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So I wanted to say you've got the idea now that although we are called the Particle Theory Group, we don't actually all work on particle theory.

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Some of us work on string theory for us, work on mathematical methods connecting to different classes of theories.

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It goes by the name of gravity duality or what we just refer to as the CFT correspondence.

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Some of us work on things relevant to the Large Hadron Collider and some others work on issues connected with astrophysics and cosmology.

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And by the way, there are only eight of us. This is not the view of a huge army of people, but what we do like is we direct with others,

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we interact with the mathematicians, we interact with the astrophysics, the particle physics.

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And although we are theorists, we also address to the experimentalists in first half of the marriage to them.

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So can we help?

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So I want to tell you about a sort of a you know, I have a day job as a theorist, but I do also have a second career as part of a experiment,

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which I was invited to join now nine years ago, which is this amazing thing at the South Pole.

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So I always like to show this thing, you know, you can teach on the back, but every direction points north here.

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Okay. And that's where there is this amazing laboratory, which is this in fact, is what's on the surface.

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The experiment extends down two and a half kilometres to the bedrock under the pool.

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And it's amazing that, well, that physicists actually managed to convince people to give them the resources to build such a detector,

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which obviously most of us, including many experimentalists,

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thought was folly for many years until the day it came up trumps, which happened quite recently.

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And that's what I'm going to tell you about. I will also tell you why theorists are interested and involved in experiments such as

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these and what bearing it has on the kind of work that we do here in the role of centre.

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So the title is Seeing the High Energy Universe.

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How do you see the universe? We see it with photons coming along with past life, and that's all we know about the universe.

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And if we plot with this in an ideal detector,

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the amount of energy we are receiving as a function of the wavelength or the energy of the photon up

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here you see that the sky is really dominated in terms of energy by the cosmic microwave background,

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and there is the characteristic brown shape.

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And we know that although our knowledge of the universe for many years has been restricted to just this optical band,

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it so happens that the sun, our star emits light in that range.

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And that's why our eyes are sensitive to that.

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We don't see any of the rest of it, but we have been able to open new eyes in the universe, and that started with radio.

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That was just during the war. And then microwaves, of course, and then we have gone through infrared ultraviolet.

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This is important because distant sources are redshifted, as we said.

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So in order to see them, you have to observe in the infrared.

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And we just heard about an application of what interesting phenomenon we might see in the ultraviolet and X-ray image.

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The final frontier here is gamma rays, but gamma rays can only take us so far.

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They could take us to something of the order of ten DV, which is roughly the energy of the Large Hadron Collider,

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because then the photons that we are trying to see on the route to us will hit

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other photons that happen to be hanging around like the infrared background,

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photons leftover from the formation of stars. And they're attenuated.

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However, if we use charged particles, cosmic rays, we can in principle see up to 10 to 11 g.

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They, too, will ultimately see the background photons as high energy particles because, you know,

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from special relativity, if I have a ten to the 11 G, the proton that's the gum affected.

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The relativistic factor is ten to the 11 because the proton visa V so to it a microwave photon which

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is ten to the minus three electron volt actually looks like you multiply that by ten to the 11,

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it looks like a ten to the eight V in other words, 180 photon.

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So you are moving so fast that even a soft photon hitting you feels like a hard gamma ray and that would break it up.

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And you have the so-called Grizzard Xuxa Guzman effect that they will create a lot of pions.

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And those pions would go into neutrinos and those will travel all through the universe.

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Nothing stops neutrinos. Therefore, ultimately, if you want to see the universe back to the early universe, you must learn to look at neutrinos.

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That's the challenge. The challenge, of course, is that they're very hard to measure.

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They're very hard to detect. So the story starts with this guy I mentioned discovering cosmic rays 100 years ago, 101 years ago now.

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And that discovery has now been extended in this plot,

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which shows you the flux of cosmic rays versus that energy through this 10 to 11 g v that I just motivated for you.

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Okay, that's amazing. You can see the number of decades of energy that we been able to explore this domain with the variety of experiments,

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some based on the ground, others flown on balloons.

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They now fly balloons down there, darting on the prevailing winds, which stay up for days at a time.

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And then, of course, on rockets and satellites and so forth.

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We still are somewhat uncertain about where this energetic particles come from, why they have exactly this ball or shape.

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Although Tony Bell, who is in the Clarendon Lab in the laser physics group,

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had a interesting theory as to how they get that spectral shape, which we might be able to soon test.

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And the real culprits, we believe, are these most violent sites in the universe,

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the centres of galaxies like ours, which have got a black hole that is accreting matter,

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shooting objects of plasma, which then evidence themselves as huge radio lobes or gamma ray bursts,

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which are the most powerful explosions in the universe, as bright as the whole galaxy.

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But we don't really know what they are, except that they also involve gravity.

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They involved ultimately the energy of supermassive black holes dragging in matter.

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That's the most efficient way to convert matter to radiation. And we believe these are implicated.

