Arthur McDonald (2016) - The Sudbury Neutrino Observatory: Observation of Flavor Change for Solar Neutrinos

Alright, so what I want to talk to you about today is basically the kind of neat things you can do if you create something that’s beyond what people have created before. In our case the creation of an extremely low radioactivity location. A location where you then are able to do experiments that otherwise you couldn’t possibly do. In Canada we have a phenomenon called the Northern Lights. And it lights up the sky due to cosmic ray particles in general. And by going a couple of kilometres underground you can avoid having your detector essentially glow like the Northern Lights do. And that’s what we did in what was originally the Sudbury Neutrino Observatory and then became, by extension, this is this over here, now what’s called SNOLAB. And when you go to a new experimental situation like that, you can study very interesting things. We studied neutrinos from the sun, particularly with the Sudbury neutrino observatory, and so this is kind of midway in our scale from the smallest particles all the way to the largest reaches of the universe. But we also were able to contribute to a knowledge of those smallest particles, neutrinos, and where they fit into the laws of basic physics. We were able to study the sun and get very good information on how it burns. And, of course, we also were able to determine parameters that relate to how the universe has evolved. And that will continue to be the case as we do further experiments in the new laboratory because I’ll say a word or two about dark matter detection, which is a substantial factor in terms of how the universe has evolved. I think the term “stand on the shoulders of giants” was used extensively yesterday and it really is true. The pioneers in this field of solar neutrino physics were Ray Davis, who received the Nobel Prize in 2002 for his work with a large tank of chlorine, and John Bahcall who did the detailed calculations of how the sun should be emitting the electron neutrinos. Davis’s detector of electron neutrinos showed three times fewer neutrinos than were calculated. The question was, were the calculations incomplete or incorrect. Similar for the experiments I suppose, but also the question of whether something else might be happening. And in particular Gribov and Pontecorvo, based on ideas that Pontecorvo and others, and I’ll show you later in more detail, had proposed that neutrinos might oscillate, might change from one flavour into another. And perhaps electron neutrinos were changing into muon neutrinos as was known as that time. And that that was the explanation for why they weren’t being detected in Davis’s experiment which could only see electron neutrinos. Well, that was back, starting in 1968. When we got into this game in 1984 we started because Herb Chen, a professor at UC Irvine, had the audacious question to Canadian funding agencies of whether he could possibly borrow, we could possibly borrow $300 million worth of heavy water to do an experiment. Actually the first thing he asked for would have been the equivalent of $1.2 billion worth of heavy water, but we got it down to 1,000 tons. And it turned out it was possible to borrow that 1,000 tons. And so we ended up forming a collaboration. Here you see the collaboration, there are 16 people started in 1984. In 1986 you see a collaboration meeting at the Chalk River Laboratories. Herb Chen and George Ewen were the original spokesmen in the United States and Canada. Dave Sinclair brought the UK into the project the next year. Unfortunately Herb Chen, although he looks great in this picture, was already suffering from leukaemia. He discovered it actually at this meeting. Six months later he had passed away from leukaemia. As you can see at a very early age. That shocked us all. But we proceeded with the project, proceeded adding additional participants in Canada and the US. I was at Princeton at the time and became the US spokesman after Herb passed away in 1987. And then I became the director of the project in 1989 and moved back to Canada for that purpose. So we started with 16 people. I’m going to show you later 273 eventual authors on the paper. The majority of those people were students and postdocs. And so one of the things we’re particularly proud of is the fact that this was not just a scientific bit of research, but it was an educational exercise as well. And so it’s a real pleasure to me to see this group of young people here today, because one of the pleasures for all of us older people involved in this experiment has been the set of young people we got to work with over the years. So if you have heavy water then you have the opportunity for two separate signatures. It turns out that about 1 in 6,400 molecules of the ordinary water you drink are D2O, instead of H2O, an extra neutron in the nucleus. And that extra neutron, as you can see here, gives you an opportunity for a measurement that has an electron neutrino specifically coming in, changing that neutron into a proton and a fast moving electron that produces via the Cherenkov process a cone of light in the forward direction. With heavy water you also have an additional opportunity to observe a reaction in which those neutrinos can simply break apart heavy water and releasing a neutron. And we had three different ways of detecting that neutron. As my colleague Professor Kajita explained earlier, neutrinos are unique particles that penetrate, well, it turns out they only experience the weak interaction, and therefore for them atoms are basically open space. They only stop or participate in a reaction if they hit the nucleus or one of the electrons head-on. So they can go through, for example, the distance that light travels in a year in lead. So one light year