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Good morning. First of all, I'd like to thank you for the very kind introduction¹. And also I want to mention that it is really the greatest honor for me to give this lecture today. Today I am going to talk about atmospheric neutrinos.
 
The outline of this talk is shown here. First I will give an introduction of the Kamiokande experiment that was the starting point of my research. Then, I want to discuss the atmospheric neutrino deficit. Then, the discovery of neutrino oscillations and recent results and the future. Then, I’ll summarize and finally I want to mention the acknowledgements.

Now for the introduction.

In the late 1970s, new theories² that unify strong, weak and electromagnetic forces were proposed. These theories predicted that protons and neutrons—namely, nucleons—should decay with a lifetime of approximately 10²⁸ to 10³² years. This is long enough to think about the real effect. However, it is short enough to observe proton decay. Therefore, several proton decay experiments began in the early 1980s, and one of them was the Kamiokande experiment³.

This is a photo of the Kamiokande experiment. It is a three-kiloton water Cherenkov detector and the fiducial mass for the masses of neutrino events, or proton decays, was about one kiloton. And I'd like to tell you the basics of this experiment. If relativistic charged particles propagate in the water, they will emit Cherenkov light⁴. And these photons' Cherenkov light is detected by photon detectors that are placed on the walls of the detector. You can see these dots are actually photo detectors.

I want to mention that initially Kamiokande was a rather small experiment. This was the, I would say, construction team in the spring of 1983. Here, you can see Professor Koshiba, who was the Nobel Laureate in Physics in 2002. And you can also see Professor Totsuka, Professor Kifune and some students maybe two to three meters behind Professor Koshiba. (laughs) One of them was me.

We had an introduction of neutrinos earlier, so I can keep my explanation short here. However, I would still like to repeat some of the main points.
 
Neutrinos are fundamental particles like electrons and quarks. They have no electric charge, and they have three types, or three flavors: electron-neutrinos (ν_e), muon-neutrinos (ν_μ) and tau-neutrinos (ν_τ). They are produced in various places, such as the Earth’s atmosphere, or the center of the Sun, and they can easily penetrate through the Earth, or maybe even the Sun. So, if neutrinos are produced here, they can easily propagate through the Earth, unfortunately also through the detector, and go into space. However, they can interact with matter very rarely. If interactions occur, a muon-neutrino produces a muon, and an electron-neutrino produces an electron. And also I want to mention that in the very successful Standard Model of particle physics, neutrinos are assumed to have no mass. However, physicists have been asking if neutrinos really have no mass.
 
Now I want to move on to the atmospheric neutrino deficit. I want to explain a little bit about how cosmic ray particles entering the atmosphere interact with the air nucleus. These interactions typically produce pions and these pions decay into muons, and then into electrons. In this decay chain, two muon-neutrinos and one electron-neutrino are produced. These are observed in underground detectors.
 

Now, I want to mention a little bit about my work in the early days. I got my PhD in March 1986 based on the search for proton decay. Of course, I did not find any proton decay. But, anyway, I felt that the analysis software was not good enough to select the signal that is proton decay from the background that is atmospheric neutrino interactions most efficiently. Therefore, as soon as I submitted my thesis, I began to work to improve the software. One of the programs was an analysis software for identifying the particle type for multi Cherenkov ring events⁵. Namely, I wanted to know, if, say, each Cherenkov ring in a multi-ring event is produced by an electron or a muon. For example, this is an event observed in Kamiokande. You can see three Cherenkov rings here. I wanted to know if they were produced by a muon or an electron.

Anyway, the new software was ready and was applied to single Cherenkov ring events, which were the easiest events to analyze. It's natural to start from the easiest thing.
 

Then, the neutrino flavors were studied for the atmospheric neutrino events. These are the typical events observed in Kamiokande. This is a typical event pattern for electron-neutrino interaction, and this is a typical event pattern for muon-neutrino interaction. We immediately found that the result was strange. The number of muon-neutrino events was much fewer than expected. At first, I thought that I had made some serious mistake. In order to know where I made a mistake, I decided to scan the events. Immediately, I realized that the analysis software was right. However, I was not optimistic yet. I thought that it was very likely that there were some mistakes somewhere in the simulation, data reduction, and/or event reconstruction. So, we, mostly Professor Masato Takita and myself, started various studies to find the mistakes in late 1986.
 

After more than one year of studies, we concluded that the muon-neutrino deficit could not be due to any major problem in the data analysis nor the simulation. Therefore, Kamiokande decided to publish this result. Basically, in this paper, we reported these numbers. The number of electron-neutrino events was 93, while the predicted number was 88.5. So, basically the numbers agreed. However, for the number of muon-neutrino events, the data was 85, while the prediction was 144. It's clear that there was a deficit in muon-neutrino events. In the paper we concluded: “We are unable to explain the data as the result of systematic detector effects or uncertainties in the atmospheric neutrino fluxes. Some as-yet-unaccounted-for physics such as neutrino oscillations might explain the data.” At this stage we still said ''might.''
 
However, I was very much excited. I want to mention my personal memory. I was mostly excited with the possibility of neutrino oscillations with a large mixing angle. Namely, muon-neutrinos seemed to oscillate maximally to the other neutrino type, which was not expected. This gave me the strong motivation to continue the study.

Now, since I mentioned neutrino oscillations, I want to explain a little about them. If neutrinos have mass, they will change their flavor (type) from one flavor (type) to another. For example, oscillations could occur between muon-neutrinos and tau-neutrinos. If muon-neutrinos are produced here, and then, if they travel long distances, they disappear and reappear. Looking at this diagram, as a muon-neutrino disappears, a tau-neutrino appears. This is neutrino oscillation. One important thing is if the mass of a neutrino is smaller, the oscillation length (L/E) gets longer. Now, I also want to mention the people who theoretically predicted neutrino oscillations: Professor Shoichi Sakata, Professor Jiro Maki and Professor Masami Nakagawa of Nagoya University, and Italian physicist Bruno Pontecorvo. They contributed greatly to this discovery.
 

So far, I have discussed results from the Kamiokande experiments, but there was another detector experiment called IMB⁶. The IMB experiment, which also used a large water Cherenkov detector, similarly reported a deficit of muon-neutrino events. That was encouraging.
 
However, we thought that a simple deficit of muon-neutrino events would not really be enough to conclude that the cause was neutrino oscillations. We wanted more positive evidence for oscillations.

Actually, the physics is simple. If neutrinos are produced in the atmosphere over the detector, then travel distances are so short that there is no time for the neutrinos to oscillate. However, if neutrinos are produced on the other side of the Earth, they need to travel long distances before reaching the detector. So, they may oscillate.

Therefore, we concluded that we should observe a deficit of upward going muon-neutrinos. In fact, we tried to observe such an event with Kamiokande, but it was clear that Kamiokande was not big enough to give us conclusive data. We need