STFC-RAL-Pickavance R22: End gets filled with the lightest pieces.
STFC-RAL-Pickavance R22: It's like a pound coin can be split into one pence pieces. You can't split a 1 pence piece into pound coins. But it doesn't mean that one is more fundamental than the other. They're just kind of different packets of energy.
STFC-RAL-Pickavance R22: something. Okay? So we've got ourselves some matter particles. We've also got some forces that act on them.
STFC-RAL-Pickavance R22: There's the the good old photon electromagnetism which acts on acts on every charged particle.
STFC-RAL-Pickavance R22: There's the gluon, which is the strong force binding together. The quarks make protons and neutrons, and then there's the W. And Z. Bosons which are the weak nuclear force which are really important. If you want to have the sunshine, you need to have the weak nuclear force turning protons into neutrons in order that the sun can turn protons into helium nuclei and give us light. So, which is the obvious force missing in this picture.
STFC-RAL-Pickavance R22: Yeah, yeah, there's no gravity. We don't have a theory that sticks together gravity. As soon as you try and make a quantum theory of gravity, you find that you run into infinite numbers of parameters being needed. It's a mess, so we don't really know how to do that. But there is also this, this Higgs boson, which is kind of stuck out here differently because it it's not really a force in the traditional sense. And this is the thing that mostly I'll be talking about today.
STFC-RAL-Pickavance R22: Okay, so we go on a little bit. The oops. I'm forgetting I don't have an active curse in my hand. The the matter particles. We're calling fermions, and the the force particles, bosons.
STFC-RAL-Pickavance R22: and the boson is something with the integer amount of spin and a fermion is something with a half integer amount of spin spin plus a half minus a half. But the idea that spin is a quantized quantity sometimes is a bit should be confusing to everyone. How come you only get quantized versions of spin? If you've got a top. Then, depending on what angle the top's pointing, you'd measure a different angular momentum.
STFC-RAL-Pickavance R22: It's a test. It's the it's the intervening one for a test.
STFC-RAL-Pickavance R22: Hi, give it 7. How many was that?
STFC-RAL-Pickavance R22: Good? Right? We've passed?
STFC-RAL-Pickavance R22: so yeah. So I thought it was worth saying a little word about how we how we know about spin. These guys, stern and Gerlach.
STFC-RAL-Pickavance R22: Well, 1st of all point out quantum mechanics. Almost always you end up with Planck's constant H. Turning up in the equations describing things. Planck's Constant describes how much quantum mechanics is different from classical mechanics. Usually, if you sort of imagine shrinking, Planck's constant down to 0, then any funny quantum effects would go away, almost all not quite all, but in general, so when you're measuring
STFC-RAL-Pickavance R22: the energy of a particle, Heisenberg's uncertainty, principle tells you the uncertainty on the momentum times, the uncertainty on the energies at least times constant over 2 prime.
STFC-RAL-Pickavance R22: Okay, so we're quantizing energies. So we're quantizing uncertainties. Or we're having limits from quantum uncertainties.
STFC-RAL-Pickavance R22: Stone ago, did this interesting experiment to find silver atoms through a variable magnetic field. You see, these pole tips here are very different shapes. So this field is spreading out.
STFC-RAL-Pickavance R22: And what happened if you, if you chuck silver atoms out of an oven through a collimator and through this magnet, you might expect that the magnets, the magnetic field of silver atoms. If they have one, it turns out they do would interact with a variable magnetic field and get pulled either up or down, depending on which way their spin is oriented, and you might expect you get a sort of
STFC-RAL-Pickavance R22: smear, that blur of silver hitting a glass plane on which the silver is being deposited, but they didn't see that they saw 2 dots, and they didn't immediately understand what they were looking at. But what we understand now is that the outer there is one unpaired electron in the silver atom is one spin half available, and if it's oriented along with the direction of the field spin up, you go up, and if you spin down you go down.
STFC-RAL-Pickavance R22: and you never get anything in between. So this is a total mystery. Why? And yet it turns up in a fairly simple apparatus. But so it's there. We have a quantized thing which works in every other way as if it were angular momentum.
STFC-RAL-Pickavance R22: I haven't put it on this slide, but pulled around, tried to write down, trying to get equations describing how particles move where they where he tried to unify quantum mechanics and special relativity, and he ended up kind of having to introduce spin. He wasn't intending to, but at least the way his equations went. Spin just sort of popped out as a property. I don't want to go into that. But
STFC-RAL-Pickavance R22: okay, so these these silver antoms are either either plus h bar over 2 sorry each, plus a half h bar over 2, or minus half time.
STFC-RAL-Pickavance R22: plus a half h over 2 pi, so h over 2 pi is usually called h bar. I couldn't find an H Bar symbol when I was writing this, so I wrote the h over 2 pi, but I want to say, H bar.
STFC-RAL-Pickavance R22: so okay, so we call these spin a half, and you either see a projection of plus or minus a half, or nothing else but good nice port of particles. The force bosons the W. And Z. For example, they have spin one, and depending on how you measure them out of the K, you're either going to get a minus one or 0 or one projection that you will see. The photons are a bit of a special case because they've got no mass and the lights gone off again.
STFC-RAL-Pickavance R22: just like I said it wasn't gonna do okay.
STFC-RAL-Pickavance R22: right? So don't worry about photons.
STFC-RAL-Pickavance R22: So we had some open questions. There's lots of things we don't understand in particle physics. A few of my favorites are on this slide here. One of them is why the universe is full of matter, not antimatter. At Cern we can keep creating matter and antimatter by banging a lot of energy together. E equals, Mc squared tells us we can turn energy into matter, but we always make equal amounts of the 2.