of lead but only a 50% chance of interaction. And so it makes it extremely difficult, these are very small cross sections that you’re dealing with here. But if you can build an experiment that makes it possible for you to observe these two reactions, you can then compare them. That same property of being hard to detect means that the neutrinos can come out from the sun with very little interaction. And so we were able to observe solar neutrinos and compare these two reactions and without reference to how many were produced in the first place, get a ratio of the number of neutrinos that had survived to the detector. In other words what we observed in fact was that only, there were only effectively one third of the total number of neutrinos that were still electron neutrinos at the time that they reached us. But we had to be extremely careful in doing this experiment because there are other processes. Any gamma ray that has an energy greater than 2.2 million electron volts, and they are emitted in the decay chains of uranium and thorium, can break apart, deuterium into neutron and proton and mimic this second reaction by producing a free neutron. And so we had to control the radioactivity extremely well. We ended up controlling the radioactivity in the heavy water in the middle of our detector such that there was less than one radioactive decay per day, per ton of water in the centre. And that was what was necessary in order to be sure that the radioactivity level was low enough that that was not what we were observing. Same time we measured that radioactivity very accurately so that we could do a subtraction of the radioactivity. We had reduced it to a factor 3 or 4 lower than the observations that we had from neutrinos themselves and we measured them to 20 to 30% uncertainty. And so we ended up with a very small uncertainty overall in the contribution from radioactivity. What was really the tour de force in this experiment was control of radioactivity. There is another reaction in which electron neutrinos, actually which all neutrino types scatter from electrons. It’s a significantly lower cross section than the other two and it’s dominated by electron neutrinos. Basically you end up with about 6 times greater sensitivity for electron neutrinos than the other types but you have a small sensitivity to electron neutrinos. And in fact the first results that we reported with about 3 standard deviation uncertainty were a comparison of this first reaction and this one as measured by the Super-Kamiokande detector. And my colleague Professor Kajita didn’t mention the fact that Super-Kamiokande has made some really significant measurements of solar neutrinos as well as atmospheric neutrinos over the years. We combined our results with them. Then when we were able to go to a full measurement of this reaction as well, we had something on the order of 5.3 standard deviations which we considered a discovery of flavour change for solar neutrinos. Now, that neutral current reaction so-called, we detected neutrons in three ways, three different phases of the experiment, because we wanted to be sure that we had the same results in all those three phases. And also as we moved from phase to phase we were improving our sensitivity. In the first phase we simply allowed the neutrons to recapture in deuterium, producing a 6.25 MeV gamma ray. And that was the observation. You notice the efficiency for neutrino capture was about 14%. We then put a couple of tons of salt in the water, in heavy water. And that increased our efficiency for detection of neutrons to 40% and also created events because of the number of gamma rays that are emitted in that capture, that were much more isotropic than the cones of light from the specifically electron neutrino sensitive reaction, that so-called charge current reaction. And then finally we put an array of about 400 metres of very low radioactivity proportional counters. They didn’t add to that central radioactivity level I mentioned of less than one radioactive decay per day per ton of water in the centre. And in this case we had explicit signals from the detection of a neutron. So this is what the detector looks like in a larger scale drawing. In the centre here is the biggest Christmas tree ornament you’ve ever seen, made out of plexiglass or acrylic. It had to be fabricated from 120 pieces that were small enough to come down on the 3 metre by 4 metre elevator or cage for the mine. It’s surrounded by about 9,500 phototubes. And each one of them had about a 25% quantum efficiency. In other words, 25% probability of observing single photon of light. The remaining area was filled with ultra-pure water, purified in the same way as the heavy water, which was in the centre here. And so what we’re looking for is a signal that comes in, where a neutrino comes in and produces a burst of light from one of those two reactions that I mentioned in the previous slide. We produced a liner on the cavity that was 34 metres high and 22 metres in diameter, and all of this was 2 kilometres below the ground. This shows you the sort of construction process where we were bonding together the last few bits of the acrylic sphere. Everyone who came in to work took a shower and put on clean, lint-free clothing, because we were maintaining about a class 2,000 clean room in the entire detector throughout the experiment. You can see here the floors were pretty clean. My mother came to visit at one point and said, “It looks pretty clean, dear.” I think she was amazed that I could be responsible for anything that was as clean as that was. Anyway, the water systems themselves were ever cleaner inside. As I mentioned we were able to achieve this goal. So the first phase of the experiment where the neutrons were capturing in deuterium gave a neutral current signal