Going Deep Underground to Watch the Stars

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Going Deep Underground to Watch the Stars
Neutrino Astronomy with Hyper-Kamiokande
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Neutrinos are “ghost-like” elementary particles that can literally go through walls. They can bring information from places that are impossible to observe through other means. This talk provides a glimpse behind the scenes of a next-generation neutrino detector called Hyper-Kamiokande – a cylindrical water tank the size of a high-rise building. I will describe some of the problems you encounter when planning a subterranean detector of this size, and explain how this detector helps us figure out why the sun shines and how giant stars explode.
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[Music] so our speaker is Jose magenta and they are PhD students and astroparticle physics from the University of code in the UK and Jose is going to talk about going deep underground to what stars please give a round of applause a huge round of applause [Applause]
we published a design report which is over 300 pages long and contains much more detail about the experiment and 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 was radioactive beta decay now in radioactive beta decay you have a nucleus of one chemical element but turns into nucleus of a different element and emits an electron or in Cubao a modern language would say a neutron decays into a proton and an electron now after vent 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 you know all sorts of new possible
explanations for what might be going wrong in these experiments but they excluded those those explanations very quickly as well so after 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 be a radioactive ladies and gentlemen Pauly
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 Polly'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 delay the experiment right now to try and detect neutrinos so to help me with this
experiment please give me 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 and nuclear fusion now these nutrients 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 hmm you don't feel any of them right so that's how goes like neutrinos are and of course physicists are clever and shortly after Polly had this idea some of them estimated how often you Trine's interact with normal matter and they found that there is no practically possible way of observing the neutrino and that remains true for over twenty 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 detect was something bit different I talked about you know beta decay and over and over the next decades physicists slowly discovered more particles they discovered that protons and neutrons are made out of a pile of quarks and in the 1970s theoretical physicists came up with some
grand unified theories basically precursors to string theory and these series predicted that the proton should decay as well so of course you know we
built detectors to look for that and a
group in Japan build a detector near the town of kameoka which they called the Cameo qey nucleon decay experiment or cameo candy for short now they didn't observe any proton decay but shortly after they build it somebody had a suggestion that if we change just a little bit about that detector if we modify it just little we would also be able to detect neutrinos
prevent so they modify the detector
switched it back on and just a couple of weeks later they actually observe neutrinos from an exploding star just outside our Milky Way and that was a birth of neutrino astronomy and for that the venn 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 twenty times bigger started running in 1996 and is still running through 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 you know 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 receive the Nobel Prize again but today
we are again hitting the limit of you know what we can learn from a detector that size so of course the next step is to build need bigger detector and we're calling it hyper cameo candy by the way
super and hyper mean exactly the same thing just one's Latin one is Greek so we are 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 sixty billion neutrinos
go through your thumbnail every second of course super-kamiokande which is running right now is much larger than your thumbnail so there's not just sixty billion but ten thousand billion billion neutrinos passing through every day and only ten or fifteen of those get detected so let's look at what that what
this detection process looks like now this is the water inside soup okay and
there's you know 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 will kick you know black 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 in a cosmic temple limit but faster than the speed of light in water and then you get basically a sonic boom but was 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 little ring this ring
of light and we've got very sensitive photo sensors 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 a neutrino came from just based on in with which directly pushed the electron so 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 kamiokande and you can see it's you know 78 meters high 74 meters in diameter and on the top left there's a track and 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 you know doesn't quite fit in there the B 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 on the ground so that all the rock on top will act as a having 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 the only neutrinos can make it through now of course to build such a huge cavern inside a 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 bore holes 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 you know take this with a grain of salt
but ok let's say you know we talk to geologists they told us where we can you know build a 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 in the mountains of Japan you know pretty far away from any major city so we have to think about stuff like like a local infrastructure like what's the electricity supply like do we need to to add a power line or what are the local roads like and do they have enough capacity for you know 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 you know 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 gonna need a Balrog early
warning system as well but ok 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 the detector so big that that little dot
there is a scuba diver but even with water you hit limits of you know how much you can get so to Phil hypercam you can do 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 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 you know even where do you get the water is is a pretty pretty important question but you need to solve and then we're not just using you know any water but we actually have a we will build our own water purification system so that we don't have any you know traces of radioactivity in there any have dust and
