 Our speaker is Jost Meghenda and they are a PhD student in astro-particle physics from the University of Shatel in the UK. Jost is going to talk about going deep underground to what the stars. Please give a round of applause, a huge round of applause to Jost Meghenda. Good morning everybody, glad you made it to your first day of congress. Now physics rarely makes any use. And even when it does, it's often treated as sort of black box, where you pour in money and scientists on one hand wait a while and knowledge drops out. So today in this talk I want to do a little bit differently. I want to give you a glimpse behind the scenes of these experiments that I have been working on for over four years now. First as part of my master's thesis and then as a PhD student. Now right over here we published a design report which is over 300 pages long and contains much more detail about the experiment than you probably want to know. So I'll focus on just some of the highlights in this talk. But before we actually talk about the detector, I'll have to introduce you to the particles we're looking for. And that story begins over 100 years ago with radioactive beta decay. Now in radioactive beta decay you have a nucleus of one chemical element that turns into a nucleus of a different element and emits an electron. Or in our modern language we would say a neutron decays into a proton and an electron. Now after that was discovered there were lots of experiments done to measure the energy of that outgoing electron. And experiment after experiment found that there was some variance in energy but was always lower than expected. And physicists at the time came up with all sorts of possible explanations for what might be going wrong in these experiments. But they excluded those explanations very quickly as well. So after a while physicists became desperate. And some pretty well-known physicists actually thought well maybe we'll just have to give up on conservation of energy. So in this desperate situation a guy called Wolfgang Pauli came up with what he himself called a desperate way out. So in this letter to a group of his colleagues which he addressed as dear radioactive ladies and gentlemen, Pauli suggested that maybe there's another particle created in this beta decay. And Pauli originally called this particle neutron. But of course two years later the particle we nowadays know as neutron was discovered. So Pauli's particle was renamed neutrino. Now you might be wondering well why didn't they observe this particle already? And the answer is very simple neutrinos are like ghosts. So what I mean by that is they can quite literally you know go through walls or through your body. And in fact we can do a little experiment right now to try and detect neutrinos. So to help me with this experiment please give me a thumbs up. Everyone? Okay. So there's two things happening right now. First of all you're giving me a massive confidence boost. But you know more importantly somewhere out there the sun is shining. And it's producing a lot of neutrinos in nuclear fusion. Now these neutrinos are flying to Earth through the roof of this building and then through your thumbnail. And right now as you're listening to me around 60 billion neutrinos are flying through your thumbnail. 60 billion neutrinos flying through your thumbnail every second. How does that feel? You don't feel any of them? Right so that's how ghosts like neutrinos are. And of course physicists are clever and shortly after Pauli had this idea some of them estimated how often neutrinos interact with normal matter. And they found that there is no practically possible way of observing the neutrino. And that remained true for over 20 years afterwards. So now that I've introduced you to neutrinos. Let's talk about building a detector to actually detect them. And the original motivation for building this detector was something a bit different. I talked about beta decay and over the next decades physicists slowly discovered more particles. They discovered that protons and neutrons are made up out of quarks. And in the 1970s theoretical physicists came up with some grand unified theories. Basically precursors to string theory. And these theories predicted that the proton should decay as well. So of course we built detectors to look for that. And a group in Japan built a detector near the town of Kamioka which they called the Kamioka nuclear decay experiment. Or Kamiokande for short. Now they didn't observe any proton decay. But shortly after they built it somebody had a suggestion that if we changed just a little bit about that detector. If we modified just a little we would also be able to detect neutrinos with that. So they modified the detector switched it back on. And just a couple of weeks later they actually observed neutrinos from an exploding star just outside our Milky Way. And that was the birth of neutrinos thrown at me. And for that the then leader of the experiment received a Nobel Prize in 2002. Now after over a decade of running physicists were basically hitting the limits of what we could do with a detector that size. So we needed to build a bigger detector. And that one was very creatively named Super Kamiokande. And it's about 20 times bigger. Started running in 1996 and is still running to this day. Now Super K did not discover proton decay. But did detect a lot of neutrinos and made very fascinating discoveries. For example they discovered that different types of neutrinos can change into each other back and forth as they