that is shown here. And the charge current signal actually follows in this case the emission, the energy spectrum for the emission of neutrinos. And what we did in order to see whether neutrinos had in fact changed their type, was to do a hypothesis test of whether there was any possibility that they had not changed their type. And there was less than 5, and that turned out to be incorrect with a 5.3 sigma uncertainty. In other words, less than 1 chance in 10 million that there was no change in neutrino type. And therefore this was a clear demonstration that neutrinos changed their type. Essentially here was the calculations of the solar models, McCall and others, this happens to be McCall’s calculation at the time in the year 2000. Our measurement of electron neutrinos from that particular reaction, the boron 8 reaction, was about one third of that calculated value. But when you measured all neutrino types you found that that matched very accurately the calculations of the sun. So at the same time we were able to do neutrino physics in terms of having again demonstrated very clearly that neutrinos oscillate, in this case for electron neutrinos. And we also were doing solar physics in the sense of having validated the calculations of the sun. Now, in order to do this we had to do a lot of careful calibrations. You see here different energies from radioactive sources that were deployed throughout the volume by using a string and pulley technique. This is not to scale, of course, but it would be about that size if it were to scale. But it’s typical of a radioactive source being placed in different locations in the experiment. And you can see the set of sources including a neutron source. So we used to do that. And in addition we measured the uranium and thorium content in the water in several ways. We sampled the water and measured it externally in terms of filtering the water to extract via several techniques the daughter products of the uranium and thorium chains. We also looked at the low energy region of the data where we were able to separate on a statistical basis, on the basis of isotropy, the difference between uranium and thorium in terms of the quantities. And here you see what our objectives were, in each case for radon. By the way, we also sampled for radon gas, having de-gassed the water very carefully in the detector. So we had measurements of radon gas in the heavy water and we had measurements of the radium isotopes from the thorium and uranium chain. These were our goals, and you can see that the data measured over the period of about 7 years when we were running we were below those, except for a few excursions that we knew about and didn’t accept that data. When we put the salt in, the signal from the OAM neutrino type, where the so-called neutral current signal, the neutrons, went up significantly compared to the data I showed you before. And that was exactly as was predicted back in 1987 when we applied for the funding for the project in the first place which we thought was quite remarkable but very satisfying. It was a situation that I think is not uncharacteristic of many experiments that you try to do that are tough. And that is you set about in the first place the idea that there is a significant piece of physics that you can do here, if you can only make the measurement adequately. And we started in 1984 as I said and eventually had the measurements in 2001 and 2002. And in the meantime what was done was to take the idea, which started back with one of our founders Herb Chen, and pushed it to the point where by doing all of the technology adequately we were able to make the measurements. This shows you the results we had after the salt phase, plotted in a slightly different way. What’s plotted is the flux of muon and tau neutrinos, we can’t tell the difference between them. But on that axis are electron neutrinos, on this axis for the charge current reaction you’re only sensitive to electron neutrinos. For the neutral current reaction you have the slope that you see here. The black bar is the Super-Kamiokande measurements of elastic scattering, given the sensitivity of that reaction. And green is our results for that. So what you see is that they all are in very nice agreement, and these are the numbers for the fluxes in each case. So two things: One, the ratio of charge current to neutral current is only about one third and measured really quite accurately. And secondly, the total flux, which is what is to be compared with the standard solar model calculations, is also in very good agreement with that. Now, the next phase of the project we put in 400 metres of low background neutron counter. So it was kind of fun for our young people working on the project because they got to run a remotely operated submarine. It turned out that nobody over the age of 23 could conceivably run that thing. We absolutely needed the video game generation in order to – well, I think there’s other qualities that deteriorate with age as well. So we had students that were, and postdocs that were flying this submarine and putting these 400 metres of neutron detectors in the middle of $300 million worth of heavy water. And they did a fantastic job which is another indication of the way in which students and postdocs can contribute significantly to even major projects. So when we eventually had the data from that phase of the project we had measurements from the helium, from these detectors that had helium 3 in them, that then detected neutrons. We were able to calibrate that with neutron sources. We were able to observe the typical background in the detectors by having helium 4 instead of helium 3 in there. And we then did a joint analysis of all three phases. And these are our final results expressed as a ratio of electron neutrinos to all neutrinos