STFC-RAL-Pickavance R22: Gravity I already mentioned. Then there's dark matter and dark energy that we don't understand, but it looks like most of the universe is some particle. We can't work out what it is, or maybe black holes or something. We don't know what it is, and then the universe is. The big band should be slowing down under the mass of all the stuff. But it's actually accelerating. So something is driving the universe apart.
STFC-RAL-Pickavance R22: These are getting well fine. We know nothing about these, really. But then there's mass before we switched on. Lhc, we didn't know where mass came from. Now we're pretty confident that the mechanism explains where mass comes from. But we have no idea why those particular values are chosen.
STFC-RAL-Pickavance R22: I'm not even sure that's a scientific question some people would like if somebody gives me a theory that predicts all the masses of all particles. I'm going. Oh, that's great! I've got an answer. But maybe they just know, anyway, why those values we don't know. And then neutrinos have mass, which is interesting, because when we wrote down the standard model, there's no need to give them a mass term, and it turns out that there's more than one way of giving mass to neutrinos. So we don't actually know how they get it yet. So there's experiments underway trying to
STFC-RAL-Pickavance R22: explore that as well.
STFC-RAL-Pickavance R22: Anyway. Mass, so if you go back to 1960 S.
STFC-RAL-Pickavance R22: We had. We had people had made ideas about about these these gauge symmetries that we are nowadays constructing all our theories out of.
STFC-RAL-Pickavance R22: But the problem with gauge symmetries was that it predicts that photons and gluons, and W. And Z. Bosons should all just pop out into existence, and that was lovely, except that they had to be massless, and we had never seen the W. Or Z. Boson, and we're going well. If it was a massless boson like the W. Charged thing, we would see it, but we haven't. So the only explanation could be. It must be too heavy for us to make in our experiments, and so we were predicting that there would be a heavy charged
STFC-RAL-Pickavance R22: W. Boson, but which we've never seen. But then it doesn't come out of these
STFC-RAL-Pickavance R22: equations because they require to be massless. So a lot of people giving up on these gauge theories. And Peter came along, and he had this idea about how to how to keep this gauge symmetries that define this thing while keeping mass while giving mass to the to the answers.
STFC-RAL-Pickavance R22: And he wrote this this paper in 1964, published in in physical review letters.
STFC-RAL-Pickavance R22: Here's page one broken symmetries and the masses of gauge bosons and
STFC-RAL-Pickavance R22: so yes, he's he's producing a mechanism which will allow a broken symmetry keeps the symmetry, but it breaks it.
STFC-RAL-Pickavance R22: and it's not a long paper. Here's page 2. That's the end of it there.
STFC-RAL-Pickavance R22: and that gets him his Nobel Prize. 40, 44 years later.
STFC-RAL-Pickavance R22: But actually, it's kind of better than that, because
STFC-RAL-Pickavance R22: up in this paragraph here, he says that. Yeah, it's true. I didn't do this in quantum mechanics. I did this using classical equations. But it's okay. Because Braut and Anglia, they've published the quantum mechanical version of this already. So hey? You know I must be right, because these guys were right. So fine.
STFC-RAL-Pickavance R22: What he did do that was unique, though, is he predicted the existence of the Higgs boson. These guys had said, yes, you can give Mass W. And Z. Particles this way. But Peter noticed that there was an additional particle that was produced. And so the Higgs boson gets named after him essentially because he was the 1st to suggest this thing might exist.
STFC-RAL-Pickavance R22: Okay, so what? What did he actually predict in that paper he predicted something really quite vague.
STFC-RAL-Pickavance R22: I don't know how happy you are with differentials, but he said that there be some field.
STFC-RAL-Pickavance R22: a field energy, density, which is a function of the density of this particle. So energy density associated with this field as a function of the density of it. We'll call this the Higgs Field, and there should be some place, some density row 0, at which the differential of the field, with the density is 0. In other words, there's some point where it's flat and the second differential better be better than 0.
STFC-RAL-Pickavance R22: It's going up at either side. In other words, there should be a minimum of it at some point which is not the origin, some row 0, which is not the origin.
STFC-RAL-Pickavance R22: Okay? So he didn't describe the actual shape of it, but he just said, Let it have those 2 parameters, and then he says, Well, that means there must be some minimum here, which is, it's going up from there. So the origin is going to be higher and
STFC-RAL-Pickavance R22: Therefore you will have a if you imagine. So. I've drawn this in terms of row. It's often I should maybe have drawn it in terms of
STFC-RAL-Pickavance R22: Phi, which is row. You could draw it with Phi going negative as well. If you draw it in terms of Phi, you'd see it as a symmetric equation. But the symmetric equation like that you start at 0 with a symmetrical equation. But you know you're going to roll down one side or the other to row, plus or minus row 0, and you end up in the having a solution which is asymmetric
STFC-RAL-Pickavance R22: despite the fact, the equation is symmetric. So that's the trick that Peter uses to preserve the symmetry in theory, but then have spontaneous symmetry breaking which allows him to introduce masses.
STFC-RAL-Pickavance R22: And so nowadays we we write this in the the simplest form you can, which is
STFC-RAL-Pickavance R22: MoD, which uses a term proportional to rho, which is negative. So you start by going down and then term proportional to rho squared, which is positive. So eventually the rho squared term wins and you go up, and if you're being alert you'll notice that I haven't started at 0.
