wherein muon neutrinos are transformed into another type of neutrinos during flight by observing atmospheric neutrinos. Before this discovery, neutrinos had been considered not to have mass.
However, since neutrino oscillation occurs only when neutrinos have mass, it was experimentally elucidated by this observation that neutrinos have finite mass. The K2K (KEK to Kamiokande) experiment was conducted from 1999 to 2004 to confirm the neutrino oscillation using artificial neutrinos.

The K2K experiment was the first long-baseline neutrino oscillation experiment in the world. In this experiment, the neutrinos produced by the accelerator in the High Energy Accelerator Research Organization, Tsukuba City, Ibaraki Prefecture, were detected with the Super-Kamiokande, located 250 km away. We observed how the produced neutrinos changed into other kinds of neutrinos during flight. As a result, the neutrino oscillation discovered by atmospheric neutrinos was confirmed with an accuracy of 99.9% or higher.

Based on the success of the K2K experiment, the T2K (=Tokai to Kamioka) experiment started in April 2009 to observe neutrino oscillation precisely using a more powerful and high-performance neutrino beam.

In the T2K experiment, the world’s strongest neutrino beam, produced at the Japan Proton Accelerator Research Complex (J-PARC) in Tokai Village, Ibaraki Prefecture, is injected into the Super-Kamiokande located 295 km away. More precise research results for neutrino oscillation have been expected using a neutrino beam that is 50 times stronger than the K2K experiment.

The T2K experiment is an international collaborative research project. Approximately 500 researchers from 12 countries, including the United States, Canada, Europe, and Japan, are participating.

Why do we need accelerator neutrinos?

To accurately measure neutrino oscillations, it is necessary to get the information at the neutrino-production point and compare it with the information after flying a long distance. For this purpose, artificially produced neutrinos are useful, because we can accurately grasp the energy and the number of neutrinos at the generated point. In addition, we can also produce neutrinos focusing on the energy range wherein the effect of neutrino oscillation is most significant.

Thus, the accelerator neutrinos are very useful for measuring neutrino oscillations precisely.


Neutrinos are believed to have played an important role in the evolution of the universe. For example, there is so much more matter than antimatter in the universe. The difference between neutrino oscillation and antineutrino oscillation may reveal the mystery of this fact. Thus far, the whole picture of neutrinos has yet to be elucidated. In the T2K experiments, we can conduct research to clarify the nature of neutrinos by using intense neutrino and antineutrino beams produced in accelerators.

Neutrino oscillations can be represented by three mixing angles (θ12, θ23, and θ13), which indicate the mixing of neutrinos, two squared differences in neutrino mass (Δm212 and Δm232), and parameter (δ), which represents the difference in oscillation between neutrinos and antineutrinos. The parameters of θ12 and Δm212 have been measured by the observation of solar neutrinos and nuclear reactor neutrinos. Furthermore, the parameters of θ23 and Δm232 have been measured by atmospheric neutrinos and the K2K experiments, etc.

Furthermore, for the values of θ23 and Δm232, which had been already measured, it has become possible to measure more accurately than before. As of 2015, the T2K experiments succeeded in measuring these parameters with the highest accuracy in the world.

At present, the finite values of θ13 have been measured. There are plans to conduct experiments where an antineutrino beam is injected into the Super-Kamiokande, and to demonstrate the mystery of asymmetry between matter and antimatter, based on the measurements of the difference between neutrino oscillations and antineutrino oscillations.

J-PARC neutrino beam is the world’s highest intensity

In the Japan Proton Accelerator Research Complex (J-PARC) in Tokai Village, Ibaraki Prefecture, protons are accelerated up to 50 GeV using the proton synchrotron. When the accelerated protons collide with the carbon target, a large number of pions are produced. The pions decay into muons and neutrinos while traveling for tens of meters. Then, the produced neutrinos are injected in the direction of Kamioka. However, since a neutrino does not have an electric charge, we cannot control the neutrino direction.

Therefore, a magnetic field is applied to the pions before they decay into neutrinos using an electromagnetic horn, and the pion direction is aligned toward Kamioka. The pions aligned by the electromagnetic horn decay into muons and neutrinos while flying in a cavity 94 m in length, called a decay volume. At the end of the decay volume, there is a hadron absorber made of graphite that can stop non-neutrino particles.