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But one way to find out what these things are doing is to look into that which is very hard to do optically because things that are obscured by dust,

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by radiation and so forth. But you can do it in neutrinos.

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And neutrinos are always produced when you have high energy particles interact with matter and they are

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produced essentially through the production of particles that you're familiar with by onset quads and so on,

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which ultimately decay to neutrinos. And those are the guys we are to look for.

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And you can see on this plot that the energy range of the CERN Large Hadron Collider is on the up to here.

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Cosmic rays nature can do a lot, lot more than that.

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And one of the big mysteries in this thing is to find out how it actually does that.

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I should also mention that these neutrinos are actually quantum mechanical.

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They oscillate. They are light particles that mix with each other as they propagate the oscillate from one type to another,

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as dictated by the quantum mechanics that you have heard about.

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And therefore, we have a laboratory for studying quantum mechanics on cosmological scales now, which would be very, very interesting.

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So David showed a version of this which is even more spectacular,

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which is that to create some of these particles of Planck energy, you'd have to build a collider as big as 100 light years.

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For cosmic rays. It is not that impressive. But you still need to build a collider the size of Mercury's orbit.

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To agree to that, to the level of the protons that you have actually observed on Earth in the period Ozone Observatory in Argentina,

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which has observed particles. And the plot here shows that to just contain such a particle of that, it was enormous energy in a ring.

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You need to have a magnetic field as strong as that and then extend as big as that.

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The product of the two must lie on that line in order for the particle to even be held in its orbit according to the equations.

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David, should you follow this to Exhilarated?

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So upstairs we have the accelerator physics said to people worry there about how to accelerate particles to high energies,

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develop new techniques, laser fields and so forth. Well,

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nature clearly has a lot to teach us that we haven't yet learned how it manages

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to do this without the benefit of thousands of trained engineers and physicists.

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Okay. But there is the challenge for you. Okay. And here is that it's our pride and joy, and here we are being put to shame.

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Okay. Now, the point here that I want to make is what, however,

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those particles accelerate these enormous energy dick protons, they would ultimately make neutrinos.

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So here is a schematic of how those neutrinos are being made through the processes that I told you about.

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And we can make very simple estimates based on how much energy these sources are dumping into the cosmic rays.

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We can normalise that to the amount of neutrinos you'd expect because roughly speaking, as much energy goes into the by-product as in the sources.

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So whatever energy we can see in the cosmic rays should also be there in the neutrinos,

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and we can make estimates of what sort of flux they should have. Obviously, I'm not going to show you the details,

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but these are the kind of calculation that we make in order to estimate how big a detector we need to do this.

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And turns out that nature has already provided us sources of neutrinos, which some of which are indicated here.

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There is the cosmic neutrino background. We just heard about the cosmic background that might also be there somewhere.

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This has not been detected. It's too challenging at the moment. But we have detected neutrinos from the sun.

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We have detected neutrinos from Supernova 1987, which many of you will remember.

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Most of my students are too young, and I tell them 1987.

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And that was, well, they're still in primary school or something.

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But we have also the relic neutrinos from past supernovae, which have yet to be detected.

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But we have neutrinos being made in the atmosphere of the earth.

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But that's not the cosmic rays which we can measure, and that is actual data from IceCube.

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And we hope that ultimately, if you go to high enough energies,

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we'll can start to see neutrinos that we have calculated must exist from active galactic nuclei,

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from gamma ray bursts, and from this process that I mentioned, where the protons run into the cosmic microwave background and create pions,

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and the process is always the same, which is shown here to gain the proton comes in, hits a nucleus in the atmosphere,

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creates a so-called extended air shower, and that creates pions that which ultimately decayed beyond the neutrinos.

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Those millions are going through us right now, as are the neutrinos.

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Obviously, we cannot detect them because we are too small to do so.

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You need a big detector.

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We'll see how big when we do the classic back of the envelope calculation, because we know that neutrinos interact very, very weakly.

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They interact with the cross-section, which is very,

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very small compared to the kind of things we are used to see for photons or electrically charged particles.

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But it you know that the typical cross-section is of ordinary bond.

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That's why we invented the unit, which is ten to the -24 centimetre square.

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This is nine orders of magnitude smaller. Okay? So therefore, this is the kind of calculation our undergraduates do.

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How many nuclei do you need? Well, Avogadro's number is big, but you still need a lot of material.

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And if you want the rate of events for neutrinos, it turns out that you detect one per unit in a kilometre to detector.

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Okay, that's how big you need to get. You have to think big. How do you detect them?

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Well, the neutrino come in, hit something and that would create a charge lepton for in this case a Bjorn monstrously long way.

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And if they are travelling through a medium, they might be going faster than the speed of light in the medium.

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And that would set up a cherenkov cone, a shockwave of light,

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and we can detect that with sensitive photodiodes or for the multipliers budding in the detector.

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And what you have done is to follow up this thing. You could do it in water.

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People have thought of doing it in other media, but we decided to do it in ice because nature has been bountiful enough to give us a lot of ice.