STFC-RAL-Pickavance R22: I put in a plus constant here, which is opposite. But constants do nothing in these energy equations doesn't actually matter what the total levels are. So that is basically the simplest equation you could write down that has this form, but it is exactly the equation you wrote down, and then a 1 and a 2 are parameters. We've had to go and measure in data in order to understand whether this theory is right or not.
STFC-RAL-Pickavance R22: Okay, but it's worth making the connection with good old Einstein, Einstein told us, E equals mc squared. This is an energy density. Here's a density of stuff. The mass you got is proportional to the density. So Einstein's equation is basically equals rho times.
STFC-RAL-Pickavance R22: The density of stuff times the volume of stuff Times, C squared. So here is what Einstein would predict, if you want to have twice as much as something, you need twice as much energy, a nice linear relation. You probably hardly think of that as being so obvious that the energy is proportional to the amount of stuff. But Peter is saying, well, let's propose some new field where, if you've got none of it
STFC-RAL-Pickavance R22: that involves having some energy. You can release that energy by going to a standard density, and then from there you need more energy either to make more of this field or to go back towards 0. You're kind of stuck at a minimum. That's a very weird sort of behavior. You've got a box. You've got no Higgs field in it. You close the box. You come back. Oh, it'll have a standard density in it.
STFC-RAL-Pickavance R22: If you put twice as much in. Close the box. Come back. Oh, it'll understand the density, and it always just settles back to this this row. 0. So it's a it's a very and non-intuitive behavior.
STFC-RAL-Pickavance R22: However, the consequences of this field can be explained usefully with an analogy which is here on this fish tank, a fish tank full of rocks and weeds, and some fish. If we take the rocks and weeds out, we see there are 3 fish, and the fish probably say they're now in an empty tank, but we go. No, no, no, you're in a tank full of water.
STFC-RAL-Pickavance R22: and it's kind of the same for us. We're stuck inside this Higgs field we're dragging around the Higgs field with us. It gives a drag on our particles, and we feel that the mass, each of the particles in your fundamental particles interacts with this Higgs field proportional to its mass. And so it's kind of dragging it around with them.
STFC-RAL-Pickavance R22: if we could manage to turn the Higgs field off or get outside it, or something. Then all the electrons in your arm would become massless. They would want to orbit on infinite radii at infinite speeds, and they fly apart and you fall to bits. So that wouldn't be much fun. But you know, we're just so used to being inside it, we can hardly tell it's here.
STFC-RAL-Pickavance R22: So if you were sitting in a tank of water and wanted to prove it well, if you kick the water hard enough, you'd see some sort of ripple, and the same is true in the Higgs field. If we kick the vacuum hard enough, kick the Higgs field hard enough. We're going to see some sort of ripple or the Higgs boson.
STFC-RAL-Pickavance R22: and that's what the Lxe was built for.
STFC-RAL-Pickavance R22: So we've got a big ring. Sure, everybody knows about it. It's 27 kilometers. I started working on this ring in the 19 eighties, although that was for preparing it for an electron glider. Lh, started up about 2010, 28, roughly 2010. My experiment is this Atlas experiment, which is just near the Cern main site. Here's the Geneva airport. It's some impression of scale. Is my house over there?
STFC-RAL-Pickavance R22: Right
STFC-RAL-Pickavance R22: 4 4 points where the protons are orbiting around underground, and there's 4 points where they collide, and 4 big detectors. But I'm going to concentrate on this one because it's the one I work on.
STFC-RAL-Pickavance R22: So the Lhc is accelerating protons up to 6,800 Gv. It was designed for 7,000. If I want to ask, we can discuss that. But 6,800 is where we are today. What is a Gev. A. Gev is about the energy to make a proton. E equals mc squared one Gb, one proton. You give 2 protons enough energy, 6,800 Gb. And in principle you can make 13,000 protons and antiprotons in colliding the 2 of these things. So that's a lot. It's a lot more 7 times what the previous best
STFC-RAL-Pickavance R22: collider achieved. One electron volt is the energy for an electron
STFC-RAL-Pickavance R22: would get by going from one end to the other of a 1 volt battery. So if we had a billion one volt batteries in a row and it didn't spark. We could get one gev by moving electron from one end to the other. A 1,000 of those towers, and we'd get to a Tev, and we still wouldn't beat the Lhc by a long way.
STFC-RAL-Pickavance R22: The speed is pretty high point 9 9 9 9 8 of the speed of light, the relativistic time dilation associated with going so fast is large. If I'm going at point 9 9 8 of the speed of light, I will get to the nearest star in time for T.
STFC-RAL-Pickavance R22: It's pretty good. However, these things are going around a 27 kilometer ring at the speed of bite. So 11,000 times a second.
STFC-RAL-Pickavance R22: They're having to reverse direction and be going in speed. Light the opposite direction.
STFC-RAL-Pickavance R22: I'm back again.
STFC-RAL-Pickavance R22: So this is this is an incredible problem, trying to get these things to keep turning around.
STFC-RAL-Pickavance R22: What isn't difficult is accelerating them. They're passing the same bits of the ring a million times in a hundred seconds.
STFC-RAL-Pickavance R22: So we can make an accelerating device that gives them
STFC-RAL-Pickavance R22: one G one time. Then we'd be getting to TV. So actually give them more like 5 gb, each time 5 mev each time, and after less than 2 min we can get them up to energy. That's not difficult.