The neutrino beam center is shifted by 2.5 degree from the Super-Kamiokande

In the T2K experiment, the direction of the neutrino beam is different from that in the K2K experiment. In the K2K, the center of the neutrino beam was directed toward the Super-Kamiokande. However, in the T2K experiments, the center of the beam is 2.5° below the Super-Kamiokande.

The state of the neutrino oscillation depends on the energy and flight length of the neutrino. For T2K, the oscillation effect is maximum at the neutrino energy with 0.5 to 0.7 GeV, for 295 km from the neutrino generation point to the detection point. By adjusting the neutrino energy in this energy range, it is possible to observe the neutrino oscillation more effectively. By shifting the center of this neutrino beam by 2.5° from the Super-Kamiokande, the neutrino energy can be focused on the desired value irrespective of the fluctuation of pion energy. This makes it possible to observe the effects of neutrino oscillations more accurately, and the sensitivity of the observation is expected to be improved.

Obtain information at the generation point.

In the T2K experiment, we observe how neutrinos change after flying long distances. Therefore, we need to obtain both the information at the neutrino generation point and the Super-Kamiokande. To obtain the information on the neutrino generation point, a near-detector is placed 280 m from the carbon target. The detector located at the beam center measures the direction and stability of the beam. The detector placed in the direction of the Super-Kamiokande measures the energy distribution, etc.

How do we find the J-PARC neutrinos in Super-Kamiokande?

We upgraded the Super-Kamiokande data acquisition system in September 2008. As a result, we can record all signals from the photomultiplier tubes. It’s about 500 GB of data obtained in a day. In this vast amount of data, various signals are included such as atmospheric neutrinos, solar neutrinos, cosmic-ray muons, and radiation from radon in the bedrock. We use the following method to distinguish J-PARC artificial neutrino signals from other signals.

At the J-PARC, neutrinos are ejected for 5 micro sec. (1 micro sec. corresponds to one-millionth of one second) once per three seconds. The neutrino beam shooting time at the J-PARC and the time detected neutrinos at the Super-Kamiokande are accurately recorded using GPS satellite radio waves. The neutrino shooting time is provided from the J-PARC to the Super-Kamiokande using the Science Information Network (SINET3). We can calculate the arrival time at Super-Kamiokande by adding the flight time between J-PARC and Kamioka (about 1/1000 second) to the neutrino shooting time at J-PARC. By selecting the data detected at this time, we can identify the neutrinos coming from the J-PARC.

At the J-PARC, neutrinos are ejected for 5 micro sec. once per three seconds.

Results and Future

The T2K experiment has precisely investigated neutrino oscillations using high-intensity and high-performance neutrino beams. In 2011, two years after the start of the experiment, we confirmed the appearance of electron neutrinos wherein muon neutrinos changed to electron neutrinos, and observed for the first time in the world the signs that θ13 has a finite value (for related articles,please see here). After that, the values were actually measured in experiments using overseas nuclear reactors and the T2K (for related articles, please see here). From these experiments, all the oscillations among the three neutrino oscillations were observed.

The candidate event of muon neutrino from J-PARC

The candidate event of electron neutrino appearance

However, there is a slight difference between the results obtained using low-energy antineutrinos in the nuclear reactor experiments and the results obtained in the T2K experiments using relatively high-energy neutrinos. The results suggest a possibility that there is a difference between neutrino oscillation and antineutrino oscillation. For the oscillation parameters θ23 and Δm232 related to the neutrino oscillations from muon neutrinos to tauon neutrinos, the T2K experiments succeeded in measuring at the highest sensitivity in the world. However, since more highly sensitive measurements are required at present, efforts are now underway to realize such measurements.

As described above, the measured values of θ13 were relatively larger than those expected by researchers, and there was a difference between the results obtained by the nuclear reactor experiments and those obtained in the T2K experiments. Therefore, the next main objective is to investigate the difference between neutrino oscillation and antineutrino oscillation. At present, an antineutrino beam produced by the accelerator is injected into Kamioka, and data to directly verify the difference in the oscillation is being collected.

The allowed region of the oscillation parameters θ23、Δm232 measured in T2K

The allowed region of the oscillation parameters θ13、δCP measured in T2K

Search for supernova relic neutrinos (SK-Gd project)

Supernova remnant in the universe

In February of 1987, the Kamiokande detector detected the world’s first neutrinos from a supernova burst. (See more)  Since then, no supernova explosion has occurred in or near our galaxy, so we have not observed any neutrinos from a supernova burst since then.