travel. That's like you buying a cone of vanilla ice cream and then as you walk out it suddenly turns into chocolate ice cream. That's really weird. And for that discovery just a few years ago they received the Nobel Prize again. But today we are again hitting the limit of what we can learn from a detector that size. So of course the next step is to build an even bigger detector and we're calling it Hyper Kamiokande. By the way Super and Hyper mean exactly the same thing just one is Latin one is Greek. So we're currently getting ready the plans to build Hyper Kamiokande and we will start construction probably in the spring of 2020. Details of the Nobel Prize are still to be determined. Now I said that 60 billion neutrinos go through your thumbnail every second. Of course Super Kamiokande which is running right now is much larger than your thumbnail. There's not just 60 billion but 10,000 billion billion neutrinos passing through every day and only 10 or 15 of those get detected. So let's look at what this detection process looks like. Now this is a water inside Super K and there's a bunch of electrons in there but I'll show just one and there's neutrinos flying through. Not just one not just a few but loads of them and most of them go straight through without leaving a trace. But every once in a while we're lucky and one of those neutrinos will actually hit the electron and give it a little kick. And that little kick you know like billiard balls basically and that little kick accelerates the electron to faster than the speed of light in water. Still slower than the speed of light in vacuum which is the absolute cosmic temple limit but faster than the speed of light in water. And then you get basically a sonic boom but with light which is this cone of light. And let's just show the animation again. So you've got this cone of light and as that hits the wall of the detector you see this ring, this ring of light. And we've got very sensitive photosensors all over the inside walls to detect this flash of light. And from how bright it is we can tell the energy of the neutrino. And we can also tell you know just like with billiard balls we can approximately tell what directions the neutrino came from. Just based on in which direction it pushed the electron. And that's the basic idea of how we detect neutrinos from the sun. Now let's talk about what's actually like to build one of these detectors. So this is a drawing of hyper cameo candy. And you can see it's you know 78 meters high, 74 meters in diameter. And on the top left there's a track kind of for comparison. But maybe a better size comparison is to compare this to buildings which you're familiar with. Like the entrance hall which you just came through this morning. Or you know the Statue of Liberty. And it doesn't quite fit in there, the arm still looks out. But you could drown the Statue of Liberty in this detector. Which nowadays is probably some sort of political metaphor. So this is a giant detector. And what's more we're building it inside the mountain. About 650 meters underground. So that all the rock on top will act as a natural shield. Against all sorts of other particles that are raining down on our atmosphere from outer space. So that all other particles get stuck and only neutrinos can make it through. Now of course to build such a huge cavern inside the mountain. That's something that we physicists can't do on our own. So we need to talk to geologists who look at the rock quality. And tell us you know what's a good place to build this cavern. Where is the rock stable enough to do that? And to figure out the rock quality they drill boreholes. In what's actually called a boring survey. Now during my years working on this experiment I had to listen to several hours of talks on these geological surveys. And I can tell you that name is quite appropriate. Though of course there's a reason I'm not a geologist. So take this with a grain of salt. But okay let's say we talk to geologists. They told us where we can build our detector. So next step is we need to actually excavate the cavern. And something to keep in mind is that we are building this somewhere in the mountains of Japan. Pretty far away from any major city. So we have to think about stuff like the local infrastructure. Like what's the electricity supply like? Do we need to add a power line? Or what are the local roads like? And do they have enough capacity for dozens of trucks every day to transport away the excavated rock? And by the way where do you store all that excavated rock? Because we will be moving something like half a million cubic meters of rock. You can't just store that in your backyard. You need to find a place where all that fits. And of course if you've listened to or watched a lot of the rings. You'll know that it's dangerous to dig too deeply, too readily. So we're going to need a ball rock early warning system as well. But okay, let's say we've got all those. And we managed to build our cavern. And now we need to fill it. And as detector material we use water. Both because it's actually pretty good for detecting neutrinos. But also because it's cheap and there's lots of it. So you can afford to build a detector this size. A detector so big that that little dot there is a scuba diver. But even with water you hit limits of how much you can get. So to fill hypercomucanth you need about as much water as 5,000 people use in a year. And that's for drinking, for showering, for washing their car, and so on. Now that's easy if you're near a