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And at the South Pole not you also have ice in Greenland.

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That doesn't seem to have the right properties. This one is perfect. Because this is actually a plateau.

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This is quite high up. Okay. And you already had the big base station did, which provides the necessary infrastructure.

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And in particular, you know, flights can go there regularly, land on skis on the strip.

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And this is where the base station is, about a couple of hundred people there, lots of experiments being done, including ones in cosmology.

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But our route to work is over here, where you have the icecube detected by deep underground, under ice, rather two and half kilometres down.

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What you do is you drill holes two and a half kilometres down with a five megawatt drill of hot water.

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And as I show you very quickly, you can implant detectors in the ice before it freezes up, takes about a day,

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and then you have there the strings in the ice, which have got the ability to see neutrinos.

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And this is just to say, show that obviously I didn't do that work.

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Lots of competent people from elsewhere did the work. They know what they're doing.

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And however, Oxford is involved in that, as you see it, we have been there for some time.

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That's along with people from elsewhere.

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So something like 11 countries, it's a relatively small collaboration as far as experimental particle physics is concerned.

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Typically, Atlas has something like 4000 scientists.

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Okay. But the director itself is a challenge to build.

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As I said, you build drill these holes, you load the strings.

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The strings carry these so-called digital optical modules, which are very sensitive to tubes made by Hamamatsu.

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And they are, of course, remember, the pressure down here can go up to 300 atmospheres.

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They have to resist that. And happy to say that apparently 97% of our modules have survived.

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And, you know, it's just like your mobile phone. Either it works or it doesn't.

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If it works, it will work until the built in obsolescence period runs out.

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In our case, that hasn't happened. But this whole area, of course, is an ice that is constantly moving.

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And so somebody calculated that by 21, 20 or something, the whole array would have drifted to the edge of the Antarctic.

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But, you know, we are doing experiments at the moment. We are fine.

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And what we have right now is all the strings in the dice, and there are 60 of these modules on each string.

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So you have a gigaton of instrumented volume now.

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Now we are talking, you know, size does matter in this game.

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We do need to have that kind of volume to be able to get a event rate that's at all interesting.

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And as I indicated, you have these detectors looking for the flashes of light, the faint blue light.

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That's what the wavelength of radiation is at.

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And you detect that in these things and the timing tells you what direction they're coming from.

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I'd show you some actual data later, but really we are looking at this so-called charge current interaction with a neutrino exchange,

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a W boson with the one of the target nuclei creates a muon that is the we are looking for and of course a local record.

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Right. If it was the electron neutrino that is, you know, the signature is different.

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We see that. So this is what the Bjorn Neutrino track looks like.

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There is a lot of information here. I'll be obviously skipping over all these things, but please feel free to ask me questions later.

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So the colour code is actually the timing that tells you which one triggered earlier, which would trigger later.

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And obviously you can reconstruct the direction very well. But because the neon is passing through, we can only get a lower bound on its energy.

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We can only measure up the amount of energy deposited while it was there.

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So here is the timing. Good red means immediately this blue means delayed.

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And from that we can infer a lot of other information as well as to the energy of the event.

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The other kind of they call it topology have objected that topological needs to articulate, but the experiment is calling them topology.

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For some reason they have a different signature.

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This is an electron neutrino that created an electron and the electron does not have much of a range.

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It is, as you know, a lot lighter than the moon. It stops almost immediately.

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So you just see a huge mushroom of light in the detector that's about 100 metres across.

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Okay, that's an energetic electron neutrino. We know the energy now pretty well, but the pointing is very pure.

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How do you know what direction that blob came from? Right. But we'll see that experimentalists are very clever and find ways to do that.

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So we are looking for different kinds of neutrinos that have different signatures.

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Some of them make charge leptons, and then they can also have interactions with the exchange, a desired boson rather than a W boson.

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These are the so-called neutral current events. Right. In fact,

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one of the first neutral current events that established the standard model was found in the bubble chambers photograph just downstairs

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with the Dallas centre now is years ago and we therefore have a long association with the history of neutrino physics here in Oxford.

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I mentioned Dawn Perkins and others earlier and we are today carrying on the

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legacy by looking for these very energetic neutrinos coming from outer space,

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which give us both tracks through events such as this, or showers, as you see here,

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or cascades, as you call them, which could be a signature of any one of these kind of things.

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But you make it all lived on, for example, or electron, but not a view on which would give you a track.

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So basically we know the taxonomy of these things.

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We know what we are looking for, we know how to identify them. And then the question is,

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are we sensitive enough to see some of these cosmic sources that we know must be out there because we see that charge counterparts, the cosmic rays?

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And this is the current status we have seen, as I said, neutrinos from supernovae and from the sun.

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We have seen the atmospheric neutrinos. We can in principle see the neutrinos from gamma ray bursts and going up to here using the IceCube detector,

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because we now have to learn a little more than the GV and MF that you've been using.