STFC-RAL-Pickavance R22: The difficult bit is bending them. So how do you get that strong thing? How do you get strong? That strong bending? Well, proton is a charged particle. How do you act on a charged particle without actually touching it? Well, electric fields turn out to be not very good, magnets are much better, and a magnet. If you're in a magnetic field, you feel a force at right angles, direction of motion.
STFC-RAL-Pickavance R22: and right angles to the magnetic field. So a vertical magnetic field passing through like this could be bent sideways, and if you can get a strong enough field to give you 10 to 17 gravities bending, then you can go around a 27 kilometer ring, and that means 8.3 Tesla magnets.
STFC-RAL-Pickavance R22: So how do you build an 8.3 Tesla magnet and 27 kilometers of it. You need well, the earth's field is
STFC-RAL-Pickavance R22: rather small in Tesla. Your fridge magnets, maybe 100th of a Tesla. I've got a couple of permanent magnets which give about one Tesla, but the record, as far as I'm aware, is 5 Tesla, for a permanent magnet structure. I don't think there's anything that gets to 8, and so you have to put current in a wire. If you're going to generate fuel, you can't use a permanent magnet.
STFC-RAL-Pickavance R22: And if you think about putting lots and lots of protons in a ring, you kind of fairly, quickly convince yourself you're going to have to have the current a few centimeters away from the protons. You don't want it microns away. If you're a few centimeters away, it's trivial calculations. You need about a million amps to provide the magnetic field with a Tesla. So all you need is a million amps. We're actually doing it with 12,000 amps turning 80 times
STFC-RAL-Pickavance R22: but even 12,000 amps is quite a lot of current, as we like to say. If you put a 1 ohm resistor in there you have 140 mega, 144 megawatt heater, so we can't cope with with a
STFC-RAL-Pickavance R22: a real resistor. If anybody happens to be in Dubna. It's a town a couple of hours drive north of Moscow. There's a accelerator center there where they've got
STFC-RAL-Pickavance R22: about the size of this room no bigger, 50% bigger a cyclotron synchron a little bit like the Nhc. But you know much smaller scale. But the magnets we come 3 quarters away to the roof of this room the wires of the size of my leg twisted around each other. The hole in the middle is, I could walk through and then next to it is the superconducting replacement twice the energy which is about that size.
STFC-RAL-Pickavance R22: using superconductor allows you to make these wires thin and cheap, and there's no losses. 0 losses inside the one at the table. So this, you know, would be possible. At least you'd be possible without superconducting magnets.
STFC-RAL-Pickavance R22: But we have to build 1,232 of them, and they're quite expensive and heavy, and so forth. We cannot talk about them if anybody's interested.
STFC-RAL-Pickavance R22: otherwise I move on to the detector.
STFC-RAL-Pickavance R22: So this is a picture of the Atlas detector. You can see this, this chap leaning on it for a for a bit of scale.
STFC-RAL-Pickavance R22: The the detector has to be very, very big, because when these protons collide debris from those collisions comes out with a decent fraction of the proton energy sometimes, and you have to stop them and measure their energies. So we need large scale detectors.
STFC-RAL-Pickavance R22: But let's go to a a 3D. Model.
STFC-RAL-Pickavance R22: which is perhaps easier to see what's what's going on.
STFC-RAL-Pickavance R22: We've got here again. The people for scale.
STFC-RAL-Pickavance R22: We've got here some magnets which are going to be bending things. We're going to watch to see how particles bend and work out their momentum from their curvature. Inside the magnets. We've got some steel and some lead.
STFC-RAL-Pickavance R22: so steel and lead, which are designed to make the particles stop and give up their energy. But they're instrumented with different sorts of detectors, so that we can measure how much that energy is. So we measure lumps of energy in these detectors, and then inside that in the red section is what we call a tracking detector, which is about a meter in radius, and that's designed to be as light as it possibly can be, and measure the particles. And that's something we built here.
STFC-RAL-Pickavance R22: or part of it here. So it looks a custom way through that. So there's your one meter radius.
STFC-RAL-Pickavance R22: and you see that we've got various layers of different detector going out. These are kind of getting coarser and coarser as you go out. These Fct. Ones I was involved in creation of. They're each about the size of my hand. They would generate about a DVD's worth of data a second if you let them. And there's a lot of them, because we've got to tile lots of barrels to surround this thing. There's 2.5,000 2,700 of these objects.
STFC-RAL-Pickavance R22: But they're they're giving us 4 hits. So we get if a particle comes out from this collision point we see 1, 2, 3, 4 little dots, and we can draw the line through it and see. That's where the thing went, and then these other bits of detector also help to
STFC-RAL-Pickavance R22: help to find the track.
STFC-RAL-Pickavance R22: So if you happen to be in the Ppd Exhibition room at the end of our one. You can see some of these things sitting around in a glass case. Yeah, I've got some very statistics on that I started describing already 6 rehab chips on the front, 6 more on the back. That gives you your Dvds a lot of data per second. Their built precision is a few microns. The precision that they can tell you hit to is about 20 microns, because they're divided into 1,236 strips.
STFC-RAL-Pickavance R22: And so this is a semiconductor semiconductor, is not. It means that the electrons are not very well tied to their atoms. And so, if so it's a silicon silicon center. So if a proton passes through this, it knocks 20,000 electrons off their atoms. We put a few 100 volts between the front and the back, and we drift the electrons up to the to the wires that we've laid out on the surface, and then they go into the readout chips, and they get offered in here for a while until we decide whether we want to look at them or not.
STFC-RAL-Pickavance R22: All right. So we built those.
