WPT University Place	Space Place		Francis Halzen, "Ice Cube: First Light"		December 14, 2010	UP535    cc >> Jim Lattis: Well tonight for our monthly guest speaker, I'm pleased to introduce professor Francis Halzen. Francis has been the head of Neutrino Astronomy here for quite some years, and the Ice Cube project is nearing a really important point in this current construction season and he's going to give us, tonight, a talk about the Ice Cube Neutrino Telescope and what he calls First Light, but it isn't actually light, it's neutrinos actually. So, Francis. >> Francis Halzen: Thank you for not mentioning how long we have been doing this. [LAUGHTER] In my case, it's over 20 years. I understand this is the fourth talk of a series, and so this is usually what I talk about, but a lot of this has been covered. So I will quickly go over the previous talks as an introduction, and then I'm going to talk to you about our first attempts to actually do some science. And so since am I a professor, there will be a test afterwards. [LAUGHTER] And so I'll first review the three previous lectures. You know by know that we don't know what a telescope is. 

We are building a particle physics detector which we thought we knew how to do and we actually know how to do because it will be finished by this weekend. So that I will remind you of this endeavor to build this huge detector. Then I will tell you why it's maybe astronomy, maybe not. And I will remind you a little bit about the technology. But most of my talk, you will have heard most of this by now, most of my talk will be about the first science that we are trying to do with this detector. My usual apology, there is no nice pictures in this talk. There are no penguins. This is the last beautiful picture. This is Antarctica for us. This is an actual picture of the geographic South Pole. 

If you look at the wrong direction and you don't see ice cube or the South Pole station, you see this. And you are standing on three kilometers of ice. And that's our interest in Antarctica. It says here ultra-transparent ice. In fact, if you take this glacier, you're standing at the South Pole, you are moving 10 meters a year on the glacier that comes of the west Antarctic mountain range. And if you go all the way to the coast, you can see this glacier and so you have to imagine that we are at the South Pole, this is three kilometers thick and we are building a detector right in the middle of this. And this is why I'm standing here. We actually discovered that light travels 100 to 300 meters in this ultra-pure ice. It was a total surprise, especially to us. In fact, you have to realize we would never have started this project if we had had glaciology books that said that this number was more like eight meters. 

When people measure the transparency of tap water, light travels about two meters before going extinct. In distilled water, it goes about eight meters. If you make bubble free ice and test it in the lab, light travels eight meters. Fortunately, we didn't know any of this or we would never have started. So we are particle physicists so we had the blessing of being totally ignorant. [LAUGHTER] But we discovered to our surprise that this ice actually is so transparent that you cannot compete with it with things you make in the lab. You couldn't make a solid that's as transparent as this ice. It's ultra-pure condensed of some hundred thousand years ago that fell on Antarctica. So it's ultra-pure and that's what counts. So that's why I'm here. 

Now comes the review. What are neutrinos? In two slides. In high school you were told that the world is made of protons and neutrons, and the protons and neutrons make nuclei like alpha particles, helium, uranium. And then when you put electrons around it, they make atoms. And that's what you learned and that was not your money's worth, wherever you learned this, because we wouldn't be here if it were not for the fact that there is also a neutrino. And how do we know this? We know this since the 1930s because when a neutron decays, it decays into a proton and an electron, and if there were no neutrino, whenever the proton goes this way, the electron goes that way. It's billiard ball physics. In fact they noticed that occasionally the proton and the electron would go that way which means something else has to go that way. And they say, well, it's a particle and it's saves momentum conservation. And in fact it doesn't interact with anything. 

And so that was it. It was like a mathematical trick. And so the neutrino was born. But you have to remember that the neutrino is kind of an agent that makes nuclear physics possible. Without changing neutrons into protons, there's no nuclear physics, there are no nuclear reactors along Lake Michigan, there is no sun that burns and there are no stars that explode. So we wouldn't have sunlight, and we wouldn't be here because the stuff we are made of is made by nuclear reactors in stars. So the neutrino is important, but people realized that it was just like light. It had no mass, no electric charge, no nothing. So it was just like photons except photons don't go through walls and neutrinos do. That's the only difference. 