and expressed as a measure of the flux from the boron-8 reaction in the sun. Now, of course, we were not the only ones measuring solar neutrinos. We were the only ones with two separate things that could be measured with respect to solar neutrinos. And so we had an advantage from that point of view. But Ray Davis’s original experiments starting in 1968 had a threshold there and measuring everything above it in 1992, well, 1989 Kamiokande started measuring solar neutrinos with a threshold very similar to ours, with the elastic scattering reaction. Gallium was used and in this case measured all the way down to the PP neutrino region. These are the fluxes on those different reactions I showed you earlier. And so at the point when we were reporting our results in 2001 and 2002 this was the situation. All of those experiments that were either exclusively or predominantly sensitive to electron neutrinos, we were seeing too few but the question was why. Our measurements ended up giving a measurement that was smaller than what Super-Kamiokande observed. And, of course, that was because they had a small component of mu and tau sensitivity. This is our all neutrino flux compared with theory. And so that’s basically what we measured. It was done with a large group of people, large set of institutions and funding agencies. It was really an international project from the very beginning. And these are the 273 names. You can’t read them but it’s important to me that those names are up there because everybody in the project contributed to it. And I’m standing up here in front of you as a Nobel Prize winner but there’s no question that the people that you see in front of you are the ones that did the work for the winning of that prize. So let’s talk for a moment about where we are with respect to neutrino physics in terms of our knowledge of the parameters associated with the oscillation of neutrinos. It was mentioned earlier by Professor Kajita that there is a formulism which is attributed in combination to Pontecorvo, Maki, Nakagawa and Sakata. The basic idea is that the electron mu and tau eigenstates are a linear combination of mass eigenstates. And that then, when they propagate it, is the kinematics of the mass that determines how they propagate. And if you work through the equations you end up for 2 neutrinos, let’s say mu is going to electrons with oscillatory behaviour as you see here and as you saw in the data for Super-Kamiokande. What’s nice about this rather complicated matrix when you go to 3 by 3 is that it breaks down into different components which are characterised by parameters, usually expressed as angles here, angles for example between mass 2 and mass 3, cosine of that angle, between mass 1 and mass 3 and so on. In solar, and it was later confirmed in reactor measurements such as KamLAND, we were able to measure the parameters for 1, 2 as well as measuring the difference in mass squared between 1 and 2 with atmospheric neutrinos, super K had measured 2, 3, both these parameters and the difference in mass. Subsequently there have been measurements of the 1, 3 parameters, and what is still uncertain is this CP violating phase, and that’s something that people are very interested in trying to pursue with larger scale experiments, particularly with accelerator beams. Now, oscillations of neutrinos are basically this set of parameters. But for a phenomenon called neutrino-less double beta decay there are a couple of additional phases that come in, but basically these are the same parameters. And so we’ve made a step forward in terms of trying to understand where we should be looking for neutrino-less double beta decay by having to find these other parameters. If you look at what has happened, if you combine all of the solar data from the various measurements you see here the different reactions that participate, you see in the coloured bars here that give the total expected flux, you see the measured fluxes in the various experiments. And what you find is, if you analyse that and particularly if you include another phenomenon referred to as Mikheyev-Smirnov-Wolfenstein effect, in which there’s a slightly different interaction of electron neutrinos with electrons in the sun, then you have for the other neutrinos, basically it’s an additional W exchange instead of just a Z exchange. The conclusion is that the electron neutrinos are essentially converted into a pure mass 2 state. And they stay in that state as they travel to the earth. You also end up determining delta m squared 1, 2 as I mentioned. But you also determine that mass 2 is greater than mass 1 because of the interactions with the electrons in the sun. So if you put together the information that we know, this is what’s in the particle data group right now, you know all of these different mixing angles with reasonable accuracy, you know differences in mass. You don’t know the absolute mass scale yet although measurements of normal beta decay and the N point have defined that to be less than about 2 electron volts. You don’t know the mass hierarchy in the sense that you don’t know whether mass 3 is greater than the other two. And that’s the subject of a number of measurements being planned. You don’t know the CP violating parameter which people find interesting in the sense that they’d like to get clues about how matter, any matter asymmetry occurred in the early universe by studying the light neutrinos, it’s not these neutrinos that would participate in that, but they’d like to understand the relationship. Neutrinos double beta decay can tell you whether a neutrino is Majorana or Dirac particle. In other words, in the case of Majorana whether it’s its own anti-matter particle. There are a variety of kind of 2 sigma results that suggest there may be a lighter right-handed neutrinos