big city. But we are not. We're somewhere in the mountains in Japan where the next biggest town has far fewer than 5,000 people. So how do we get enough water to actually fill our detector? And we could use rivers nearby. We could use springs. We could wait for the end of winter and for the snow in the mountains to melt. And use that to fill our detector. But if you use melting snow to fill the detector you can only fill it once a year. So even where do you get the water is a pretty important question that you need to solve. And then we're not just using any water but we will build our own water purification system. So that we don't have any traces of radioactivity in there. Any kind of dust and stuff in there. And let me just tell you just how pure this water will be. So this is my supervisor who when he was a PhD student worked in the detector on some maintenance work. So he was working on a boat doing the work. And then at the end of his shift he leaned back in the boat and just a tip of his long hair fell into the water. Which he didn't notice, didn't think about too much. Until at the end of his shift he went home, went to bed, fell asleep and then woke up in the middle of the night with his whole head itching like mad. Now what had happened there? The ultra pure water had sucked all of the nutrients out of the tip of his hair and then through osmosis over time those had sucked the nutrients out of the rest of his hair and then his skin. So that's how pure that water is. Now I said all over the inside walls and here's kind of a photo of the inside of the detector and all these kind of golden hemispheres those are what we call photomultiplier tubes or PMTs for short and those are basically giant, very sensitive pixels and we will have 40,000 of those lining the inside walls of the detector. Now in smaller detectors you could just have from each PMT one cable leading to the top of the detector and then have your computers there to analyze the signal. With a detector this size you just can't do that because you would need 40,000 cables some of which are over 100 meters long that wouldn't work. So we have to put some electronics in the water to digitize the signal and combine signal from multiple PMTs into one and then use just a single cable to bring that up to the top where we analyze the signal. But that means we have to put electronics into the water so that creates a whole bunch of new problems. For example we need these electronics to be watertight and I'm not talking the level of watertightness you'd expect from your smartwatch where it survives, you know, you're standing under a shower for five minutes I'm talking below 60 meters of water for 20 plus years. These electronics also need to be very low power because we can't heat up the water too much otherwise these pixels, these photomultiplier tubes would become noisy and this would kill our signal. And then also because we don't want one defective cable to kill a whole section of the detector we need to implement some sort of mesh networking to introduce some redundancy. Now each of that by itself is not a hard problem each of these problems can be solved. It's just a lot of additional work you suddenly have to do because your detector is that huge and it gets even worse. This is what one of these PMTs looks like it's about 50 centimeters in diameter and inside that glass bulb is a vacuum. So it's under a lot of pressure plus we add 60 meters of water on top of it which adds additional pressure. So you need to make absolutely sure when you're manufacturing those you don't have any weak points in the glass. And they don't just have to withstand that water pressure but there will probably be some PMTs that have some weak points some air bubbles or something in the glass or some structural weakness. And the neighboring PMTs don't only have to survive the normal water pressure they also need to survive one of their neighboring PMTs imploding and sending out a pressure wave. And that's not just a hypothetical. That actually happened 18 years ago sorry 17 years ago in Supakamyokande and that was in seconds killed more than half of the PMTs we had in there and it took years to restore the detector to full capacity. So lots of problems to solve and one group, one university can't solve all these on their own. So we've got this multinational collaboration with over 300 people from 17 different countries marked in green here and across many different time zones. So right now here it's about just before noon in Japan people have already had dinner going to bed soon. In the US people haven't even got up yet in the morning. So good luck finding a time for phone meetings which works for all of these people. So that's kind of a glimpse behind the scenes of what it's like to work on this detector and to actually build it but now I want to talk about what we use the detector for. And I've got two examples but of course there's a whole bunch more that we do which I just don't have time to talk about today. So first example, why does the sun shine? That seems like such a simple question, right? And yet it turns out it's really difficult to answer. So in the days of the Industrial Revolution when burning coal and steam power was all the rage people thought that maybe it's a giant ball of burning coal but when you do the math it turns out that would burn for a few thousand years maybe. So that definitely doesn't work. A bit later physicists suggested that maybe the sun is just slowly shrinking and shrinking and it's that