And so that may be good actually because if they go through walls maybe they come out of the inner cores of black holes and that allows you to do astronomy you cannot do with slide. That was kind of the fake idea. This sounds all good until you hear the rest of the story. [LAUGHTER] In fact, the person who discovered the neutrino as a mathematical object said that I have done a terrible thing, I have invented a particle that cannot be detected. [LAUGHTER] And when we were in the beginning trying to build AMANDA I would always tell my collaborators we're going to prove him right. [LAUGHTER] It never went over very well. Now this slide you have seen before, I'm sure. It gives you an introduction in two slides again to multi-wavelength astronomy. This is the explosion of the cap, 1,054. When this happened, you could read your newspaper, or whatever they wrote in, for three weeks during the day. 

And so that's what it looks like now, a Hubble picture. There is a pulsar in the center, and this is a picture of the explosion a thousand years later. And so that's the same picture in infrared and in ultraviolet. So the next case this is a -- picture taken with a satellite. And you see this picture is only a few years old, and you see the streams of high energy particles coming out that have never been seen before. And so what we want to do is take a picture of the crab, the neutrinos. Although we're almost sure that the crab doesn't emit any neutrinos, and if it does it will take us 10 years to find them, but we will find them if they can be found in 10 years, don't worry. This is the excuse we went around the world with. You know, as soon as we told --, we went around saying we can build a neutrino telescope. So isn't this great? Look, whenever we build a telescope, look at all these surprising discoveries we made. 

And so you know we build this and we don't know what we'll see but it will be great. [LAUGHTER] Just like in the past. And I said this once in the wrong place, in Washington. And someone came over and said, you know, it was from the Office of Management and Budget, and he said, what you're saying is that you're going to take government money, gamble with it and we'll see what comes out. Doesn't sound very good. I said, what do I say instead? And this is what he told me. [LAUGHTER] There are administrators with a sense of humor. Okay, so you get the idea. The summary is almost finished. The last thing is how we detect neutrinos. 

Now, I told you neutrinos for 20 years were a mathematical concept until this man, Reines, he had to wait another 20 years to get a Nobel Prize for it, but he knew that neutrinos are produced where there are nuclear reactors. He went to the Savannah River nuclear reactor and put a small detector about a meter. And you see here it's basically water and photomultipliers. And so he put it close to a nuclear reactor. And you have this enormous flux of neutrinos so you can do it with a small detector. And he did it. Got a Nobel Prize for it. He told me that as soon as he detected neutrinos he had the idea that you could do astronomy with it. In 20 years nobody had told about it. But it's strange, once you know something is real you suddenly get this idea that nobody had had before. 

And in fact he was not the only one. Someone else had the idea, and a man in Russia named Markov had the idea. All in the 1960s. This experiment was 1956. Unfortunately, this beautiful dream went home when people started doing some calculations and realized that you needed a kilometer scale detector. So what we need is this detector but instead of one meter, one kilometer. That's all. And so since the 1970s we have been trying to do this. And actually, next weekend, unless something goes wrong, it will be a reality. In fact we have a detector that's much bigger than a kilometer cube even if we don't get the next two strings in. 

So this is Markov who had the idea of how to detect these neutrinos. So this is what he wrote in his paper in 1960: We propose to install detectors deep in a lake or in a sea to determine the direction of charged particles with the help of radiation. It doesn't matter. You understand this. That's what I will explain in a minute. But why did it take so long? It is because he talked about water which was the wrong medium. And so we had the idea in '87 after people had been trying this in water, forget it. We are going to do this in ice, not knowing that ice may not be very transparent. And so you heard the story. We drill holes in the ice, put light sensors in it and all this is beautiful and easy. 

And so that's the idea. You go a mile deep under the glacier at the South Pole. You go a mile deep and put a kilometer cube worth of light sensors. It's totally dark so you can't detect light, and here comes your neutrino. And what does it do? It goes right through your detector. It has already gone through the earth, it goes through your detector, but these are light sensors. So here is a, it's called a photomultiplier and it detects light. Buy it in Japan for a thousand dollars and put five thousand of them in the ice. And so once in a while, actually about once in a million times, instead of going through the ice, a neutrino, ice is made of protons and a bit of oxygen, a neutrino will crash head on into a proton. And then a nuclear reaction happens. 

