Takaaki Kajita (2016) - Atmospheric Neutrinos

Good morning. It’s an honour to speak in this meeting. So this morning I want to discuss atmospheric neutrinos. The outline of this talk is like this: A brief introduction on neutrinos and also atmospheric neutrinos. Then I want to describe the discovery of neutrino oscillations. And then I move on to neutrino oscillation studies with atmospheric neutrinos. And future atmospheric neutrino experiments. And I will summarise. Now I want to describe what are neutrinos. Well, neutrinos are elementary particles like electrons and quarks. They have no electric charge. They have, like the other particles, I mean other quarks or leptons, neutrinos have 3 types or 3 flavours, namely electron-neutrinos, muon-neutrinos or tau-neutrinos. And neutrinos are produced in various places, such as the Earth’s atmosphere, or the centre of the sun, or many other places. And they can easily penetrate through the Earth. However, of course, they interact sometimes, although it’s very rare. But anyway, it’s important they interact. And a NuMu interaction produces a muon and a NuEpsilon interaction produces an electron. And therefore we are able to study the neutrino flavour by observing a muon or electron. And also I want to mention that in the very successful Standard Model of particle physics, neutrinos are assumed to have no mass. Now, in this talk I’m going to describe atmospheric neutrinos. So I want to describe, what are atmospheric neutrinos. Well, as you know cosmic ray particles enter into the atmosphere from somewhere in the universe. And they interact with the air nuclei and produce these pions. Then these pions decay to a muon and then to an electron. During this decay chain 2 new neutrinos are 1 electron neutrino are produced. And they are observed in a neutrino detector in the underground. The study of atmospheric neutrinos began more than 50 years ago. And in fact, in 1965, atmospheric neutrinos were observed for the first time by detectors located very deep underground. Well, these neutrinos were observed in these 2 experiments, one in South Africa and one in India. They were, indeed, located extremely deep. And they observed muons produced by atmospheric neutrino interactions. Now, this was about 50 years ago. But the general interest in atmospheric neutrinos was not so high until 35 years ago, no, 30 years ago, sorry. I want to move on to the discovery of neutrino oscillations. Before describing the discovery of neutrino oscillations, I want to describe the background for this discovery. In the 1970s new theories that unify Strong, Weak and Electromagnetic forces were proposed. This is a very appealing theory. Fortunately, the theories predicted that protons and neutrons should decay with the lifetime of about 10^28 to 10^32 years. This is, of course, a very long lifetime, but this is an observable lifetime. Therefore several proton decay experiments began in the early '80s, and one of them was the Kamiokande experiment. Kamiokande was Kamioka Nucleon Decay Experiment. Well, this is the schematic of the Kamiokande experiment. It is a 3 kiloton water Cherenkov detector. And if a charged particle is produced somewhere in the tank, then a Cherenkov photon is emitted on to these directions And these Cherenkov photons are detected by the photo detectors, located at the detector wall. This photo shows the Kamiokande construction team. Well, of course, we had to construct the Kamiokande detector in the mine, so we had to work like this. So we had the safety hat and the working clothes. And so this was our style. Actually, somehow I liked this kind of work. And, in fact, I am one of the graduate course students who was standing behind Professor Koshiba, who was actually my thesis advisor, and who was the 2002 Nobel Prize Laureate. Anyway, I liked this kind of work, so I enjoyed the construction of the detector in the mine. I received my PhD in March 1986. The thesis topic was on proton decay. And well, of course, I didn’t find any proton decays, although I worked hard. And during this thesis studies I found that, maybe, we can improve the analysis of proton decay searches by improving several analysis softwares used in the Kamiokande experiment. Therefore, soon after getting my PhD, I began to improve the analysis software in ’86. And one of them was a software to identify the particle-type for a Cherenkov ring. Namely, I wanted to know if a Cherenkov ring is produced by an electron or a muon. And here, in this picture, this is the typical electron neutrino event. And this is a typical muon neutrino event. So we wanted to identify, if the Cherenkov ring is produced by an electron or a muon. And this was, I have to say, this was for the improvement of the proton decay searches. However, of course, if we developed software, we have to test the new software with the simplest application. Therefore, as a test, I have tested the new software with the simplest atmospheric neutrino events. And, in fact, I checked the neutrino flavour, that is if the event is a NuEpsilon or a NuMu. And I found that the number of NuMu neutrino events was much fewer than expected. Of course, that couldn't be right. There must have been some serious mistake somewhere in the analysis, or simulation, or data reduction. We thought that it’s very important to find out where is our mistake. So we started various studies to find the mistakes in late 1986. Well, we really worked hard, but even about after 1 year of studies no mistake was found. So we concluded that the NuMu deficit, muon neutrino deficit, cannot be due to any major problem in the data analysis, nor the simulation. And therefore we decided to publish our