gravitational energy which is released as light and that would give you a lifetime of a few million years but then you've got pesky geologists coming along and saying no, no, we've got these rock formations or I don't know fossils maybe that are more than a few million years old on Earth. So the sun has to live longer than a few million years and the arguments from the 19th century between Lord Kelvin and the geologists back then are just amazing to read if you find those somewhere. But of course nowadays we know that the answer is nuclear fusion and here's a bunch of reactions which lead to the energy generation in the sun but now the question is how can we check that? How can we check that this is actually what's going on and the answer is neutrinos because many of these reactions produce neutrinos and we can detect them. So the one we typically detect in super and later hypercomucandes is the one on the bottom right here called boron 8 neutrinos because those have the highest energy so they're easiest for us to detect and the rate of various of these processes depends very much on the temperature. So by measuring how many of these neutrinos we see we can measure the temperature inside the core of the sun and we have done that and we know that it's about 15.5 million Kelvin plus or minus 1%. So we know the temperature in the core of the sun to less than 1% uncertainty. That's pretty amazing if you ask me. And I said that we could detect the direction that neutrinos were coming from so we can actually take a picture of the sun with neutrinos. Now this is a bit blurry and pixelated and not as nice as what you'd get from an optical telescope but this is still a completely different way of looking at the sun and what this tells us is that this giant glowing orb in the sky that's not some optical illusion that actually exists. Okay. So onto our next topic exploding stars or supernovae which is what my own research is mostly about. So supernovae are these giant explosions where one single star like in this example here can shine about as bright as a whole galaxy consisting of billions of stars and the rule of thumb is this however big you think supernovae are you're wrong. They're bigger than that. And Randall Monroe of XKCD fame had this excellent example of just how big supernovae are. So he asks which of the following would be brighter in terms of the amount of energy delivered to your retina? Option one a supernova seen from as far away as the sun is from Earth or option two a hydrogen bomb pressed against your eyeball. So which of these would be brighter? What do you think? Yeah, you remember the rule of thumb we had earlier? The supernova is bigger than that. In fact, it's a billion times bigger than that. So supernovae are some of the biggest bang since the original big bang. They also leave a neutron star or a black hole which are really interesting objects to study in their own right. And the outgoing shockwave also leads to the creation of lots of new stars. But maybe most importantly supernovae are where many of the chemical elements around you come from. So whether it's things like the oxygen in your lungs or the calcium in your bones or the silicon in your favorite computer chip life as we know it and congress as we know it could not exist without supernovae. And yet we don't actually understand how these explosions happen. And while we... I have no idea what's happening. Okay. So life as we know it could not exist without supernovae. And yet we don't actually understand how these explosions happen. And even observing them with telescopes doesn't really help us. Because telescopes can only ever see the surface of a star. They can't look inside the core of the star where the explosion actually takes place. So that's why we need neutrinos. And we've served tens of thousands of supernovae with optical telescopes. With neutrinos we've observed just one. This one in February of 1987. And we've seen two dozen neutrinos which you see here on the right. That's what we know. So we know basically that many neutrinos are emitted during the first one second or so and then fewer and fewer for the next 10 seconds. We know that neutrinos make up most of the energy of the explosion of the supernova with the actual energy of the explosion and the visible light making up just a tiny fraction. We know that the neutrinos arrive a few hours before the light. And that's all. That's all we know. And still about these two dozen events, these two dozen neutrinos, more than 1,600 papers were written. That's more than one paper week for over 30 years. So this gives you an idea of just how important this event was and how creative physicists are. Or I guess you could call desperate, but, you know, I prefer creative. In fact, this one supernova we observed was such a big deal that 30 years later, February of last year, we had a conference in Tokyo on supernova neutrinos and we had the 30th anniversary celebration. So there we were, about 40 or 50 physicists looking over the skyline of Tokyo, having dinner. There's the now leader of the super-kamiokande experiment, who was a PhD student back then when it happened. And in his hand, he's holding the original data tape with the events that he himself analyzed back then. So there we were. And at one point that evening, we actually started to sing Happy Birthday. Um... So it goes, you know, Happy Birthday to you, Happy Birthday to you, Happy Birthday dear supernova 1987. Eh... You know, it absolutely doesn't work. But it was still amazing. So that's