And nuclear reactions produce light. And in fact, in the cases we are most interested in, it does this in a very interesting way. It actually produces a slash of nuclear particles, but among these particles there is this muon and this muon can travel a long way. And when it travels through your detector, it leaves this wake of light and it's blue light and you use your sensors to map this wake. From this wake you never see the muon, but this wake tells you where the muon came from and the neutrino comes from the same direction. So you know to only detect neutrinos, you know where they come from and you have a telescope. You know where they come from, somewhere on the other side of the earth. So from the South Pole we are looking at the sky above us, looking for neutrinos that travel to the earth. So if you didn't quite get this, let me explain this a bit better. Blue light from a nuclear reaction, actually, you have to realize this is a very faint nuclear reaction. 

And most of the time what we detect is not a lot of light. In fact, typically we detect 10 photons. That's all. And these light sensors typically detect one photon. So we require that eight of them fire and that's what we call an event. Okay? So it's not as easy as it sounds. This is a nuclear reactor, not a neutrino. And you see it's covered with water. And all the nuclear particles coming out of the nuclear reactor, including very few muons, they made the water glow just like these particles make the ice glow. And so this is the color of the light we see in the ice. 

And so where does this wake come from? The wake, first of all I already explained, it's very important. This muon at high energy travels kilometers so we can actually see neutrinos outside our detector. And so the detector is actually much bigger than the kilometer cube instrument with photomultipliers. But the wake is what gives away the direction of the neutrino, and that's what makes the astronomy possible. And this is the physicist's explanation, but this is the one that's easy to understand. This is the muon. The muon travels at the speed of light or very, very close. So it tickles the atoms in the ice and the atoms emit light. But that light is slowed down in the ice. So it's going at the speed of light in ice which is significantly less than the speed of light. 

So it goes at the speed of light but the light waves go out less than the speed of light which means you get a wake. And if you measure this wake with our photomultipliers, you know where the duck is going. That's it.  Okay, so we started somewhere in the early '90s, in fact I gave my first talk to this audience when it was still in Ponderosa Steakhouse. And so we started at that time, we were thinking about building a detector we called AMANDA. So this is the South Pole, and this detector was completed by the year 2000. And it's the same story. You go 1500 meters deep and then there was 500 meters worth of photomultipliers and we started trying to do this. We knew this detector was much too small. When you read the proposal it says we want to test whether we can build a kilometer cube detector. 

And in fact we give the price. And if you correct for inflation, it's exactly what we spent. We're very proud of this. So now we see neutrinos with AMANDA. And I can remember the day I saw the first neutrino event from AMANDA. It was one of the great thrills of my life. I won't say where it hangs because it may upset a lot of people. But this is a neutrino. And once you find it, you know it's a neutrino. You can bet your house on it. 

So you see, remember you want to see a track coming through the ice. It comes from the bottom, from under the detector. Each of these dots is a photomultiplier tube. So the length of this track, you track from about here to Ponderosa Steakhouse, it's about that far. And you see the way we are going to look at a lot of these events, that's my job here tonight, you see these colored dots. Whenever you see a colored dot it means that the photomultiplier detected one photon. So you see these little dots, that's one photon. The red is early, the purple is late. So you follow the rainbow and that tells you the direction where the track is going. And when you see big dots, it means that the photomultiplier detected a lot of photons. 

You can see the size of a dot here so this dot is like 12 photons. But you see without looking at the size of the dots, anyone can tell that this track comes in here and goes out 300 meters later at the top. That's a neutrino. So it worked. So then we started, we proposed in 1999, in fact a year before AMANDA was finished we knew this work. And we proposed the ice cube. And this you can seen many times by know was the inevitable Eiffel Tower. And this is the AMANDA detector which worked for nine years until it became insignificant and we turned it off a year ago. And so it's just a kilometer cube filled with 5,160 photomultiplier tubes between one and a half and two and a half kilometers. And what it does is, in fact it's almost complete so we know this was good precision, it detects 220 neutrinos per day, but it also detects 1.7, almost a billion cosmic muons per day. 