STFC-RAL-Pickavance R22: Why have we got all these different sorts of bits of detector? I said. There's different layers to the thing.
STFC-RAL-Pickavance R22: And the point really is that when protons collide protons are a bag of quats and gluons that stick them together.
STFC-RAL-Pickavance R22: Lots of stuff can come out quarks and gluons, and sometimes you get other particles like photons, light electrons, neurons, the heavy copy of the electron when these particles come out and they hit the detector, they interact in different ways. If they're charged, they leave a track in that 1st tracker, but if they're neutral like a neutron or a photon, you don't see anything there. But then the 1st layer of the calorimeter stops electrons and photons. It happens they interact quickly.
STFC-RAL-Pickavance R22: whereas protons and neutrons go deeper and muons go right through the detector so we can see which sort of particle we've got by comparing the signatures of the different detectors. And we didn't have any of these things to start with muons, electrons, or photons. So if we see those, then we go. Ha! We made something new here. We didn't just smash a proton and send stuff flying around. We've made something that didn't exist before. So that kind of gives us a
STFC-RAL-Pickavance R22: a tag, and and we can measure the energies. We measure these energies. Well, we can measure these curvatures of these tracks in a magnetic field, well, measure their momentum so we can measure these particles and measure them energies and momentum rather accurately. These are a bit tough.
STFC-RAL-Pickavance R22: Right?
STFC-RAL-Pickavance R22: Oh, and the neutrinos. If you make a neutrino, it just zips through the detector, and you see nothing. But okay, fine.
STFC-RAL-Pickavance R22: So the Lhc started back in 2010
STFC-RAL-Pickavance R22: and the Higgs discovery was in mid 2,012. And sometimes people go. What have you been doing since then? It must be all over.
STFC-RAL-Pickavance R22: It's amazing. There's loads of searches that nobody's yet even got around to. I'm working on something which nobody has tried yet. Who knows? So we are. We are exploring the data, and there's just a lot to do with it.
STFC-RAL-Pickavance R22: This was the sort of data sets the orange and blue that we had with the initial discovery. Sorry?
STFC-RAL-Pickavance R22: Yeah. Sorry. Sorry. The red and orange. So that was the point of discovery. We had that much red and that much orange. We now have 400 inverse. So 20 times what we generated in the 1st runs. And this means we've got a lot of particles.
STFC-RAL-Pickavance R22: The when the protons collide with Lhc, there's lots of possible interactions they can have, and they end up generating billions of W. And Z. Bosons, hundreds of millions of tops, millions of heapses. And so, for all those particles we've seen far more of them at this accelerator than the rest of particle physics ever has put together. So my experiment box is 2 weeks old. This number we published 1,394 papers, looking at all sorts of different properties of these things, and searching for new particles and whatever.
STFC-RAL-Pickavance R22: So it's a very, very varied acceleration. It can do lots.
STFC-RAL-Pickavance R22: This is a collision event from Run 3 run 3 is ongoing. Now we're taking run through data. We're going to run for one more year, and that's the end of it.
STFC-RAL-Pickavance R22: in 1 3 we're colliding the proton beams. I should have said the proton beams about the length of my finger. 2 of them pass through each other. 60 proton pairs get smashed. So what you're looking at there is the debris of 60 proton pairs expanding in the tracking detectors. All these charge tracks have to be sorted out. This is one of the biggest problems at Lhc is trying to run the software that finds these tracks reliably. The CPU cost is a real limit on many, many things we do.
STFC-RAL-Pickavance R22: But look how wonderful the neon detectors are! All these lead and steel filters out nearly all the particles. This neon goes shooting out the outside, and you can just see one neon track at the outer edge before we built this thing decades ago. And people talking about, how can you possibly build a detector like this? There was a proposal just to make this a big ball of steel absorb everything in here and just put some neon detectors around the edge. We're doing much better by instrumenting this, but neons themselves by themselves are pretty good.
STFC-RAL-Pickavance R22: So here's another event which happens to have 2 muons in it, one going up, one going down. You look at this view, and they're going back to back. Well, that's sort of already interesting. That kind of suggests there might be some connection. But if we turn the thing sideways and blow it up. Then we can see lots of proton collisions happening. But these 2 Muons are coming out the same collision, so it seems pretty clear. There's some connection between these 2 Muons.
STFC-RAL-Pickavance R22: What can we? What can we say about it? Well, what we can do is we can calculate. If some object has broken into mu plus mu minus. What would it have weighed? We can calculate the mass from having measured the momentum of these guys.
STFC-RAL-Pickavance R22: So we do that.
STFC-RAL-Pickavance R22: and then we get another event with 2 muons in it, and another event with 2 muons in it, and we keep stacking up events with 2 muons in them. This is actually an ancient plot with very, very little data in it. But even in this ancient plot, with very, very little data in it. You've seen millions of entries in bins here.
STFC-RAL-Pickavance R22: And you're seeing that the mass of those pair of muons could be like any number. It could be 1, 1,100. It's not going to be 20,000, because we've only got 13,000 in the accelerator, and very rarely, actually over 100. But nowadays, with more and more data, we're filling in this region.
STFC-RAL-Pickavance R22: So you can get any number out if the 2 Muons don't have any particular connection. But if, for example, you made a z boson on the Z. Boson had decayed to me, plus v minus you, you'd measure 91. So at 91, we see a spike for z boson going to be plus v minus sitting on top of a background of unconnected stuff.