all we know. And then there's what we think we know. And most of that comes from computer simulations of supernovae. But the problem is, those are really, really hard. You know, it's one of these extremely rare situations where all four fundamental forces, gravity, electromagnetism, and the weak and the strong nuclear force all play a role. You know, normally, in particle physics, you don't have to worry about gravity. And in pretty much all other areas of physics, you only have to worry about gravity and electromagnetism. Here, all four play a role. You've also got non-linear hydrodynamics of the gas and plasma inside the star. You've got the matter moving relatively at 10 or 20% of the speed of light. And you've got extreme pressures and extreme temperatures that are sometimes beyond what we can produce in the laboratory on Earth. So that's why these simulations, even in 2018, are still limited by the available computing power. So we need to do a lot of approximations to actually get our code to run in a reasonable time. But that produces some problems. And in fact, the week I started my PhD, one of those groups doing these supernova simulations published a paper saying that there is a long list of numerical challenges and code verification issues. Basically, we're using these approximations, and we don't know exactly how much error they introduce. And the results of different groups are still too far apart. And that's not because those people are dummies. Quite the opposite. They're some of the smartest people in the world. It's just that the problem is so damn hard. In fact, in many of these simulations, the stars don't even explode on their own. And we don't know whether that means that some of these approximations just introduce numerical errors, which change the result, or whether that means that there are some completely new physics happening in there which we don't know about, or whether that is actually realistic. And some stars in the universe don't explode, but just implode silently into a black hole. We don't know. We just don't know. So take any results of simulations with a grain of salt. That said, here's our best guess for what happens. So we start out with a massive star that's at least eight times the mass of our Sun, and it starts fusing hydrogen to helium and then on and on into heavier elements until finally it reaches iron. And at that point, fusion stops because you can't gain energy from fusing two iron nuclei. So that iron just accretes in the core of the star while in the outer layers, here in orange, nuclear fusion is still going on. But as more and more iron accretes, and that core reaches about one and a half solar masses, it can't hold its own weight anymore, and it starts to collapse. And inside the core in nuclear reactions, you're starting to form neutrinos, which I'm showing as ghost emoji here. Now let's zoom in a bit. The core continues to collapse until at the center, it surpasses nuclear density. And at that point, it's so dense that the neutrinos are actually trapped in there. So even neutrinos, which literally can go through walls, cannot escape from there because the matter is so dense. And the incoming matter basically hits the wall because the matter in the center can't be compressed any further. So it just hits the wall and bounces back. So from that collapse, you suddenly get an outgoing shockwave. And in the wake of that shockwave, suddenly you get a whole burst of neutrinos, which escape the star quickly. Now that shockwave moves on and slows down. And as it slows down, the matter from outer layers still falls in, and in this kind of collision region, while neutrinos in the center are still trapped, in that collision region, neutrinos are being produced at a relatively steady rate. And the shockwave has pretty much stopped and just wavers back and forth. And we see some neutrino emission. Now after about half a second, maybe a second, neutrinos from the center are slowly starting to escape. Most of them are still trapped, but some are making their way outside. And some of those actually manage to leave the star, while others interact with matter in this shockwave layer and give that matter a little energy transfer, a little push, and heat it back up so the shockwave gets revived and the star actually explodes. And all of that took just one second. And then over the next 10 seconds or so, the neutrinos remaining at the core slowly make their way outwards and then travel away at the speed of light, hopefully to Earth, to our detector. While that shockwave moves much slower than the speed of light, it slowly makes its way outwards, and only a few hours later, when that shockwave reaches the surface of the star, do we actually see something with telescopes? So remember earlier, the neutrino signal we saw was something like a bunch of neutrinos in the first second, and then fewer and fewer neutrinos for 10 seconds. Not a lot of detail. But what we might see is something like this. A brief and intense burst when the matter hits the wall and is thrown back in this first shockwave. Then as the shockwave stagnates, we might see some wiggles corresponding to the shockwave sloshing around aimlessly. Until the shockwave is revived, the explosion starts, and then over the next 10 or so seconds, we would see fewer and fewer neutrinos as the star cools down and as the neutrinos escape. So if we have good neutrino detectors, we should be able to watch millisecond by millisecond what exactly happens inside the star. Now luckily, we've