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These giga be sped up and you have to go further to exa electron volts and then to better electron volts and finally to z to electron volts,

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which is ten to the 11 G. Okay, so that's how we load plus six here.

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We just extend the energy they give it. Okay.

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And we also have the low energy frontier is equally interesting because this is where we can look for dark matter,

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which Felix gives such a wonderful talk to us about. One of the synergies you mentioned was then violation of dark matter elsewhere in the galaxy.

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And of course you are talking about searching for photons, but photons can get absorbed, neutrinos don't.

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So we can look, for example, at the sun, which has been sweeping up dark matter for the last 5 billion years,

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which are annihilating inside the sun right now as I speak.

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And if we see a single high energy neutrino from the sun, one high energy neutrino C of energies in that range, that would be a tremendous,

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stupendous discovery because there is no way that any astrophysical process in the sun can make that it would be a signature of dark matter.

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And we are also trying to do fundamental physics with this by starting this neutrino oscillations,

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by lowering the threshold of our detectors to a few GV because then we can do experiments

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using the free beam of neutrinos that nature has given us made in the atmosphere,

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which allows us to probe certain fundamental aspects of the neutrino mixing and the quantum mechanical oscillations

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such as the mouse hierarchy and possibly one day the CP violation that might be there in the usual sector,

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whether there are single that this new kinds of neutrinos, etc., etc.

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So that was all. This was all you know what you put in the proposal and you hope for the best that somebody will fund you to build a detective,

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as the did, and so on and so on. But of course, the payoff is after years and years, you finally see them.

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And these were the first two events that you saw. You can look at the tremendous energy.

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This is, you know, two orders of magnitude beyond that at sea already.

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And, of course, being the serious people we are, we call the Bert and Ernie.

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Okay? And they made the cover of physical review letters.

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Okay. And then, of course, that opened up this possibility that we are seeing cosmic neutrinos.

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But the case wasn't yet clear because here is the plot of what we expect to see, for example,

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from the process with the protons of cosmic rays run into the cosmic microwave background and that they are called cosmic neutrinos.

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And this calculation was done by Marcus Aurelius, who was our post-doc here, and some of us.

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And we made the most flexible assumptions we could make about this process, and nonetheless, we could push it up above that red line.

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And you see these events like clear above it, not that much clear.

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The axis is only about three sigma. It is not enough to call it discovery,

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but it's certainly better though rapid date intruders that we are able to see high energy events

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for which we do not have an explanation in terms of the so called guaranteed cosmology in flux.

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These must be to do with the sources. Okay, so we decided we have to do something about it.

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We have to look for these guys.

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And the problem is that normally what we do is we look through the earth because when we look up, the cosmic rays are creating so much background.

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When you look up, we are blinded by it. Would see that you have to reject one in 10 million events in order to see what you're looking for.

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That's because these cosmic rays are creating this huge flux of neutrinos.

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And also, of course, they're companions in the process which have a huge event rate upwards.

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We need to look down to see them, but then we are not sensitive to the bulk of the signal which is upwards, right?

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So therefore what we are really doing is we are following this track of this cosmic

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rays that we know do exist and trying to see if somewhere new physics comes in,

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a new signal comes in. There is also a signal from the production of so-called heavy flavours like,

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you know, button quarks or job quarks, which we know must be there at some level.

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We have not seen it yet. But what we are hoping to see is some new flux coming in here, which in this scale, which is energy squared times,

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the flux would come out of the horizontal line as distinct from the steeply falling spectrum.

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That is the theoretical calculation, that's the measurement. Very good agreement, you have to agree.

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But we are looking for something like this and this is our got involved now the name of the of the this

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this is okay this may not be as much of a challenge as the dark matter surge that Felix talked about.

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But it is challenging all the same because if I look in Zenith Angle,

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this is looking up South Pole, this is looking down through the earth towards the North Pole.

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And you see that the background of atmospheric nuance is huge.

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Okay, so we have to somehow reject these guys.

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If you look up, we don't want to see the ones from the cosmic ray directions.

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We want to look at the closely Filipinos that are coming behind them.

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So we have to basically reject one in a million events because the thing we are looking for is down here and the background is up here.

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Okay. Looked impossible. But in fact, it is not impossible.

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What we can do is that we can use the detector itself as its own veto.

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We can sacrifice part of the altar detector and basically demand that what you are looking

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for is not accompanied by a view on which would have been made in the same direction.

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And that must be it that produce the neutrino. So we basically throw out any event which does not start in the detector.

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So basically, if it if there is any action on this periphery, then we just throw it out.

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We just look for what is happening inside the detector.

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The argument being that it has to be a neutral particle in order to enter without giving any signal there and then do something spectacular in here.

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Right. And we had managed to do this, that when I say again, we I mean, the competent experimentalists on this experiment have managed to do that.

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The energy threshold has been lowered down to 40 TV, which is a low energy for us,

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although it is still about four times higher than the energy of that at sea.