And that's a problem which I'll come back to later. That's the hard part of the story. And we'll ignore this at the moment and concentrate on the 220 neutrinos per day. I'll tell you about the muons later. This is a really annoying background. Two thousand per second. They come from a buff. No interest in it. They're cosmic muons. You have to deal with it. 

You have to measure them and reject them. And that's what we do every second, 2,000 times. So on top of the ice there is a big computer center that takes care of this. Where are we? I've referred to next weekend. Well, here is ice cube looked from the top. So if you look on top of the ice. This was the first string deployed in January 2005. And this is what the detector looked like a year ago. And in fact as of today, these strings have been put in, and there are only two more strings that have to be put in at the center here. In fact, I have to show some pictures of people drilling. 

I mean, the drill was maybe the hardest part of the project. It's 4.8 megawatts. It drills holes to two and a half kilometers. Melts the ice in two days. It moves three and a half centimeters every second. And it's a masterpiece. So here the drill comes out, the photomultipliers are ready, 60 to go in the hole. And then you see them go in, one every 17 meters, one after the other. And if you looked at my computer three hours ago, and this is what I was seeing. This is time versus depth. 

And this was the drill that finished the hole about a day ago. Three hours ago they were drilling hole number six, seven. And you see they are already at 500 meters there and now probably close to a kilometer. And then you can see them deploy a string. These little red dots is each time you attach a module one of these lines is drawn, and as time goes on the string sinks in the ice. You are looking at the data upside down. And three hours ago the string reached the bottom, reached zero which means it's at the bottom of the hole. So by this weekend, this will be finished. Two more strings to go. Now before we looked at events, here is, in case you didn't get it, here it is again, the movie. 

So look at the bottom here. You'll see a neutrino arrive. As you know neutrinos are red. [LAUGHTER] And I think they are red, I'm not sure. Yes, they are red. They hit an atom, make a muon, the muon makes light in the detector. This light is actually transformed in a digital signal. The digital signal goes up on a cable and goes through the computer that reconstruct the event. Okay? You get the idea. So here we go. 

At some point we had 22 strings, and that's what a neutrino looks like. Then the next year we had 40 strings, and that's what a neutrino looks like, three of them actually. They all look the same, right? So here, this is a neutrino that you see it doesn't make much light. That's a low energy neutrino. It's one we can barely detect. It's energy is one TeV. It doesn't matter what a TeV is. This is the same energy as the Fermilab accelerator in Chicago. So these are the highest energies we can produce on earth except they can go a bit higher in Geneva now. But this neutrino is the same energy as the protons accelerator, that Fermilab. 

But this, for us, is wimpy stuff. This is a real neutrino. You can think of this, this is a real nuclear explosion. In fact, it's not a nuclear explosion. The neutrino makes a nuclear explosion but this muon when it travels through the detector makes a series of nuclear explosions. So you see bombs going off. So you see a lot of light, and this is a high energy neutrino. In fact I'll show you one, this one, you see red again, you follow the rainbow, it goes to green, this one is a hundred times the energy of the Fermilab accelerator. This one I have to change units. It's 10,000 times the energy of the accelerator. 

And we actually don't understand this event very well. We cannot quite pin down what made it, but we only have one like it. And it produced 250,000 photons. Not the eight or ten I was talking about. So it still may be an atmospheric neutrino, but its probability is 10 to the minus three. We have more data by now, we'll figure this out. But I want to tell you, I always show this slide and this has nothing to do with me. Developing this technology was a masterpiece, and so in fact we made it to the cover of Science magazine without ever having discovered anything. This has never happened before as far as I know. But it was in recognition of having built a kilometer cube detector. 

In fact we hadn't at that point, but it was clear we were going to. We also got elected one of the Seven Wonders of Modern Astronomy. That's when we were just building AMANDA. I am not sure we had seen a neutrino by then. But the category is kind of questionable. You see. [LAUGHTER] Maybe you cannot see this very well. So let me now go back and use the rest of my time just going to look at maps. You have the idea, each time you detect a neutrino, to point this way, you make a dot where it comes from on the other side of the earth. As they, most of them, all of them as far as we know, are produced in the atmosphere, you find a totally uniform sky. 

