Atmospheric Neutrinos
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Lindau Nobel Laureate Meetings244 / 340
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Transcript: English(auto-generated)
00:15
Okay, and good morning. It's really an honor to speak in this meeting.
00:23
So this morning, I want to discuss atom-fake neutrinos. The outline of this talk is like this. A brief introduction on neutrinos and also atom-fake neutrinos.
00:43
Then, I want to describe the discovery of neutrino situations. And then, I'll move on to neutrino situation studies with atom-fake neutrinos and future atom-fake neutrino experiments, and I'll summarize.
01:02
Okay, now I want to describe what are neutrinos. Well, neutrinos are elementary particles like electrons and cokes. They have no electric charge. They have, like the other particles, I mean, other cokes or electrons.
01:24
Neutrinos have three types or three flavors, namely electron neutrinos, mu neutrinos, or tau neutrinos. And neutrinos are produced in various places, such as the Earth's atom sphere,
01:48
or the center 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.
02:03
But anyway, it's important they interact, and mu mu interaction produces a muon, and a nu E interaction produces an electron. And therefore, we are able to study the neutrino flavor by observing a muon or an electron.
02:23
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 atom-fake neutrinos.
02:45
So, I want to describe what atom-fake neutrinos, well, as you know, cosmic ray particles enter into the atom sphere from somewhere in the universe, and they interact with the air nuclei and produce pions,
03:06
then these pions decay to a muon, and then to an electron. And during this decay chain, two muon neutrinos and one electron neutrinos are produced,
03:20
and they are observed in a neutrino detector in underground. The study of atom-fake neutrinos began, well, more than 50 years ago.
03:42
And in fact, in 1965, atom-fake neutrinos were observed for the first time by detectors located very deep underground. Well, these neutrinos were observed in these two experiments, one in South Africa and one in India.
04:07
They were indeed located extremely deep, and they observed muons produced by atom-fake neutrino interactions.
04:21
Now, this was about 50 years ago, but the general interest in atom-fake neutrinos was not so high until 35 years ago. No, 30 years ago, sorry.
04:45
Now, I want to move on to the discovery of neutrino oscillations. Now, before describing the discovery of neutrino oscillations, I want to describe the background for this discovery.
05:05
Now, in the 1970s, new theories that unified strong, weak, and electromagnetic forces were proposed.
05:21
This is a very appealing theory. Fortunately, these theories predicted that protons and neutrons should decay with the lifetime of about 10 to 28 to 10 to 32 years. This is, of course, a very long lifetime, but this is an observable lifetime.
05:47
Therefore, several proton decay experiments began in the early 80s, and one of them was the Kamiokande experiment. Kamiokande was Kamioka nucleon decay experiment.
06:01
And, well, this is the schematic of the Kamiokande experiment. It is a 3-kiloton water Cherenkov detector. If a charged particle is produced somewhere in the tank, then the Cherenkov photon is emitted to these directions,
06:23
and these Cherenkov photons are detected by the photo detectors located at the detector wall. Okay, now this is the, this photo shows the Kamiokande construction team.
06:46
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.
07:08
And actually, well, somehow I liked this kind of work. And, well, in fact, I'm one of the graduate school students who was standing behind Professor Koshiba,
07:29
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.
07:51
Anyway, I received my Ph.D. in March 1986. Well, the topic, thesis topic was on proton decay, and, well, of course, I didn't find any proton decays.
08:08
But, well, I worked hard and joined the thesis studies. I found that maybe we can improve the analysis of proton decay searches by improving several analysis software used in the Kamiokande experiment.
08:33
Therefore, soon after getting my Ph.D., I began to improve the analysis software in 1986.
08:45
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 these pictures, this is the typical electron neutrino event, and this is a typical muon neutrino event.
09:14
So, we wanted to identify if the Cherenkov ring is produced by an electron or a muon.
09:23
And this was, I have to say, this was for the improvement of the proton decay searches. However, of course, if we developed a software, we have to test the new software with the simplest application.
09:42
Therefore, as a test, I have tested the new software with the simplest atom cell neutrino events. And, in fact, I checked the neutrino flavor, that is, if the event is a new E or new mu.