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We look for events which are extremely bright, which triggers, you know, a very large number of output tubes,

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which I forgot to mention since I skipped over the details, digitise the signal right there.

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So it comes to the surface. It is in a form that I could analyse on my laptop, in fact.

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But my shift was a few weeks ago. We can actually run the S.A. from a laptop and wi fi.

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Okay. I don't need to go to the pool and, you know, to to to run it.

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I mean, this is remarkable. This is the nature of. And it also is good for the ozone layer.

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I think so. That's fine. Now what we have here is a possibility of looking at this neutrino flux against this background by doing this clever trick.

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But of course, when we do this, we also suppress track like events because track like events are coming through there and predicting them.

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So ten to I would expect to find mostly events due to electron and neutrinos which have come in here without an accompanying view.

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And I've created an event in my detector. I have sacrificed out of 30%, but that's still a lot of detector left.

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And what you see here is I mean, I thought I would really show you some actual data.

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So you get a feeling for, you know what, we really work with the kind of analysis what needs to do.

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This is the background. And what we have managed to do is to separate this little part here with more than 6040 electron heads,

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which do not leave any trace in the outer part of the V2, which allows us to say that this cannot be part of that and this is something new.

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This is the signal that we are looking for. Okay.

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And although this is not theoretical physics, what actually goes into doing this separation is theoretical physics.

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The algorithms that are used have interesting connections to mathematical structures that are used in other parts of theoretical physics.

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I could tell you about that in some detail. But let me show you some pretty pictures, because ultimately it's these things that we live for.

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So when we did this analysis, first thing we did was we discovered Bert and Ernie, but we found 26 other events.

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Okay. And now we have some significance because we see 28 events.

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But that expected background is only about of the order of ten events.

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So now we do have something which is stronger than nearly five standard deviations.

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And if you include those two, it is actually above five. So it's time to well, time to get excited.

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Right. And you see that most of these events are blobs. They are not things which have got accompanying beyond tracks.

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This one is emission drag, but this is electron or it is, you know, or it could be a neutral colour individual GMO neutrino.

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We cannot tell just by looking at that. We have to do a little more work.

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We have to ask what that distribution is, energy versus the declination angle.

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They seem to be mostly showers, as you see here, only a few attracts, but that's as we expected.

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Here are Bert and Ernie sticking out at the Beav.

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I mean, this is a tremendously high energy and it tells us that we have found something new in nature.

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Nobody has been able to create particles of that energy before, at least in neutrinos.

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And you can even from those blobs, this is a very busy plot.

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But just to sort of show you that this blob here, you wonder which direction the painted neutrino came from.

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Well, if it came from this this direction, according to the red line,

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then you would expect the timing of the hits and the photo multipliers to be according to the red line.

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If it came the other way, then that would be the blue line.

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Then the timing would be according to the blue line. And you can see that the data actually allows us, by looking at the so-called reforms,

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to pick out what direction it came from, by looking at this timing.

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And therefore, we are able to identify what direction in the sky this apparent blobs came from,

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which is, I think that the clever because that then allows us to start doing astronomy rather crudely.

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So the first thing is that we have this events which are above the expected background.

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Those are those points that you see here. This is the expected background with very conservative uncertainties.

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And those tell you a bit more about that in a minute. That's where our contribution came in in estimating that uncertainty.

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But, you know, things are not clear. Is that cut off beyond that point?

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These are upper bound. So there is no events there. And there also seems to be a upper bounds here.

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Is that a gap there? At higher energies, there is the so-called glashow resonance where the electron neutrino can come in, create a W boson.

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And are we seeing that? These are all questions that we have yet to answer.

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But meanwhile, it has been very rewarding.

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Your reported breakthrough of the year as a science, as a physics result, and the science of it put us on the front cover.

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That event that I showed you earlier, which is now called Big Bird,

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and the distribution of these events in declination, in the direction of the sky that they're coming from.

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I showed you already. That's the South Pole. This is the North Pole.

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And we see the distribution is in fact quite different from what you would expect according to the background atmospheric neutrinos,

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which is this blue line. In other words, this is not coming from a particle that is being made in the atmosphere by some exotic process.

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That is a possibility. We can rule that out. They are definitely consistent with something which is isotropic about us.

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So that's the first sign that we are on to something that is of potentially, well,

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cosmological or cosmic at this interest that might be coming from the halo around our galaxy, which is also symmetric about us.

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But they might be coming from extragalactic objects which would by definition be isotropic about us.

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And there is a lot of physics that goes in here that I can only allude to at high enough energies.

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Neutrinos will be absorbed in the atmosphere. This is showing that attenuation for different energies and that allows us to

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make estimates of how many we should expect as a function of the zenith angle,

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knowing the cross-section.

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And then we can compare that with the data and then we can conclude that what we are seeing is definitely the extraterrestrial neutrinos.

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They're not being made in the atmosphere of the earth and they are probably coming from extragalactic objects.

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All do currently. We cannot be sure of that. They could be coming from within our galaxy.

