Oscillation of Atmospheric Neutrinos
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Transcript: English(auto-generated)
00:14
And this morning, I want to discuss the oscillation of atmospheric neutrinos.
00:22
And by the way, this is the photo of the Super-Kamiokande detector. And this photo was taken last year when we opened the Super-Kamiokande tank and we had some improvement work of the detector.
00:43
So, today, essentially, I'm going to talk about the neutrino studies with this detector. So, this is the outline. First of all, I'd like to discuss briefly about neutrinos.
01:00
I understand some of you are not neutrino physicists. So, I have a brief introduction. Then, I want to discuss the activities in Kamioka. Kamioka is the town where the Kamiokande and Super-Kamiokande existed or exist.
01:23
Then, I move on to the discovery of neutrino oscillations. And before summarizing, I want to briefly discuss why we think that the small neutrino mass are important.
01:41
And also, I'll briefly discuss what will be the near future of the neutrino experiments. And I'll summarize. Neutrinos are fundamental particles like electrons and cores. But still, of course, you do not know what are neutrinos.
02:02
And I want to say that neutrinos are something like electrons without electric charge. And in fact, this feature has a significant consequence. Neutrinos can easily pass through even the Earth.
02:24
But sometimes, they can interact with matter. And therefore, we are able to study neutrinos. And I have one more page. Neutrinos have three types, namely electron neutrinos, mu neutrinos, and tau neutrinos.
02:45
They are shown here. And in the very successful standard model of particle physics, neutrinos are assumed to have no mass.
03:02
Of course, this was the assumption. Therefore, physicists have been asking, neutrinos really have no mass. OK, so this is the introduction to neutrinos. Now, I want to move on to our experiment.
03:25
OK, now I want to talk about the first experiment in Kamioka. Well, the name of the experiment is Kamiokande.
03:40
And in fact, this experiment was initially not a neutrino experiment. Let me tell you the brief history. In the 1970s, new theories called grand unified theories
04:07
predicted that protons should decay with the lifetime of about 10 to 30 years. And in fact, this was a very important prediction.
04:22
If we observe proton decays, that means forces should be unified at a very high energy scale. Therefore, several proton decay experiments began in the early 80s,
04:42
and one of them was the Kamiokande experiment. And this is the sketch of the Kamiokande detector. It was a 3,000-ton water detector. Well, the dimension was about 16 meters in diameter and 16 meters in height.
05:03
And if a proton decays, well, if one of the protons in the water in the Kamiokande detector decays, then there should be typically two or three charged particles emitted,
05:24
and each particle propagates in the water, and while propagating in the water, they emit Cherenkov photons, and we detect these photons by photo detectors instrumented on the surface.
05:42
So this was the principle, and with this detector, we wanted to observe proton decay. And in the spring of 1983, we came to the experimental site and started the construction work of the Kamiokande detector.
06:04
This photo was taken in the morning in the spring of 1983, when our spokesperson, Professor Koshiba, came to the site. Well, he was in the middle, in front, and in the back side.
06:23
And there are four students, and these four students, in fact, worked underground in the mine for several months to construct the Kamiokande detector. By the way, you notice our way of working is slightly not like scientists.
06:50
We have safety hat, rubber boots, and so on. Anyway, we come into the mine, 1,000 meters underground,
07:03
and we worked to construct the detector. And this is one of the photos we took during the construction of the Kamiokande detector. Well, I said the detector had the height of about 60 meters,
07:22
and we had to install the photo detectors onto the detector surface. So the issue was how we installed the photo detectors on the wall. And, well, after some discussion, we decided to use these rubber boats.
07:46
Unfortunately, the working condition was not so great with these rubber boats. But, well, I think we were fortunate. No one dropped from the boat. Anyway, the construction work was successful,
08:04
and the experiment began in July 1983. And as I said, we really wanted to observe proton decays. Unfortunately, we found proton lifetime is longer than expected.
08:27
We had no evidence for proton decay. And, well, we simply observed background events to the search for proton decays.
08:44
They are neutrino interactions in the Kamiokande detector. And these neutrinos are created in the atmosphere of the Earth. Well, cosmic ray particles come into the Earth.
09:03
Then these particles interact with the nucleus. Then, typically pions are produced, but pions are unstable. Therefore, they decay to other particles. And during this decay chain, neutrinos are created.
09:20
And these neutrinos, of course, most of them simply pass through the Earth. But sometimes they interact in the Kamiokande detector. And these are the most serious background for the proton decay searches.
09:40
And, well, in fact, after, say, three years, we still had no evidence for proton decays. But we didn't give up. So we decided to improve the proton decay analysis software.