referred to as sterile neutrinos. So there’s a lot still to do. And we’re fortunate in Canada to have been able to expand our laboratory. It’s right now until the laboratory in China hits full stride, a laboratory that’s the deepest one available. The one in Jinping is a little bit deeper. So I think there’s going to be significant interest in the future in this laboratory, in Gran Sasso, in Kamioka, in the types of physics that I’m going to talk about. There are a number of experiments that are being cited or have been cited in the SNOLAB, so-called laboratory, including a recycling of the SNO detector to do neutrino-less double beta decay. Speaking about that the idea is to put a liquid scintillator in place of the heavy water which actually has been returned, and load that liquid scintillator with about 4 tons of a Tellurium compound that is dissolvable and transparent when it is dissolved. That will give us a very good sensitivity. Tellurium has about 34% natural abundance of one of the isotopes that can give you neutrino-less double beta decay. There’s only a few that can give you that. And we expect to have a very sensitive experiment when we start in 2017. A number of different techniques are being applied in our laboratory to look for dark matter. There are, of course, many others being studied around the world. Others tell you about, I’ll show you a couple of pictures of SNO+ and also one or two of the dark matter experiments. In SNO+ because the liquid scintillator is lighter than water, as opposed to heavy water, we had to then hold down the central vessel. That has now been established, it’s the biggest macramé project I think that’s ever been done, but these things that come over the top are in place and hold it down. And we’re in the process of starting to fill with ordinary water and hope to have a liquid scintillator in by the end of the year. Just to show you what comes in to neutrino-less double beta decay, you have to have a finite mass for a neutrino and you have to have then the sensitivity for the half-life which is 10^26 years or greater if the mass is less than 0.1 electron volt. You see that the expression involves an effective mass which is made up of the actual m1, m2, m3s and the parameters I mentioned that have been measured in oscillations but in addition to a couple of other phases. And what you end up with is these potential regions where you might observe an effective mass depending upon the 2 hierarchies. Present limits, lowest one is about 200 milli-electron volts, and we would hope to get down into this region. As you can see in the next transparency this is what peak would look like from neutrino-less double beta decay, at the end of the 2 neutrino spectrum, if you were at a value of neutrino mass, effective neutrino mass that’s equivalent to the current lowest limit. We actually hope to get down to much lower levels. Matrix elements for the nuclear physics affect our sensitivity there, too. Ok, I will simply say that if you use liquid argon you have the opportunity to do a nice discrimination between dark matter particles that give you 10 nanosecond pulses in your detector and other background that gives you 10 microsecond pulses in your detector ionisation. If you use germanium or silicon detectors, you can operate them as bolometers and the difference in sensitivity in bolometry and in ionisation enables you again to discriminate against the ionising pulses compared to nuclear recoils. But just to show you what the situation is now with respect to dark matter detection, there’s a series of limits that have been placed by various experiments. These are the solid lines coloured in here. There’s a limit as to how well you can make these measurements, ironically created by neutrinos, the neutrino background. Neutrinos are now background rather than the thing you’re trying to measure. But with 4 tons of liquid argon in the deep 3,600 experiment, which we’re filling, we have about 800 kilograms of the eventually 3,600 kilograms in place, and we hope to be taking data very soon. We can get into this region and with SuperCDMS they can get into this other region of lower mass. And so the hope is that we may perhaps see dark matter particles interacting directly or at least set further limits that are down in the region that you might expect from supersymmetric theories predicting wimps of that nature. So I simply wanted to say to you that if you reduce radioactivity to its lowest levels, you have some great opportunities to do exciting physics. And those things that you can do in particle astrophysics and neutrino physics answer some of the most basic questions that we can ask about our universe. Thank you very much.

Arthur McDonald (2016)

The Sudbury Neutrino Observatory: Observation of Flavor Change for Solar Neutrinos

Arthur McDonald (2016)

The Sudbury Neutrino Observatory: Observation of Flavor Change for Solar Neutrinos

Abstract

The Sudbury Neutrino Observatory (SNO) was a 1,000 tonne heavy-water-based neutrino detector created 2 km underground in an active nickel mine near Sudbury, Canada. SNO has studied neutrinos from 8B decay in the Sun and observed one neutrino reaction sensitive only to solar electron neutrinos and others sensitive to all active neutrino flavors. It found clear evidence for neutrino flavor change that also implies that neutrinos have non-zero mass. This requires modification of the Standard Model for Elementary Particles and confirms solar model calculations with great accuracy. Future measurements at the expanded SNOLAB facility will search for Dark Matter particles thought to make up 26% of our Universe and neutrino-less double beta decay, a rare form of radioactivity that can tell us further fundamental properties of neutrinos.

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