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Right. And so at the moment, the bottom line is just this.

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What does that distribution look like on this guy? Well, this is what is shown here.

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This is called the log likelihood. This is a measure of the probability that the event actually came from that direction.

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The fuzziness is because the reconstructions are not accurate.

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I told you that. Plus minus ten, 15 degrees. So this is hardly astronomy.

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But, you know, you have to take a first step. And this is the same thing being shown in different coordinate systems.

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So this in this coordinate system, which is the so-called equatorial system, that is the galactic centre,

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and there appears to be a clustering of events around the Galactic Centre, as you would expect for something to do the galaxy.

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Maybe this neutrinos are from the decay of dark matter. But in fact, one of the highest energy events did come from that direction.

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But again, the same. This is the Galactic Centre. Now in galactic coordinates where it's of course at the centre of the picture.

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But at this point, you go and talk to your statistics chums and they sort of throw cold water on you and they say,

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well, yes, that's what it looks like to you.

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But it's not actually true because, you know, if you spend enough computer power doing random realisations of the sky scanning in right ascension,

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you find that 8% of the time you'll get a clustering like that just by chance.

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Right. And 8% is not small enough for us to think that it is significant.

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So what's the answer? Well, you could continue debating about what sort of statistics we should use, or better still, get some more data.

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Right. And that's exactly what we are now doing. So we'll soon know whether the source is a galactic or actually extragalactic.

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There has been a profusion of heretical ideas about what these events could be due to,

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and they are so too small for you to read from the back deliberately.

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Okay. Because. Because some of them are very baroque and I would not want to.

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So here is testing the DVD with high energy neutrinos. Sup with neutrinos from unidentified sources, the super heavy particle origin.

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And you know, some of them have more than one sort of implausible thing died in.

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But you know so there are a lot of. Well, I just want to say, people theorists always get excited when these things appear,

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because now is a chance to try out some of these new ideas we have had about new physics and to see this survive,

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or indeed, they might be responsible for some of these things. So let me just give you the current picture.

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I showed you this already. This these have been seen for some time.

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These are just conventional neutrinos from beyond on condition that most of you.

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But now we are seeing the spectrum which looks flat because this is plotted in energy squared times, the floods.

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So if spectrum that goes as one over the energy squared would look horizontal.

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And we are seeing this. And this is roughly where you expect events to be.

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President Trump Gamma Ray Bursts from active galactic nuclei.

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We obviously don't have enough information yet to test those theories in detail,

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and we are heading towards the energies where you would expect this guaranteed flux of

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neutrinos from the interaction of cosmic rays with the cosmic microwave background.

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So things are getting very, very exciting, though.

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But let me just tell you for a second, you might be wondering, this is all very fine stuff, but what is a particle theorist doing in this experiment?

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Well, uploading a flux here you have some events.

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But to convert an event into a flux, you need to know the probability of the interaction of the particulate matter.

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Okay. How often does it interact? Now we know, of course, at what rate neutrinos interact when they're at laboratory energies.

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That's how you measure it. Okay.

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But we are not talking about energies which are 2000 times, 10,000 times higher than anything we have ever had in the laboratory.

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So you have to extrapolate how do you extrapolate from what we know in a meaningful and sensible way and estimate the uncertainties?

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Because that is crucial to establishing whether you have a signal that is significant or not.

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Right.

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So I just want to give you a plug very quickly, because I realise we are already over our scheduled time of how it makes sense to be in a department.

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But fortunately our neighbours have been doing things which turn out to be very relevant to this.

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In particular, particle physicists here have been involved in an experiment called Hera at Hamburg,

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which has been measuring a process called deep inelastic scattering,

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which is exactly what you are measuring in IceCube, except they have been doing it with electrons and protons that neutrinos,

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you can't accelerate neutrinos, but they do it with electrons.

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But electrons for most purposes are like neutrinos that are good electromagnetic interactions,

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but otherwise they are particles that interact with protons without strong interactions.

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And that diagram shows the technical king and are deliberately kept in all this stuff that I'm not going to explain.

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Just to impress on you that there is some hard core physics behind all this.

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You have to do a lot of work to measure. The probability of a neutrino interacting with the so-called protons within the nucleus, you know,

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the quarks and the gluons and their distribution within the nucleus is parameterised by things called proton distribution functions,

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which are these quantities here. And those, in turn are related to the measurements that you can make at the large at the heat machine.

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So what you have here is a measured in those kinematic variables that I had written on the page,

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did the momentum transfer in that interaction and the fraction of the momentum of carried by

385
00:36:32,650 --> 00:36:38,740
the struck button of what range has been investigated by experiments and these experiments,

386
00:36:38,740 --> 00:36:44,770
H1 and Zuse Zuse was the experiment here with people involved as investigated a

387
00:36:44,770 --> 00:36:48,940
much deeper kinematic range than before and found something very surprising.