STFC-RAL-Pickavance R22: and we can look across here like a history of particle physics. If we hadn't, if we'd done this experiment in the 19 fifties you'd be going. Wow! We found the E to the rho, the omega, the 5. It's like the psi prime, the psi. 1, 2, 2. All these things are just sitting there as clear bumps and telling us something is happening that liberates that much energy only. So there's a particle of that mass.
STFC-RAL-Pickavance R22: But those had all been discovered before that wasn't new.
STFC-RAL-Pickavance R22: Here's a few of the measurements we've we've made from Atlas. Each of these points here is measuring something. In this case. I'm looking at the production of a Z with 2 jets. The data point is a square, and just behind it you can maybe just about make out the edge of a gray band, which is the theory prediction.
STFC-RAL-Pickavance R22: So we've got theory predictions for production of Zw, top pairs, Higgs's 2 w's and a Z or W. 3 photons, 4 tops quarks together also, and they all match well, almost all match. If you've got so many plots, you expect something somewhere. It's not going to quite hit the theory prediction. But the agreement is, is very impressive. Thanks to all the amazing number of
STFC-RAL-Pickavance R22: hours, years of work going into all these measurements and all these theory predictions to do this, but we know the standard model is a fantastically good model. And note also the scale on the Y-axis. Here this is the rate at which these things are produced. We've got a rate of total proton collisions, that is, a few times 10 to the 11
STFC-RAL-Pickavance R22: cross section, technically of 10 to the a few times 10 to the 11. And we're measuring processes with rates of 10 to the minus 3. So it's 14 orders of magnitude difference in rate trying to find these guys when they are
STFC-RAL-Pickavance R22: a hundred 1 million times less common than the typical collisions means. You've got to have some very good software and and understanding of what you do.
STFC-RAL-Pickavance R22: Alright time is moving so oh, it's moving very fast.
STFC-RAL-Pickavance R22: So I won't say how we make exclusions.
STFC-RAL-Pickavance R22: You make them, but they almost immediately decay.
STFC-RAL-Pickavance R22: Higgs, bosons decay usually into bee quarks, but they actually have quite a lot of different decay modes that could go into bee quarks are very common when we're trying to find the Higgs. But at Lhc things made of quarks are produced all the time. It was one of the most common things on the previous slide that was even on it. So we don't actually look. We didn't actually discover it through bee quarks. We looked for Higgs going to 2 Z's or Higgs going to 2 photons. It was a tiny little yellow sliver, but it happens to stand out. Well.
STFC-RAL-Pickavance R22: so yes.
STFC-RAL-Pickavance R22: Well, my personal comments actually drop because we've I've run. I've spoken too much.
STFC-RAL-Pickavance R22: and just point out that this is a an example of an event with 4 leptons in it.
STFC-RAL-Pickavance R22: 2 muons heading out to this detector, and 2 green photons caught inside the calorimeter. And so if you look at the energies unrolled and kind of flatten the detector onto a 2 g plane, you see these 4 lumps of energy which is the stuff we're after. They all come from the same point.
STFC-RAL-Pickavance R22: and you can calculate the mass of that. And you get about 125. So this looks like a Higgs turning into 4 Leptons because the Higgs weighs 125. So this was the Higgs discovery slide back in 2012, and we were pointing to the fact that we'd seen.
STFC-RAL-Pickavance R22: if we make the mass of all the events with 4 leptons in them that we've seen, we get this mass spectrum. The dotted points is the data. The red is the background prediction. And here you can see that in this region we expected to get about 5 or 6 events per 10 gb, but here, in this bin we've got 13
STFC-RAL-Pickavance R22: expected 5, or 6, 6, or 13. Would you want to go and tell the world's media that yes, I've made an incredible discovery.
STFC-RAL-Pickavance R22: It was a bit of an interesting discussion what to do with it, but fortunately we didn't just have that. We had the Higgs going to 2 photons, which was the other discovery channel. So here we've got a couple of photons. We can again calculate the mass and make the mass spectrum.
STFC-RAL-Pickavance R22: But
STFC-RAL-Pickavance R22: you know photons are light. If you make a high energy collision, sparks come out. Light is not surprising. So you get rather more background in this particular search.
STFC-RAL-Pickavance R22: and you've got a rather continuous background. But you've also got a little bump at 125. If you subtract off the background, then maybe looks like a convincing bump. I don't know if you take the red lines away. Really, it's very hard to see anything in this plot, but by the time we've put the guidelines on for your eye you can be persuaded. But statistically, this was actually slightly more powerful than the full Lepton, but the 2 together goes exactly to a 5 Sigma discovery. So we claimed, that's it. We've got it
STFC-RAL-Pickavance R22: and Cms. Our competitors have exactly the same. So that made it easy.
STFC-RAL-Pickavance R22: because we knew we we looked a few days before what each other's results were.
STFC-RAL-Pickavance R22: So since then we've been busy trying to analyze this thing. We found it in Zz and Gamma Gamma, but we've now seen Higgs to tau pairs, Higgs to W. Pairs, Higgs to Bb. Pairs. Last week we announced evidence for Higgs to type 2 muons. My competitors have announced that last year earlier, anyway. But at least we caught up with them on that one
STFC-RAL-Pickavance R22: evidence. Not quite clearly. It's definitely happening yet. But
STFC-RAL-Pickavance R22: and we can look at these these plots like the Higgs going to 4 Leptons, and we're no longer looking at a few events in a plot. We're looking at something with a nice clear peak and a clear background well described by the background predictions. And so we can read off the rate. We can also read the mass off and see that we're reading this thing to a precision of 0 point 2 gb. In 1 24. So I'm getting rather precise on the masses. So that's nice. The Higgs to Gamma Gamma Channel.