stuff in there and let me 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 you know on the 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 you know didn't know just didn't think about too much until at the end of his shift he went home you know went to bad fell asleep and then woke up in the middle of the night was so had itching like mad now what had happened there the ultra-pure water had sucked all of the nutrients out the tip of his hair and then through osmosis over time those had sucked the nutrients out of the rest of his hair and met 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 of PMT's for short and those are basically you know 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 you know from each PMT one cable leading to the top of the detector and then have your computers there to analyze the signal for the detect of this size you just can't do that because you would need you know 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 PMPs into one and then use just a single cable to bring that up to 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 water Titans you'd expect from your SmartWatch where it survives you know you send another shower for five minutes I'm talking below 60 minutes 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 you know these these pixels these photomultiplier tubes would become noisy and this would kill our signal and then also because we don't want you know 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 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 detectors is that huge and getting worse
this is what one of these PMPs looks like it's about 50 centimeters in diameter and inside that last bulb is 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 that 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 PMT's that have you know some weak points some air bubbles or something in the glass or some structural weakness and the neighboring PMT's don't only
have to survive the normal water pressure they also need to survive one of their neighboring PMT's imploding and sending out a pressure wave and that's not just a hypothetical that actually happened you know 18 years ago
sorry 17 years ago in in super-kamiokande and that you know within seconds killed more than half of the PMT's we had in there and it took years to restore the detector to full capacity
so lots of problems to solve and you know one 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 here right now here it's about you know just before noon in Japan people have already had dinner and uh going to bed soon in the u.s. 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 to actually build it but now I want to talk about what we use a 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 turns out it's really difficult to answer so in in the days of the Industrial Revolution when you know burning coal and then steam power was all the rage people thought that you know maybe it's you know a giant ball of burning coal but when you when you you know do the math it turns out that with your burn for a few thousand years maybe so that definitely doesn't work a bit later the physicists suggest 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 you know pesky geologists coming along and saying no no we've got these rock formations or I don't know fossils maybe but I'm 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 you know the 19th
century between la Kelvin and the geologists back then are just amazing to read if you find those somewhere but of course nowadays we know but the answer
is nuclear fusion and here's you know a bunch of reactions which 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 hypercam you can do is the one on the bottom right here called boronate 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 fifteen point five million Kelvin plus or minus one percent so we know the temperature in the core of the Sun to less than one percent uncertainty that's pretty amazing if you ask me and you know 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 you know blurry and pixelated and not as nice as what you'd get from a you know from the optical telescope but this is still a completely different way of looking at the Sun and what this tells us is that this you know giant glowing orb in the sky that's not some optical illusion that actually exists okay so
onto our next topic exploding stars of supernovae which is what my own research is 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 you know
randall munroe 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 the 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 this supernova is bigger than that in fact it's a billion times bigger than that so supernovae are some
of the biggest bangs 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 of 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 understand how these explosions happen and even up serving members 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 have 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 2,000 neutrinos which you see here on the right that's what we
know so we know basically that you know
many neutrinos are emitted during the first one second or so and then fewer and fewer for the next ten seconds we know that neutrinos make up most of the energy of the explosion of the of the
supernovae was the actual energy of the explosion and the visible light making up at 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 2,000 events least two dozen neutrinos more than
1,600 papers were written that's more than one pay per 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 you know 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 in 30th anniversary celebration so then we were about 40 or 50 physicists looking over the skyline of Tokyo having dinner there see now leader of these 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 where we were and at one point that evening we actually started to sing happy birthday so it go you know how it goes happy birthday to you happy birthday to you happy birthday dear supernova 1987a you know it absolutely
doesn't work but we're 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 nonlinear hydrodynamics of the gas and plasma
inside the star you've got the matter moving around to vistacal II at ten or twenty percent of the speed of light and you've got extreme pressures and extreme temperatures but uh sometimes beyond what we can produce in a 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 gives you something that you know 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 this approximations and we don't know exactly how much error they
introduced 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 you know completely new physics happening in there which we don't know about or whether that is actually realistic and some stars you