388
00:36:49,300 --> 00:36:52,540
When you look at the universe at very low volcanics x,

389
00:36:52,540 --> 00:37:00,580
this is the parameter that characterises the fraction of the momentum carried by the struct part on the nucleus is basically made of glue.

390
00:37:00,610 --> 00:37:07,239
This is the glue on structure function which you see here has been divided by 20 in order to bring it down to this scale.

391
00:37:07,240 --> 00:37:12,880
Otherwise it would be up there. The nucleus is basically do this constituent golf that you've heard about,

392
00:37:13,060 --> 00:37:17,500
which make up a proton are entirely unimportant when you look at it in this kinematic region.

393
00:37:17,740 --> 00:37:24,730
But because you can create space in pairs of gluons, it is one of the processes which essentially swamp the nucleus.

394
00:37:24,940 --> 00:37:31,630
So we have to understand how neutrinos interact with that stuff in order to be able to interpret the findings of IceCube.

395
00:37:31,840 --> 00:37:32,890
And we have done that.

396
00:37:33,010 --> 00:37:40,330
We can calculate the cross section for neutrinos to interact with the charge current, for example, as a function of energy with that precision,

397
00:37:40,630 --> 00:37:47,650
by taking the data that I just showed you and extrapolating it to the energies that which are now observing icecube neutrinos,

398
00:37:48,010 --> 00:37:50,319
that acceleration induces some uncertainties.

399
00:37:50,320 --> 00:37:58,630
You have to use the machinery called the dotted group of other parts of the equation, and that uncertainty is reflected in that band there.

400
00:37:58,840 --> 00:38:05,020
And so a lot of hard work has gone into it, a large part of it done by my extraordinarily.

401
00:38:05,020 --> 00:38:11,950
But in order to calculate that quantity and that is now the standard input into all these experiments like IceCube and others,

402
00:38:12,160 --> 00:38:15,580
which are seeking to determine whether cosmic neutrinos exist.

403
00:38:16,030 --> 00:38:19,540
And there is also the possibility that we might be able to turn the table.

404
00:38:19,900 --> 00:38:27,430
Our colleagues who work on the dynamics of quantum thermodynamics tell us that this is what the nucleus

405
00:38:27,430 --> 00:38:32,110
looks like in the terms of the model that we are familiar with in walks rattling down somewhere.

406
00:38:32,380 --> 00:38:38,020
But in this kinematic range, the density of gluons is growing so much that the thing is becoming opaque.

407
00:38:38,020 --> 00:38:42,700
It's like a black disk. You might have phenomena which are non-productive in nature.

408
00:38:42,910 --> 00:38:46,870
You might form a new phase of matter, something called the colour gluon condensate.

409
00:38:47,500 --> 00:38:51,010
How do you test these ideas? Well, let's gather neutrinos of this, guys,

410
00:38:51,010 --> 00:38:56,169
and see whether the predictions differ depending on what theory is the correct theory

411
00:38:56,170 --> 00:39:00,610
of this thing here and here you see that there are clearly differing predictions.

412
00:39:01,000 --> 00:39:05,500
Now, this is an energy of ten to the seven gbps we have measured so far, ten to the six.

413
00:39:05,770 --> 00:39:10,000
But all these models give the same answer. But if you can go up to these energies,

414
00:39:10,210 --> 00:39:20,170
then we can tell the difference between what the physics of low excuse CCDs a field of great interest to people doing one of two dynamics.

415
00:39:20,320 --> 00:39:24,700
And how are you doing that? By measuring neutrinos down at the South Pole under ice.

416
00:39:25,000 --> 00:39:32,620
I really like this this interplay of, you know, very different spheres of activity, which all bear on the same theoretical issues.

417
00:39:33,660 --> 00:39:37,240
But Ice Cube's physics program is broader even than that.

418
00:39:37,260 --> 00:39:41,729
We have already mentioned this part here at the search for extragalactic signals,

419
00:39:41,730 --> 00:39:46,559
point sources, gallery births, etc., etc. We also look for dark matter.

420
00:39:46,560 --> 00:39:55,440
As I mentioned in passing, we look for exotic particles like monopoles which might be caught in the ice because if they are slowly enough moving,

421
00:39:55,440 --> 00:40:00,030
they can get ionised and they can. They might lose energy and get stuck.

422
00:40:00,390 --> 00:40:04,590
We look for neutrino oscillations and not give you much further with this.

423
00:40:04,830 --> 00:40:09,240
But just to mention this dark matter idea, which I alluded to earlier,

424
00:40:09,480 --> 00:40:14,160
the dark matter particles annihilating in the sun can generate neutrinos that we look for.

425
00:40:14,370 --> 00:40:17,880
At least you know where the sun is. So you can just look at the sun all the time.

426
00:40:18,180 --> 00:40:24,240
It just goes above and below the horizon at the South Pole and look to see if you see any high energy neutrinos from it.