They come from everywhere. So this is after we operated AMANDA for one year. And so we had 600 neutrinos in one year. And so you look at this map, by the way these are muons, this is this annoying background coming from the southern hemisphere. And so if you look at that, you can see this here. This is the type of thing you're looking for. This is the atmosphere, and this is a star that you see through the atmosphere. It's actually a statistical fluctuation. We knew that. We expected to see a fluctuation like that in this map. 

We understand statistics. Nobody got excited. In fact, we took data for another four years and you get this map. And there's actually a hole in the sky where this X has appeared and now there's an X somewhere else, it doesn't matter. But you get the idea as time goes on, it's like in the old days when you developed a picture in the darkroom you just see it become clearer and clearer, except for us it takes years. And remember AMANDA was a hundred times too small to see anything we anticipated. And so we were establishing the technology. So here this was AMANDA's final sky plot, and it delivered a beautiful sky with nothing in it as it was supposed to do, almost 7,000 events. Then we went to ice cube and this is ice cube of two years ago. And it delivered by now of instead of 7,000, in one year it produced more events than AMANDA in its eight years of existence. 

And you see 14,000 neutrinos in this map. These are the neutrinos. Now, I am not going to discuss this, but this detector is so powerful that we begin to be able to look at the northern sky. We begin to find neutrinos in this huge and annoying background of atmospheric muons. And so at the moment, is there an interesting point in this map? There always is. It's there. There's no way you can see it. I know where it is. You have to find it by computer. And so now we understand this detector. 

And this is what our sky map will look like in five years. I'm a theorist so you can run a computer program, and you can see here you will see the sources appear. You see these sources? That's what I think we'll see first. It's almost inevitable. Let me try to explain this. Since 1932 people have realized that about the only thing that can energetically produce galactic cosmic rays is a supernova explosion. Actually like the cap, they look like a cap although the cap is not a good one. And so detectors have been looking for these in gamma rays. And this is an experiment that mapped the galactic plain in gamma rays at the energies we are working. So there you see the candidate sources except they look like any other gamma ray sources. 

There is no way to prove that these produce cosmic rays. But we have. In fact, if you take a lot of particle physics courses, then you can derive this identity. That's for every two gamma rays they see, we have to see a neutrino and an antineutrino which we cannot tell apart. So basically, if these are the sources of the cosmic rays, which everybody already believes although there is no evidence for it, then we have to see one neutrino for every gamma ray they see, and that gives us this map. And you know you can see the sources we see arise. It's incredibly easy. And in fact, you can ask what about this map we just saw? So of course here it is. I couldn't resist plotting this out and there are the candidate sources, they are now in a different coordinate frame, but the biggest one is this star here. And you see this cluster of red dots. 

And so this is maybe the beginning of a very beautiful future. However, these red dots here are two sigma fluctuations. So there's nothing to get excited about. But this is on track to in five years, in fact the predictions are that we should get to where we would declare we have discovered something to which we have agreed on a minimum statistical significance. This could take as little as three years. It could take forever. If it takes forever then this man will be very unhappy because he's the one who predicted in 1934 that supernova remnants make cosmic rays, this is Zwicky, and unfortunately he will not be around but I'm sure wherever he is when he sees these sources appear on the map he will smile. He won't look like this. So I have been telling you about this annoying ongoing muon sky, and so that's the end of the story. This is the detector. 

If you looked at the detector at any time, this is of course you realize when you see these events, these develop at the speed of light. So I clearly have the speed of light significantly slowed down. And so instead of coming at 2,000 seconds, they come at like one every few seconds. So you get one event after the other. But there's something interesting. Look at these events. These are just like neutrino events. They are also muons like the muons made by neutrinos. A muon is a muon. But look here, the red is there and the green is here. 

