Super-Kamiokande

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.

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.

Fig.1

Fig.2

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!

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.

Fig.5

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.

Fig.6

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.

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.

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.

Purpose

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.

For the remaining parameter of θ₁₃, even in 2009, only the upper limit value had been obtained by antielectron neutrino observations from nuclear reactors. In the T2K experiments, since a large amount of high-quality data could be obtained, it was expected that the value of θ₁₃ would be measured by discovering undiscovered oscillations from muon neutrinos to electron neutrinos. Then, the measurement of θ₁₃ values was successfully achieved for the first time in 2011 by observing the appearance of electron neutrinos where a muon neutrino changes to an electron neutrino (for details, please see here). Finally, in 2013 the definitive evidence for the appearance of electron neutrinos was able to be obtained. This allowed us to confirm all three neutrino oscillations experimentally.

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 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

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.

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