study. And this is the essential data we published in 1988. Here, in this publication, we simply compared a number of observed muon neutrino events with the simulated number of muon neutrino events. And also we did the same thing for electron neutrino events. And obviously, you can see, that for the electron neutrino events the data and the simulation are going quite well. However, it’s clear that for the muon neutrino events there is a significant deficit. So, basically, in this paper, this is the only data we presented. And we concluded that we are unable to explain the data as the result of systematic detector effects, or uncertainties in the atmospheric neutrino fluxes. Some are, as yet, unaccounted for physics such as neutrino oscillations might explain the data. That was our conclusion in 1988. Well, this was the publication, but for a few years we were unable to get the supporting result from other underground experiments. It was only in 1991/92 that we got the same result from another experiment. The same result came from another large water Cherenkov detector, it was an IMB experiment. They also observed the deficit of muon neutrino events. And this graph shows the amount of the muon neutrino deficit compared with the calculated, or expected, value. So both experiments observed a significant deficit. Well, if there is no deficit then the data should be consistent with unity. So that was quite interesting. However, I have to say that the observation of the muon neutrino deficit was not really enough to conclude that this was due to neutrino oscillations. We needed more strong evidence. Now, in order to study the strong evidence for neutrino oscillations, what should we do? And for this I think I need to describe what are neutrino oscillations: If neutrinos have masses, neutrinos change their flavour, or neutrino type, from one flavour to the other. For example, a muon neutrino produced at the point can change to another flavour, that are tau-neutrinos. As shown in this graph, a muon neutrino produced at this point might change, well their survival probability may change in this way. And if they fly further, their survival probability comes back to unity and goes back to very small number. This way the survival probability oscillates. And also when the muon survival probability gets lower, then the tau-neutrino appearance probability gets high. So we need to observe this effect. What will be the effect that we can observe? If we assume some neutrino mass, then we can imagine that neutrinos produced in the upper atmosphere do not have enough time to oscillate. Therefore they are observed as muon-neutrinos. However, neutrinos produced on the other side of the Earth, they have long distances to travel. Therefore they have time to oscillate to another flavour. So if we observe this kind of effect, that can be a very strong evidence for neutrino oscillations. Yes, that is the observation we should have carried out. And, in fact, in Kamiokande we tried to observe this effect. But we realised with Kamiokande, that a 3 kiloton water Cherenkov detector was too small to study this. So it was clear that we needed a much larger detector tan 3 kiloton water Cherenkov detector. That is the Super-Kamiokande experiment. It is a 50 kiloton water Cherenkov detector. And this is an international collaboration at present: researchers from 8 countries are collaborating in Super-Kamiokande. And this is a very large experiment detector. Therefore, in order to construct the Super-Kamiokande detector, we really need to work hard with collaboration-wide effort. So this is a photo that we took while constructing the Super-Kamiokande detector, in the spring of 1995. Typically, these people worked in the mine every day, almost for one year. And you may imagine that, well, in the construction of the Super Kamiokande detector we hired a lot of workers. But that’s not true. Most of these people on this photo are our collaborators, maybe 80%, 90% of them are Super-Kamiokande collaborators. So we really worked hard for 1 year to construct the detector. And this is a photo we took while we filled the Super-Kamiokande detector with pure water. At that time, that was January ’96, pure water was filled to almost half of the height, or depth, of the Super-Kamiokande. And, in fact, this experiment worked well from the beginning. And from the beginning of the experiment we continuously observed this kind of events. The left side is the typical single Cherenkov ring muon neutrino event. And the right side is the single Cherenkov ring electron neutrino event. Well, I don’t know if you can clearly distinguish these 2 patterns. But, well, to me it’s clear that they are clearly different. Anyway, the separation is done by software, so we don’t have any bias. But it’s clear that we were able to analyse these neutrino events quite efficiently. And only in 2 years we were able to come to a very important conclusion, that is the evidence for neutrino oscillations. And this is a presentation at Neutrino ’98. The upper figure is the electron neutrino event and the lower figure is the muon neutrino event. The number of events are plotted as a function of zenith angle. This means, neutrinos coming from the upper atmosphere are around 1, and neutrinos coming from the other side of the earth are around -1. So you can see there is a clear deficit observed for upward-going muon neutrinos. And, well, statistical significance is very high, therefore this cannot be a statistical fluctuation. And in order to explain this data, it was clear that we needed neutrino oscillations. So Super-Kamiokande concluded that the observed zenith angle dependence gave evidence for neutrino