10:08
So we developed new software. And, well, in fact, we developed several new software. And one of them tried to identify the particle propagating in the water.
10:26
Namely, we wanted to know if a particle propagating in the water Oh, I cannot do this well. OK.
10:40
We'd like to know if a particle is an electron or a muon by the detailed analysis of the pattern of the chain of photons. And, of course, if you write a new software, you have to extensively test the new software.
11:04
And indeed, we did several tests. And finally, at the final test, we studied the neutrino type or neutrino flavor of atmospheric neutrinos so far we observed in Kamiokande.
11:25
Then we found that the number of muon neutrino events was much fewer than expected. Well, at that stage, we were simply testing the new software.
11:43
And the software told us the data is something strange. That means, most typically, there is some mistake in the software. So, we started very serious studies.
12:05
Where is the mistake? Well, of course, this is some kind of background events. Therefore, in principle, you may say that,
12:23
well, I can forget about this, but forgetting about some problem is not really a good way. So, we started very serious studies to find mistakes.
12:43
That was the end of 1986. Well, I think, in the 80s, we had a lot of time. We studied to find out mistakes for about a year.
13:06
Well, unfortunately, we didn't find any serious mistake in our software package. And therefore, we concluded that muon neutrino deficit cannot be due to any major problem
13:23
in the data analysis nor the simulation. So, therefore, at that stage, we think that it is our duty to report this strange data to the community.
13:41
So, we wrote a paper, and this is the essence of the paper we wrote. We simply reported the number of observed muon neutrino event, which was about 85,
14:01
then compared with the simulation, which was slightly more than 140. So, clearly, the data show a big deficit of muon neutrinos. And also, we did the same comparison for electron neutrino events.
14:22
And in case of electron neutrinos, the data and the simulation agreed quite well. So, we simply reported this observation data Unfortunately, while the community's reaction to this publication or to this data were not so positive,
14:49
people discussed what was the mistake Kamiokand made. Well, but, as I said, we worked quite seriously for about a year
15:03
to find out mistakes, but we didn't find out any mistakes. So, we had some confidence that there must be something new in this data.
15:21
On the other hand, with simply the deficit of muon neutrinos, we had no clear idea what was the cause of the deficit. But anyway, I was most excited with the data.
15:41
And, well, before finding this problem, I was seriously searching for proton decays, but I changed my research completely from proton decay to neutrino studies, and I wanted to know what is happening in neutrinos.
16:06
Anyway, if I recall my research career, I think this time was the most exciting time to me.
16:21
Although, as I said, we had no clear idea what was going on. Well, in fact, I said we had no clear idea what was going on, but as a possibility, we thought,
16:42
maybe, at that time it was still maybe, but maybe neutrino oscillations are the cause of the deficit. Well, I simply began to say neutrino oscillations, and I want to tell you what are neutrino oscillations.
17:03
If neutrinos have mass, neutrinos change their type from one type to the other. For example, here I show you the, say, muon neutrino survival probability, which is shown blue, as a function of, essentially, distance.
17:27
So, this graph says a muon neutrino is created at 0.0, then if they propagate to the right direction,
17:43
then after, say, 500 kilometers, the probability that muon neutrinos to remain muon neutrinos goes down substantially. But if they propagate further, at around 1,000 kilometers, the probability comes back to unity.
18:03
If they propagate further, the probability goes down, goes up, goes down, and so on. So, this is the muon neutrino survival probability, and when muon neutrino disappeared,
18:20
then the neutrino oscillation theory tells us that at that time, tau neutrino is appearing. So, this is the neutrino oscillation, and we thought maybe we are observing neutrino oscillations.
18:42
However, of course, there were several other possibilities. Therefore, we thought we have to separate the neutrino oscillation from the other possibilities.
19:01
And for this, we thought this way. Well, I said, neutrinos are created in the atmosphere of the Earth. Then, some of them are created just above us,
19:21
maybe 10 to 20 kilometers above us. And these neutrinos, after traveling 10 to 20 kilometers, they come to the detector, and they may interact there. But the flight distance is quite short. Therefore, these neutrinos may have no time to oscillate.
19:45
On the other hand, neutrinos created in the other side of the Earth, they have to travel long distances, typically 10,000 kilometers. So, these neutrinos have really long time to oscillate.
20:07
So, if this picture is right, then we thought we should observe the up versus down asymmetry of the atomic muon neutrino flux.
20:24
Okay, this was the idea. Unfortunately, the Kamiokande 3000-ton detector was too small to observe this effect.
20:41
Simply, the number of neutrino events we were able to observe in Kamiokande was too small to test this effect. So, we needed a much larger detector, and that was Super Kamiokande. Okay, now I want to move on to the Super Kamiokande experiment.