STFC-RAL-Pickavance R22: It's now a very clear bump. Even if I took the lines away you wouldn't be daging this one. But because there's a lot more events in it. You can break it into lots of different production modes. I don't want to go into what these 30 different measurements are. Note that they're mostly very bad measurements. 0 will be nothing seen at all. One would be standard rate. 2 would be double the standard rate. Most of these measurements are really coarse, but they do agree with the prediction, and some of them are quite more precise. If you average them all, you get quite a precise answer.
STFC-RAL-Pickavance R22: So Higgs, Gamma, Gamma Higgs Center. In fact, we've established quite a lot of interactions with Higgs boson.
STFC-RAL-Pickavance R22: So if we plot the Higgs the particle mass on the X-axis
STFC-RAL-Pickavance R22: and on the Y-axis. We look at the interaction. We cheated a bit because bosons and fermions have slight different interactions.
STFC-RAL-Pickavance R22: What we see, what we've seen is that in each case there is an interaction which is scaled with the mass.
STFC-RAL-Pickavance R22: the top part interacting with most and all the things going in strength. So there's no question that the X is doing what it's supposed to do. It's interacting with a lot of particles proportional to their mass. And it seems to explain where the masses come from we're beginning to see. So at the moment we have good evidence for the W's and Z's, and for the 3rd generation particles. We're now beginning to get the mu one coming into the picture as well with some evidence for it.
STFC-RAL-Pickavance R22: but we don't have any hints of the 1st generation. Electron neutrino meet up and down. Quark!
STFC-RAL-Pickavance R22: Do these get their mass from Higgs? Boson? Well, it seems logical. They probably do. But we've no proof.
STFC-RAL-Pickavance R22: We may never have proof.
STFC-RAL-Pickavance R22: Okay, but actually, the important thing in this whole story is the Higgs field.
STFC-RAL-Pickavance R22: So let's have a have another look at it at the way this thing works.
STFC-RAL-Pickavance R22: So the standard model uses this
STFC-RAL-Pickavance R22: form, V, minus rho plus rho squared with a couple of of constants to fix
STFC-RAL-Pickavance R22: which we fix by measuring the
STFC-RAL-Pickavance R22: by measuring the Higgs boson mass, and by measuring the rate at which which the the the Ion decays essentially or well, MW, mass.
STFC-RAL-Pickavance R22: never mind, we may we fix these things. We measure these things that predicts the shape, and that's the the red line fine. But
STFC-RAL-Pickavance R22: Peter just made this up. There was no logic for saying row plus rho squared. You can perfectly well stick in a row cubed term if you want, provided you make sure that what happens near the minimum looks like the original form.
STFC-RAL-Pickavance R22: What we're doing as we as we test things at the moment, is testing some of the properties near this minimum. But we're not testing any of the properties away from it, so it could turn down either on the right or on the left. If you stick in some different form for the equations, and we would not today know the difference.
STFC-RAL-Pickavance R22: So all of these things meet Peter's original paper, you know, a flat minimum which a flat minimum appearing. So any of these things could be correct. So you know, why are we so convinced that we understand the way this, this field potential works and the whole field model?
STFC-RAL-Pickavance R22: And this matters, because
STFC-RAL-Pickavance R22: actually, the Higgs potential is not quite as simple as I was making it out to be. If you change the temperature which you're analyzing it, if you heat the Higgs field up, you change the shape of the field.
STFC-RAL-Pickavance R22: So, for example, if you go to a really really hot field, then the density doesn't just keep rising forever. Then it starts to turn over and drop down again.
STFC-RAL-Pickavance R22: And that's kind of interesting. Because if we're sitting here in this minima.
STFC-RAL-Pickavance R22: If somehow we manage to increase the Higgs density a lot and push ourselves over this hill and down the other side.
STFC-RAL-Pickavance R22: Well, then, the heat density would increase some more, and release energy and increase, some more, increase, some more increase, some more increase, some more. It goes down forever.
STFC-RAL-Pickavance R22: So you would get a lot of energy being emitted which would heat up the Higgs field around it.
STFC-RAL-Pickavance R22: and then you get a super dense state, expanding at the speed of light, destroying everything it touches.
STFC-RAL-Pickavance R22: and that will be the end of the universe.
STFC-RAL-Pickavance R22: So this is what our equations currently tell us. The universe is doing. We're stuck in this metastable state. One day the Higgs field will find, get kicked and bend over that edge.
STFC-RAL-Pickavance R22: and then the universe will destroy.
STFC-RAL-Pickavance R22: There is some good news, though. One is that you'll never notice. It'll happen at speed of light, so nobody's gonna have to panic. It'll just be over.
STFC-RAL-Pickavance R22: It'll be no indication. It's coming.
STFC-RAL-Pickavance R22: So that's good. Another is the expected lifetime is way more. I think it's like 10 to 50 years. It's huge. It's not going to happen in your lifetime unless you live a very long time.
STFC-RAL-Pickavance R22: But I think more fundamentally
STFC-RAL-Pickavance R22: these details of these curves, these extrapolations. They depend upon us, understanding how the particles work with each other. The top current mass, for example, is very important in this calculation. If there's any change in the equation particle content, or the forces or anything. It's going to change that interaction. And you know, we know we're not complete. We know we don't know what dark matter is, we know how gravity interacts. Almost certainly these equations are not going to actually cause the destruction universe, in my opinion. But this is what our math currently tells us the universe is going to die.