got many more neutrino detectors by now. Probably the biggest one is Super Kamiokande up in green, which would see about 4,000 events from a kind of average supernova in our Milky Way. And then we've got a bunch of other detectors which was typically hundreds of events. And some of these detectors are part of something called the supernova early warning system or SNUZ. And SNUZ is meant to act as a wake-up call to astronomers. So when these detectors observe neutrinos, which are probably from a supernova, they will send out an alert to astronomers to get their telescopes ready to be able to see that supernova from the very beginning. And then of course, just in the past few years, we've also had rotation wave detectors like LIGO in the U.S., Virgo in Italy, and in just a few years we will get another one which is located in Japan, actually inside the same mountain as Super Kamiokande. So they're literally next door neighbors. And then we might get another detector in India, maybe one in China, in the future. So that's three completely different ways of looking at supernovae. So when we observe a supernova, it will be headline news. And now you know what's behind those headlines. So I've introduced you to neutrinos. I've told you a bit about what's like to work on such detector and the challenges of building a detector with scale. And I've showed you how with neutrinos we can observe things that we can't directly observe otherwise, like the interior of exploding stars. And with that, I want to thank you for your attention and please let me know if you have any questions. Thank you, Jost. Thank you for using talk. We have plenty of time for questions and there are two microphones. Microphone 1 is on the left side of the stage. Microphone 2 is in the middle, so queue up. And we're going to take some questions. First question from Microphone 2. Yeah, thank you. But I do have a question. I come from a mining area and I just looked up how deep other mines go and I'm wondering why do you dig into useless rock if you can just go to some area where there are mines that are no longer used. In my area, they go as deep as 1200 meters, I think. I just looked up and was surprised that the deepest mine on earth is almost four kilometers in South Africa, an active gold mine. So why don't you use those? So part of what we're using in that particular location is because we used it for Super-K and for Kamiokande before and the mountain that Kamiokande was in actually is a mine. So we had some previous infrastructure there. And then there's I guess some trade-off between the benefits you get from going deeper and deeper and the additional cost. I think. Thank you. We have a question from the internet that's going to be narrated by our wonderful signal angel. Hello internet. So the question, I didn't understand the whole question but something about earthquakes. Okay, does the earthquake affect the detector? Or there's two parts to the answer. Part one, I'm not a geologist. Part two, I think the earthquakes are mostly centered on the east coast of Japan and we're about 200, 300 kilometers away from that. So the region we're in is relatively stable. And in fact, we've been running for since 1983 and we haven't had problems with earthquakes. And during the Fukushima earthquake our detector was mostly fine but we've actually had so in addition to what I was talking about we're also producing a beam of neutrinos at an accelerator which we shoot at the detector. And that accelerator is right at the east coast. So the only damage from the Fukushima earthquake is to that accelerator not to the detector itself. I hope that answers your question. Next is from microphone too. Hello, thanks for an interesting talk. Do you or does have science, any theory if the neutrinos who hit the electron affected themselves from this hit? Are they like directed in a other direction sort of energy itself or just hit the electron and pass? True. So conservation of energy and of momentum still holds. So they would lose some energy as they give the electron a little kick. Thank you. One more question from Mike too. Hello, thanks for your talk. My question is you said that the only supernova where we detected some neutrinos from is from the 80s. So what is so special about that supernova that with all the new detectors built there was never another detection? So the special thing about that one is that it was relatively close. So it was in the large magnetic clouds about 150,000 light years away which is on cosmic scales our next door neighbor. Whereas as a supernova which we observe can be millions of light years away. We can easily see them at that distance but we can't detect any neutrinos. Okay, thank you. And we expect about between one and three supernovae in our Milky Way per century. So we're in this for the long term. Okay. Thank you. Microphone 2 again please. Hi, thanks for your talk. My question is you said changing the water once a year is not often enough? How often do you change the water? How often do we change the water in the detector? Yes. So we completely drain and refill the detector only for repair work which happens every depending on what we want to do but typically every couple of years to ten plus years. And apart from that we recirculate the water all the time to purify it because there will always be some traces of radioactivity from the surrounding rock which make their way into the water over time. Thank you. And that would be all that was a wonderful start to the congress. Thank you.