21:07
Well, Super Kamiokande is simply a much larger version of the original Kamiokande detector. It is a 50,000-ton water chain detector,
21:22
and the dimensions are about 40 meters in diameter and 90 meters in height. And again, this is located 1,000 meters below the surface of the mountain.
21:40
And this is an international collaboration. At present, we have about 170 collaborators from 10 countries. And in fact, this detector is still observing neutrinos but the construction was in the 90s.
22:07
The Japanese government approved to construct this detector in 1991, and the construction period was five years. And in the last year, physicists came to the detector in the underground
22:26
to construct the Super Kamiokande detector. And this photo was taken in the spring of 1995 when we constructed the detector. Namely, we installed the photo detectors onto the detector wall.
22:44
And, as I said, Super Kamiokande is much larger than the original Kamiokande detector, and that means we need much more people to install photo detectors onto the detector wall.
23:02
And, in fact, typically these many people worked in the underground to construct the detector, and the construction period was almost one year. Anyway, this work was quite successful.
23:22
Oh, by the way, I should say about 70 to 80 percent of these people are the collaborators of the Super Kamiokande experiment. So, really, physicists worked in underground for about a year to construct the detector.
23:47
I'm spending too much time. Okay, this photo was taken in January 1996 when we essentially finished the detector construction. And from April 1996, the detector worked quite well,
24:06
and we usually daily observe these events. On the left side is the typical electron neutrino event, and the right side is the typical muon neutrino event, and we analyze this data as a team.
24:26
And in two years, we were able to report the analysis result, and this is one of the slides we presented at the Neutrino Conference in 1998.
24:41
And I want to briefly explain what is written here. Here, we show the neutrino arrival direction distribution. Cosine theta one means down-going neutrinos,
25:01
minus one means upward-going neutrinos coming from the other side of the Earth. And these black data points with the error bars show the data, and these squares show the Monte Carlo prediction. And you can see for down-going neutrinos, the data and simulation agreed quite well.
25:26
On the other hand, for upward-going neutrinos, data show almost a factor of two deficits compared with the expectation. And this data was very naively explained if we assume neutrino oscillations.
25:47
And in fact, this way, we were able to convince the world community that neutrinos oscillate. Well, if I had time, I wanted to show this, but I have clearly no time, so I simply skip.
26:03
Sorry. Now, I want to tell you why physicists were so excited with this small neutrino mass. Well, here, I show you the mass of charged leptons and quarks.
26:21
And, well, after more than 20 years of neutrino oscillation studies and other related studies, we have now a fairly good idea on the neutrino mass.
26:41
So I want to show you the approximate neutrino mass on this. And they are here. And first of all, you clearly see that neutrino mass are in fact small. But you have to be careful about the horizontal axis.
27:03
And indeed, you find that neutrino mass are approximately, or even more than, 10 orders of magnitude smaller than the corresponding mass of quarks and charged leptons. 10 orders of magnitude different.
27:21
And we believe this is the key to better understand particles and even the universe. Well, but now, in 20 years, we understand neutrino oscillations fairly well.
27:41
But still, we do not understand everything related to neutrinos. And these are the agenda items for the neutrino experiments in the future. But here, today, I only concentrate on one thing. That is, we'd like to know if the oscillations of neutrinos are identical to the oscillations of anti-neutrinos.
28:10
In fact, this is really a very important issue. This might be related to the baryon asymmetry of the universe.
28:21
That means quarks in the universe, we have matter, but no matter. And maybe neutrinos can tell us. And in fact, there are next-generation experiments designed to study this effect.
28:40
And in fact, there are two projects. One is in the United States and one is based in Japan. And, well, I simply want to say that the next-generation experiment is still going to be even bigger. And in fact, what people are thinking for the future neutrino experiment is this big,
29:06
and it's going to be about eight times bigger than Super K, and we are going to do various studies. Well, okay, my time is up. I want to summarize. An atonic muonutrino deficit was observed by proton decay experiments unexpectedly.
29:26
And subsequently, after 10 years, Super Kamiokande discovered neutrino oscillations, which shows that neutrinos have mass. And the discovery of 9-0 neutrino mass opened a window to study physics beyond the standard model of particles
29:42
and neutrinos with small mass might also be the key to understand the fundamental questions of the universe. And I feel that I was very fortunate. I had very good advisors, colleagues, and I was always involved in very good projects.
30:06
And finally, this is the message to young physicists. If you like physics, and if you are lucky, nature may kindly tell us the secret.
30:22
So, let's enjoy physics. That's all. Thank you very much.