10:03
And I found that the number of muon neutrino events was much fewer than expected. Of course, that cannot be right. There must be some serious mistake somewhere in the analysis or simulation or data reduction.
10:30
So, we thought that it's very important to find out where is our mistake. So, we started various studies to find mistakes in late 1986.
10:47
Well, we really worked hard, but after about one year of studies, no mistake was found. So, we concluded that the muon neutrino deficit cannot be due to any major problem in the data analysis nor the simulation.
11:08
And therefore, we decided to publish our study. And this is the essential data we published in 1988.
11:24
Here, in this publication, we simply compare the number of observed muon neutrino events with the simulated number of muon neutrino events. And also, we do the same thing for electron neutrino events.
11:41
And obviously, you can see that for the electron neutrino events, the data and the simulation agrees quite well. However, it's clear that for the muon neutrino events, there is a significant deficit.
12:00
So, basically, well, 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 atom-seq neutrino fluxes. Some as yet unaccounted for physics, such as neutrino oscillations, might explain the data.
12:28
That's 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.
12:46
It was only 1991-92 that we get the same result from the other experiment. The same result came from another large water channel detector IMB experiment.
13:05
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.
13:24
So both experiments observed a significant deficit. Well, if there is no deficit, then the data should be consistent with the unity. Okay, so that is quite interesting.
13:41
However, I have to say that the observation of the muon neutrino deficit is not really enough to conclude that this is due to neutrino oscillations.
14:00
We need more strong evidence. Now, in order to study the strong evidence for neutrino oscillations, what we should do? And for this, I think I need to describe what are neutrino oscillations. If neutrinos have masses, neutrinos change their flavor or neutrino type from one flavor to the other.
14:27
For example, a muon neutrino produced at a point can change to other flavor that is muon tau neutrinos. So, as shown in this graph, muon neutrino produced at this point might change, well, the survivor probability may change to this way.
14:53
And if they fly further, the survivor probability comes back to unity and goes back to a very small number.
15:01
This way, the survivor probability oscillates, and also when the muon neutrino survivor probability gets lower, then the tau neutrino appearance probability gets high. So, we need to observe this effect.
15:22
Now, so what will be the effect that we can observe? Well, if we assume some neutrino mass, then we can imagine that neutrinos produced in the upper atmosphere do not have enough time to oscillate.
15:49
Therefore, they are observed as muon neutrinos. However, neutrinos produced in the other side of the earth, they have long distances to travel, therefore they have time to oscillate to other flavor.
16:09
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 carry out.
16:24
And in fact, in Kamiokande, we tried to observe this effect, but we realized Kamiokande, three kiloton water chilenko detector was too small to study this. So, it's clear that we needed much larger detector than three kiloton water chilenko detector.
16:45
That is the Super Kamiokande experiment. It is a 50 kiloton water chilenko detector. And this is an international collaboration at present. Researchers from eight countries are collaborating in Super Kamiokande.
17:05
And this is a very nice experiment detector. Therefore, in order to construct the Super Kamiokande detector, we really need to work hard with the collaboration-wide effort.
17:25
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.
17:45
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 collaborators. Maybe 80%, 90% of them are Super Kamiokande collaborators.
18:07
So, we really worked hard for one year to construct the detector. And this is a photo we took while we filled the Super Kamiokande detector with pure water.
18:27
So, at that time, that was January 1996, pure water was filled to almost half of the height or depth of the Super Kamiokande.
18:44
And, in fact, this experiment worked well from the beginning. And from the beginning of the experiment, we continuously observed these kind of events. The left side is the typical single chain-coffering muon neutrino event, and the right side is the single chain-coffering electron neutrino event.
19:06
So, well, I don't know if you can clearly distinguish these two patterns, but, well, to me, it's clear that they are clearly different.
19:21
Anyway, well, the separation is done by software, so we don't have any bias. But anyway, it's clear that we are able to analyze these neutrino events quite efficiently. And only in two years, we are able to come to get a very important conclusion.