know in 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 you know any results of some 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 and to 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 retreats and that core reaches about 1/2 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 us 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 it matter is so dense and the
the incoming matter basically hits a wall because you know the matter in the center can't be compressed any further so just hits the wall and bounces back so from that collapse you suddenly get an outgoing shock wave and in the wake
of that shock wave suddenly you got a hole burst of neutrinos which escape the star quickly now that shock wave moves on and slows down and as it slows down the matter from outer layer still falls in and in this keV collision region while neutrinos in the center are still you know trapped in that collision region neutrinos are you know being produced as a relatively steady right and the shock wave you know 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 you know most of them are still trapped but some are making their way outside and some of those actually mentioned leave the star while others interact was matter in this shockwave layer and give that meThe matter little little energy transfer a little push and heat it back up so the shock wave gets revived and the star actually explodes and all of that took just one second and then over the next ten seconds or so the neutrinos remaining at the core slowly make their way outwards and then you know travel away at the speed of light hopefully to earth to our detector while that shockwave moves much slower than the speed of light you know slowly makes its way outwards and only a few hours later when the shock wave reaches the surface of the star do we actually see something
with telescopes so remember earlier the
neutrinos signal we saw was something
like a bunch of neutrinos in the first
second and then fewer and fewer neutrinos for ten seconds not a lot of detail but what we might see is something like this a brief and intense burst when matter hits the wall and is thrown back in this first shock wave then as a shock wave stagnates we might
see some some Wiggles corresponding to the shock wave you know sloshing around aimlessly until the shock wave is revived the explosion starts and then over the next ten or so seconds we would see fewer and fewer neutrinos as a star
cools down and as neutrinos escape so if we have good neutrino detectors we should be able to watch no 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 the super-kamiokande I've been green which would see about four thousand events from a curved average supernova in our Milky Way and then we've got a bunch of other detectors which was typically you know hundreds of events and some of these detectors a part of something called the supernova early warning system or snooze and snooze is meant to act as a wake-up call to astronomers so when when these detectors observe neutrinos which are probably from a supernova they will send out an alert to astronomers to get that 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 a gravitational wave detectors like LIGO in the u.s. Virgo in in Italy and in just a few years we will get another one called cadra which is located in Japan actually inside the same mount in
a super-kamiokande so they're literally next-door neighbors and then we might get another detector
in India maybe one China in 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 bit about what it's like to work on on such a detector and you know the challenges of building a detector of this scale and I've showed you how with nutrients we can observe things but 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 used it was an amazing talk we have 20 of time for questions and there are two microphones microphone one is on
the left side of the stage microphone two is in the middle so queue up and we're going to take some questions first question from microphone - 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 take into useless rock if you can just go to some area where there are mines that are no longer used my area tako as steep as 1,200 meters I think I just looked up and was surprised that
the deepest mine on earth is almost 4 kilometers in South Africa an active coal mine so why don't you use those so part of what we're using you know that particular location is because we use it for super K and for cameo kinda before and the the mountain that kamek and it was in actually is a mine so we had some you know 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 okay 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 I didn't understand the whole question but something about earthquakes okay does the earthquake affect the detector well there's two parts of the answer
part one I'm not a geologist part two I think the earthquakes are mostly centered in the cave on on the east coast of Japan and we're about you know 200 300 meters away from that so the region where in is relatively you know stable and in fact we've been running for you know since 1983 and we haven't had problems with the 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 was to that accelerator not to the detector itself I hope that
answers your question next is from microphone - hello thanks very 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 other direction or loose some sort of energy itself or just hit the electron and pass true they so
conservation of energy and of momentum still holes so they would lose some energy as they give the electron a little kick thank you
thank you one more question from my tune hello thanks for your talk and my
question is you said that the only supernova where we the tactics of 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 special what special thing about that one is
that it was relatively close so it was in the Large Magellanic Clouds about 150,000 light-years away which is you know on cosmic scales our next-door neighbor whereas other supernovae you
know which we observed can be you know millions of light years away we can easily see them at that distance but we can't detect any neutrinos and we expect about you know between 1 and 3 supernovae in our Milky Way per century
so we're in this for the long term okay thank you microphone 2 again please thanks for
your talk and my question is you said that changing the water once a year is not often enough how often do you change the water so how often do we change the
water in the detector yes so we we completely drain and refill the detector only for repair work which you know happens every you know depending on you know what we want to do but typically every couple of years to you know 10 plus years and apart from that we recirculate the water all the time to purify it because there will always be some you know traces of radioactivity
from the surrounding rock which which make their way into the water over time thank you thank you and that would be all that was a wonderful start to the
Congress Thank You youth thank you
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