So they go the wrong way. 2,000 a second, you measure them, throw them away. How do they get there? Well, cosmic rays hit the atmosphere everywhere, make pions and these pions decay into these annoying muons which you can see here. So you get constantly bombarded by these muons. Here, to tell you how annoying this gets, here in four microseconds you see five of these muons go through the detector. And it's very easy to fool yourself in this mess and reconstruct a fake neutrino some way or another. But we managed to do this. Remember, they are a million times more frequent than neutrinos. So if you mess this up one in a million times you have no experiment. So we are doing this now like one in a billion times we make mistakes. 

I think you get the idea. The muons come from above, which is from below at the South Pole, and the neutrinos come through the earth. So what we are going to do, I explained this already, but here several billion, oops that's not right. Atmospheric muons, yeah seven billion per year and about 400,000 neutrinos a year. So what we decided at some point, I'll tell you why in a minute, we decided we are not seeing anything this way, the detector is too small, let's look up where we have billions of events. They shouldn't point any where but let's look up. And so you cannot see the dots anymore. You cannot see a billion dots on a screen. This is actually a probability distribution you are looking at, but you may think of it as a billion dots. In fact this is four billion dots. 

And we looked at this and you got the idea these muons come through the ice from above and they tell us about the direction of this cosmic rays which are produced in our galaxy. So the first thing we saw was the moon. You know wherever the moon is, the flux of the cosmic rays is absorbed. And you know the moon is a half of degree on the sky. And here is the moon. It makes a deep hole in our detector of less than one degree, it's about 10 standard --. So we saw the moon. So now the next thing what we actually wanted to see was this. The cosmic rays are attached to our galaxy so we try to see the solar system move through the wind of the cosmic rays. Because our detector is on earth, so six months we are moving into the cosmic rays and six months we are moving out. 

We didn't see it. What we did actually see, so we thought maybe our detector doesn't work, what we did see is let's try if we can see the sun go around, the earth going around the sun. And there it is. Beautiful dipole. This, by the way, has also never been seen before. So we now know that the earth moves. We have seen the moon and we know that the earth moves. So anything can happen next. [LAUGHTER] But this detector worked beautifully. In fact, we have much better than a degree resolution now. 

So here is the map. And so what we saw is, so we didn't see any of these fancy motion of the solar system but we saw that there's a big excess of cosmic rays coming from one direction in the sky. Now cosmic rays are charged. They are not neutrinos, they are not light, they are not supposed to make a map. In fact among friends I can tell you that all this structure in this map we know by know is real. Nobody knows what this is. The only thing, in fact you can see this is with 22 strings, this is with 40 strings and I have seen 59 strings and you see how all this structure reappears in these maps. They look identical. The only clue we have is that detectors in the north, the Vela detector I mentioned before, it also has annoying muons that it mapped. And our access, in fact in the latest map, the hot spot in this map points right at Vela, which is the closest supernova to the earth. 

And so is this the explanation? Not possible. There's no way 10 TeV cosmic rays can travel in straight line to the galactic field from Vela to here. Not possible. So it's like there is no galactic field between us and there. So this is not an explanation but I don't have one. We promise surprises, well, this is the first one. And so this is, there is something wrong with our understanding of the magnetic field or there's something wrong with the fusion of charged particles in the galaxy. God knows. So keep tuned. But if we detected Vela, we have already proven Zwicky's theory. 

But we don't claim this yet. The only thing we claim is that we discovered something we don't understand. Okay, so that's the end of my talk. So everybody, the first question is always will we discover atmospheric earth, atmospheric we have, will we discover cosmic neutrinos? And a few years ago there was a web page, this is always in Britain right, where you could bet on big projects like what's the probability that LIGO detects cavitational waves, AMANDA detects cosmic neutrinos, and others--. And in fact they gave six to one that AMANDA discovered cosmic neutrinos, which is crazy. But six to one. In fact, we started betting on LIGO where they gave 500 to one. And I thought that was a good bet so when we started betting on it they took the page down. [LAUGHTER] You see the normalization here. Finding Elvis alive was a hundred to one, and the Loch Ness Monster is 66 to one. 

I would actually have bet on the Loch Ness Monster instead of AMANDA. [LAUGHTER] But after this talk, after the question period, I'll be in the back of the room, cash only, I give on Ice Cube two to one. [LAUGHTER] Thank you. [APPLAUSE]