427
00:40:24,600 --> 00:40:30,989
So far, the answer is no. And we are therefore able to put a restriction on the rate at which dark matter

428
00:40:30,990 --> 00:40:35,220
particles scatter with ordinary matter in order to be captured in the sun,

429
00:40:35,550 --> 00:40:44,160
which are very competitive with the diet experiments that Philip talked about, that that about Felix talked about earlier.

430
00:40:44,370 --> 00:40:50,249
As you see here, our constraints are way below that. And so far we haven't seen anything.

431
00:40:50,250 --> 00:40:56,940
But who knows very soon. Right. And we can also measure neutrino oscillations because they are coming through the earth.

432
00:40:57,330 --> 00:41:01,080
And these neutrino oscillations have been measured in terrestrial experiments.

433
00:41:01,080 --> 00:41:07,320
But you can see them over the baseline much longer than anybody else, as I again have to skim through this for lack of time.

434
00:41:07,560 --> 00:41:14,730
But you can see that we are able to distinguish between the normal oscillation case, which is this and the oscillations that you find here.

435
00:41:15,000 --> 00:41:21,900
And this allows us to determine the characteristic mass, different square of neutrinos versus their mixing with each other,

436
00:41:21,930 --> 00:41:27,749
one mechanical and the controls that we currently have at this once the red ones here not yet

437
00:41:27,750 --> 00:41:32,070
competitive with the one from the terrestrial experiment minnows which is the flagship bit,

438
00:41:32,340 --> 00:41:41,460
but we are catching up and very soon will have a new extension of IceCube called Penghu, which is the precision IceCube next generation upgrade,

439
00:41:43,260 --> 00:41:48,830
and that promises to be able to do things that nobody can do at a long baseline experiment.

440
00:41:48,840 --> 00:41:55,320
So, you know, we have now got stuck in two fundamental neutrino physics, starting with looking for neutrinos from gamma ray bursts.

441
00:41:55,770 --> 00:41:59,400
And just to end something that I'm particularly interested in.

442
00:41:59,940 --> 00:42:03,690
Neutrinos are produced in this ratio in they're produced to pound decay.

443
00:42:03,690 --> 00:42:07,320
You basically make million neutrinos. No electron neutrinos, no doubt neutrinos.

444
00:42:07,740 --> 00:42:12,510
But as they move through space, they mix with each other and they oscillated to each other.

445
00:42:12,780 --> 00:42:16,050
So after travelling far enough, that issue should be democratic.

446
00:42:16,320 --> 00:42:21,420
One, two, one, two, one. And we can tell the difference between electron beyond and down neutrinos.

447
00:42:21,960 --> 00:42:30,630
So this is what we should see at Earth. But one can imagine lots of processes that violate or disturb the coherence that is

448
00:42:30,630 --> 00:42:34,830
necessary in order for this quantum mechanical oscillations to give you that number.

449
00:42:35,100 --> 00:42:41,520
Right. And they could be something as fanciful as the fact that the neutrinos are coming to us to what looks like empty space,

450
00:42:41,520 --> 00:42:47,070
but especially empty when you look at the blank skin that we heard about from David,

451
00:42:47,310 --> 00:42:50,700
maybe it's the seeding mid-stream of black holes appearing and disappearing.

452
00:42:50,910 --> 00:42:54,300
What happens? Evolution of into a black hole is information lost.

453
00:42:54,510 --> 00:43:00,959
We are touching on some of the most fundamental and discussed questions currently theoretical physics and we have a way to

454
00:43:00,960 --> 00:43:09,270
experimentally probe that by checking whether this neutrino flavour issue is as expected without such decoding influences.

455
00:43:09,510 --> 00:43:14,920
So this is something I'm very excited about. It might take a while to do this, but certainly it's on the cards.

456
00:43:15,450 --> 00:43:17,129
So let me sum up.

457
00:43:17,130 --> 00:43:25,920
We have the first analysis from this amazing experiment which has shown us, I think, the first glimpse of an astrophysical neutrino flux.

458
00:43:26,820 --> 00:43:30,990
And that, of course, as usual, raises a number of questions.

459
00:43:30,990 --> 00:43:34,320
We still don't know any of the details. We don't know the spectrum very well.

460
00:43:34,530 --> 00:43:39,900
The sky distribution is consistent with isotropic. We don't yet know if they're galactic or extragalactic.

461
00:43:40,680 --> 00:43:44,700
We don't know that yet. But we will have twice more data in a couple of months.

462
00:43:45,060 --> 00:43:49,530
We are doing something very interesting to look at moon tracks coming from through the earth

463
00:43:49,770 --> 00:43:55,950
all the way 8000 kilometres across and we are able to do various technical improvements.

464
00:43:56,550 --> 00:44:01,590
So I think it is not farfetched to say that we are really seeing the beginning of a new astronomy,

465
00:44:01,590 --> 00:44:06,000
a new window on the universe where you look not in photons but in neutrinos.

466
00:44:06,540 --> 00:44:13,680
And this in turn interests us because it also provides as possible new proofs of physics beyond the standard model.

467
00:44:13,740 --> 00:44:14,130
Thank you.