19:45
That is the evidence for neutrino oscillations. And this is a presentation at Neutrino 98. And the upper part, upper figure is the electron neutrino event, and lower figure is the fomino neutrino event.
20:03
And the number of events are plotted as a function of zenith angle. This means neutrinos coming from the upper atmosphere are around one, and neutrinos coming from the other side of the earth are around minus one.
20:20
And 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 need neutrino oscillations.
20:46
So, Super Kamiokande concluded that the observed zenith angle dependence gave evidence for neutrino oscillations. And, well, fortunately, at that time, there were two other atomic neutrino experiments going on, and
21:05
these experiments also observed the zenith angle dependent deficit of muon neutrinos and confirmed neutrino oscillations. Now, in the last five, six minutes, I want to describe neutrino oscillation studies with atomic neutrinos.
21:26
Well, I mentioned the discovery of neutrino oscillations in 1998, and this is the 1998 data. Of course, Super Kamiokande is continuously updating the data, and this is the, well,
21:47
already two-year-old data, but this is the Super Kamiokande data in 2015. So, compared with 1998, the amount of the data is improved by a factor of 10.
22:04
So, with this huge data statistics, Super Kamiokande has been carried out various studies of neutrino oscillations. And I just want to mention one example, that is the detecting tail neutrinos.
22:23
So far, I have been described the neutrino oscillations by the observation of muon neutrino deficit. However, if the oscillations are between new muon and new tail, one should be able to observe new tail interactions, and the typical new tail interaction in the Super Kamiokande detector is like this.
22:42
So, this is a complicated event pattern, and actually it's not possible for Super Kamiokande to identify new tail events by event-by-event basis. Therefore, we need statistical analysis knowing that new tails are upward going only.
23:06
And in fact, we carried out these special new tail search analysis, and this is the result. Well, I cannot go into detail, but this part, shown gray, is the part that we need the new tail appearance.
23:27
And this gray part, if we integrate, is total of 180 events, and the expected number of new tail appearance was 120. So, we have the new tail evidence for new tail appearance at 3.8 sigma.
23:48
Well, okay. So far, I have only discussed the experiment. I never discussed why neutrino masses are relevant.
24:01
So, in this slide, I want to describe why we think neutrino masses are important. This figure shows the masses of quarks and charged electrons for the first generation, second generation, and the third generation.
24:27
Now, because of the neutrino oscillation studies, at present, we know the neutrino masses if we assume something.
24:42
So, anyway, 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 the quarks and charged electrons.
25:02
In fact, neutrino masses are approximately more than 10 billion, or 10 orders of magnitude, smaller than the corresponding masses of quarks and charged electrons. This is important, and we believe this is the key to understand the nature
25:22
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.
25:47
Now, briefly, I want to describe the future atomic neutrino experiments. Well, although there has been tremendous progress in the neutrino fields after
26:00
the discovery of neutrino oscillations, there are still things to be understood. So, I just want to mention one example. I showed this graph. I said, if we assume something, in fact, this assumption is important.
26:23
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.
26:43
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. So, there are a lot of new ideas, new projects, to observe the neutrino mass pattern.
27:08
And these are the experiments that are proposed in preparation to observe the neutrino mass patterns. And among them, I want to mention that these four experiments are primarily
27:24
trying to use or observe atomic neutrinos to understand the neutrino mass pattern issue. Okay, I'll summarize. About 50 years ago, atomic neutrinos were observed for the first time.
27:45
And proton decay experiments in the 80s observed many contained atomic neutrino events and discovered the atomic neutrino deficit. In 1998, Super Kamiokande discovered neutrino situations, which shows that neutrinos have mass.
28:03
Since then, various experiments, including solar neutrino experiments, have studied neutrino situations. And I'm sorry, this part was not discussed in my talk. The discovery of non-zero neutrino masses opened a window to study physics beyond the standard model of particle physics.
28:27
Well, I see many young people, and I want to emphasize there are still many things to be observed or studied in neutrinos.
28:40
And finally, because the title of this talk is atomic neutrinos, I want to mention that atomic neutrino experiments are likely to continue contributing to neutrino studies. That's all. Thank you very much.