Supernova explosions in our galaxy may be fairly rare, but supernovae themselves are not. On average, there is one core collapse supernova (ccSN) somewhere in the universe each second. The neutrinos emitted from all of these ccSN since the onset of stellar formation have suffused the universe. We refer to this thus-far unobserved flux as the Diffuse Supernova Neutrino Background [DSNB], also known as the “relic” supernova neutrinos.

The detection of the supernova relic neutrinos enables us to investigate the history of star formation, a key factor in cosmology, nucleosynthesis, and stellar evolution. Furthermore, the study of supernova bursts, which produce and disperse elements heavier than helium, is vital to understand many aspects of the present universe.

New method to detect supernova relic neutrinos

The flux of the supernova relic neutrinos is expected to be several tens per square centimeter per second. It is very weak compared to the flux of higher energy neutrinos: six million per square centimeter per second. Theoretical models vary, but as many as five supernova relic neutrinos per year above 10 MeV are expected to interact in Super-Kamiokande. However, in order to separate the rare DSNB signals from the much more common solar neutrinos and other backgrounds, we need a new detection method.

Supernova bursts generate all types of neutrinos, however, because of its larger cross section, anti-electron neutrinos are the most copiously detected neutrinos in a water Cherenkov detector like Super-Kamiokande. About 80% of the detectable supernova neutrino events are inverse beta interactions: an anti-electron neutrino interacts with a proton, ending up with a positron and a neutron in the final state. Super-K can detect the relativistic positron because it emits Cherenkov light. But to identify the signal as coming from an anti-electron neutrino, we need to detect not only the positron but also the neutron.

New method to detect supernova relic neutrinos

We are considering dissolving a 0.2% concentration of a gadolinium compound in Super-Kamiokande in order to detect the neutron. The cross section of gadolinium to capture neutrons is very large, and the gadolinium then emits a cascade of observable gamma rays after the capture reaction. The coincident detection of a positron’s Cerenkov light, followed shortly thereafter in roughly the same place by a shower of gamma rays, will serve to positively identify inverse beta reactions in the detector.

Once we add gadolinium to Super-Kamiokande, we expect to record up to 20 supernova relic neutrino signals – with almost no background – after five years data. This will be the world’s first observation of the DSNB. The same coincidence technique will also allow Super-K to make a very high statistics measurement of the anti-electron neutrino flux and spectrum from all of Japan’s nuclear power reactors, yielding the world’s most accurate determination of the mixing parameters connecting the first two generations of neutrinos.

Proof-of-Principle Experiment is underway

Super-Kamiokande wants to continue its precise observation of solar, atmospheric, and man-made neutrinos, so it is necessary to confirm that adding the gadolinium will not negatively affect other neutrino observations. Therefore, we have excavated a new experimental hall for dedicated gadolinium R&D studies; it is located near the Super-Kamiokande detector in the Kamioka mine. The gadolinium test facility consists of a 200 ton stainless steel tank containing 240 50-cm photomultiplier tubes (227 50-cm tubes are of the same type as in Super-K and 13 of them are prototypes for Hyper-Kamiokande), DAQ electronics, calibration equipment, a water attenuation length measurement device, and a selective water filtration system needed to keep the water clean yet retain the dissolved gadolinium in solution.

The new laboratory for the gadolinium test facility was completed in March 2010, and the main 200-ton tank was completed in June 2010. Performance tests of the innovative selective water circulation system, both with and without dissolved gadolinium sulfate in the main tank, were carried out from 2011 through early 2013. In August 2013, 240 photomultiplier tubes were installed in the tank and connected to a DAQ system, turning the 200-ton test tank into an operational detector.

The gadolinium test facility

In early 2014 the water filtration system was upgraded, and starting in November of that year the operational 200-ton detector was loaded in steps with gadolinium. By late April 2015 we had reached the target concentration of 0.2% gadolinium sulfate by mass. 

Inside of the main 200-ton tank

We have since confirmed that the water attenuation length in the fully Gd-loaded test detector is maintained at the same level as the attenuation length of the ultra-pure water in Super-Kamiokande. Other calibration efforts have allowed us to observe the gamma ray cascades emitted by the dissolved gadolinium following neutron capture, and to confirm that our simulations of this new water Cherenkov technology are accurate and reliable. Based on these achievements, Super-Kamiokande collaboration approved the “Super Kamiokande-Gd” project in June, 2015.