oscillations. Fortunately, at that time, there were 2 other atmospheric neutrino experiments going on. And these experiments also observed the zenith angle dependent deficit of muon neutrinos, and confirmed the neutrino oscillations. In the last 5 or 6 minutes I want to describe neutrino oscillation studies with atmospheric neutrinos. I mentioned the discovery of neutrino oscillations in 1998. And this is the ’98 data. Of course, Super-Kamiokande is continuously updating the data. And this is the – well, already 2 year’s old data - but this is the Super-Kamiokande data in 2015. Compared with ’98, the amount of the data is improved by a factor of 10. So with this huge data statistics Super-Kamiokande has carried out various studies of neutrino oscillations. And I just want to mention one example, that is detecting tau-neutrinos. So far I have been describing the neutrino oscillations by the observation of muon neutrino deficit. However, if the oscillations are between NuMu and NuTau, one should be able to observe NuTau interactions. And the typical NuTau interaction in the Super-Kamiokande detector is like this. So this is a complicated event pattern. And actually it’s not possible for Super-Kamiokande to identify NuTau events by event-by-event basis. And therefore we need statistical analyses, knowing that NuTau is upward-going only. And, in fact, we carried out these special NuTau search analysis and this is the result. I cannot go into it in detail, but this part, shown in grey, is the part that we need for the NuTau appearance. And this grey part, if we integrate it, is a total of 180 events. And the expected number of NuTau appearance was 120. So we have the NuTau evidence for NuTau appearance at 3.8 sigma. So far I have only discussed the experiment. I never discussed why neutrino masses are relevant. So in this slide I want to describe why we think neutrino masses are important. And this figure shows the masses of quarks and charged leptons for the first generation, second generation and the third generation. Because of the neutrino oscillation studies, at present we know the neutrino masses, if we assume something. So under some assumption I can plot the neutrino masses, which is here. So you can clearly see that neutrinos are much, much, much lighter than quarks and charged leptons. In fact, neutrino masses are approximately, or more than, 10 billion, or 10 orders of magnitude, smaller than the corresponding masses of quarks and charged leptons. This is important, and we believe this is the key to understand the nature of the smallest, that is the elementary particles, and the largest, that is the universe. So we are really excited with these small neutrino masses. And this could be one of the keys to go beyond the Standard Model of particle physics. Now, briefly, I want to describe the future atmospheric neutrino experiments. Well, although there has been a tremendous progress in the neutrino fields after the discovery of neutrino oscillations, there are still things to be understood. I just want to mention 1 example. I showed this graph. I said, if we assume something - in fact this assumption is important. I assumed that the third generation neutrino mass is heavier than the second generation one. But we do not know. Maybe the truth could be like this: the so-called third generation could be the lightest. We do not know. So we have to measure if the third generation neutrinos are really the heaviest. And this is really one of the important issues to be observed in the neutrino community. There are lots of new ideas, new projects, to observe the neutrino mass pattern. And these are the experiments that are proposed, or in preparation, to observe the neutrino mass patterns. And among them I want to mention, that these 4 experiments are primarily trying to use, or observe, atmospheric neutrinos to understand the neutrino mass pattern issue. I will summarise: About 50 years ago, atmospheric neutrinos were observed for the first time. And proton decay experiments in the ‘80s observed many atmospheric neutrino events and discovered the atmospheric muon neutrino deficit. In 1998, Super-Kamiokande discovered neutrino oscillations, which showed that neutrinos have mass. Since then, various experiments, including solar neutrino experiments, have studied neutrino oscillations. I’m sorry this part was not discussed in my talk. The discovery of non-zero neutrino masses opened the window to study physics beyond the Standard Model of particle physics. Well, I see many young people and I want to emphasise: there are still many things to be observed, or studied, in neutrinos. And finally, because the title of this talk is 'Atmospheric Neutrinos', I want to mention that atmospheric neutrino experiments are likely to continue contributing to neutrino studies. That’s all, thank you very much.

Takaaki Kajita (2016)

Atmospheric Neutrinos

Takaaki Kajita (2016)

Atmospheric Neutrinos

Abstract

Atmospheric neutrinos are produced by cosmic ray interactions with the air nuclei in the atmosphere. The interactions of these neutrinos have been observed in underground detectors for about 50 years. In 1998, neutrino oscillations were discovered by the detailed studies of these neutrinos with the Super-Kamiokande experiment, a water Cherenkov detector with the total mass of 50 kilo-tons. In this lecture, I will discuss the experimental studies that led to the discovery of neutrino oscillations, the present studies, and the possible future neutrino experiments for further studies of neutrino properties. The implications of the small neutrino masses will also be discussed briefly.

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