STFC-RAL-Pickavance R22: I just don't believe it myself.
STFC-RAL-Pickavance R22: I just wanted to point out that it could be very good, or it couldn't explain why we're here. Perhaps
STFC-RAL-Pickavance R22: if we do, the other sort of culture we take, we say.
STFC-RAL-Pickavance R22: and this does not come out of just heating Higgs Field. Here you have to go and change the Higgs. You have to go and fiddle with the potential, but it was made up. So I mean, you know, I can make up something just as well as Peter. I can put in a little wobble here. So the universe, when it starts doesn't just roll down to the minimum gets stuck here for a bit
STFC-RAL-Pickavance R22: until somewhere there's a bubble and expansion and the drop into the lower state and the universe kind of collapses into that point. The point about that is that doing so releases a lot of energy, and that energy might be exactly what's needed to help explain why we have a matter antimatter, asymmetry in the universe. Virtually all models I know about for explaining this require some sort of collapse of a field. Maybe it's the Higgs field with a bit of a change to it.
STFC-RAL-Pickavance R22: So I'm very interested in trying to pursue what what this looks like, and perhaps it's going to explain why we have matter, not antimatter.
STFC-RAL-Pickavance R22: We don't know much about it. We know it's incredibly dense. It's nonsense, dense. It's a pint of water is the mass of Mount Everest. The mass should collapse the field. But we don't have a quantum theory of gravity, so we don't know what's going on there. You can maybe dismiss it as an integration. Constant, perhaps, but gravity usually fuels energy so not clear what's happening there.
STFC-RAL-Pickavance R22: And really, what we want to do is we would like to prove that it exists by looking at the way 2 Higgs bosons interact
STFC-RAL-Pickavance R22: because that it's the interaction between Higgs Bosons that is generating this field. So we want to check whether Peter's mad equation was
STFC-RAL-Pickavance R22: was right. So we can make Higgs bosons at Lhc by a diagram where you just make one Higgs boson and then make another separate one. And that's a bit boring, because we've already been studying that.
STFC-RAL-Pickavance R22: But this one would be very interesting making a Higgs that splits into 2. But but we need to find 2 Higgs at once, and we don't even have any proof that 2 Higgs ever get made at once. At this point we're starting to see just the tiniest hints. But
STFC-RAL-Pickavance R22: so phase phase one of Lhc is stopping next year. It's going to deliver more than 300 predicted. But it's over. But we're going to start again in 2030, trying to get 3,000 inverse centi bomb 10 times as much, and that with that we really ought to be able to see pairs of Higgs, Bosons and start to learn something about how they interact and check whether it's possible. But
STFC-RAL-Pickavance R22: but the detector wouldn't cope. The accelerator can't do it. They're both being majorly rebuilt. We're busy rebuilding or building a new tracking detector. This is out sitting outside my cern outside the pit head with above the accelerator. We're just getting ready support structures. We're busy over in our 12 clean room.
STFC-RAL-Pickavance R22: not in our 12 team room just next to our 12 team room, busy making silicon detectors to try to stick onto these things. They're more granular. They're more radiation resistant than the original ones, so that we can cope with a higher density of particles and radiation.
STFC-RAL-Pickavance R22: But we are running out of time
STFC-RAL-Pickavance R22: when they do. Okay, never mind.
STFC-RAL-Pickavance R22: So where are we? We found a Higgs field. This Higgs field is really something different. Written up on the wall just outside, beneath the library is a quotation from Newton, saying, there are, therefore, in nature strong particles of matter and strong interactions between them, and is a business experimental philosophy to explore them.
STFC-RAL-Pickavance R22: And that idea of particles and forces worked from 17 0, 4 to 2012. But now we know we have to embed it in Higgs Field. We're going to get a full understanding. So we've really got 3 things, particles, forces and Higgs Field, which are all different with different behaviors.
STFC-RAL-Pickavance R22: So we're gonna explore the the Higgs field.
STFC-RAL-Pickavance R22: And I'm hoping we're going to make unexpected discoveries along the way. Who knows?
STFC-RAL-Pickavance R22: Thank you.
STFC-RAL-Pickavance R22: So I've spoken too long, but we've still got a few minutes, so if there are any questions, I would be very happy to
STFC-RAL-Pickavance R22: try to answer, Archie, you said giving away this might be work experience students you've seen before
STFC-RAL-Pickavance R22: gluons. So the Higgs interacts with them with particles proportional to their mass.
STFC-RAL-Pickavance R22: so you should not expect it to interact with massless particles and photons and gluons are massless particles. So it's a bit odd that Higgs decays to photons and gluons, but it does it with a loop diagram. It really decays into maybe a pair of quarks, and the quarks annihilate, and either give off 2 photons or 2 gluons as they go.
STFC-RAL-Pickavance R22: And so you see that but at Lhc we will never see. Higster came to gluons. Lhc is basically a gluon collider gluons fly everywhere all the time. We have no idea about. Well, we know how to look for that. We have to build an electron positron collider. If you want to look for that
STFC-RAL-Pickavance R22: excellent, I mean the the basic interaction is proportional to this, this Kappa parameter
STFC-RAL-Pickavance R22: in the field. The cap approach is what's giving the mass to the particle. So what's written up here is the interaction strength of the particle. So the top quad has an interaction strength which is nearly one, and people point to that and go well, that's quite interesting. When you've got interaction strength. It's about one that sort of might mean something. But to me that's just numerology.
STFC-RAL-Pickavance R22: the actual numbers they're just invented. There's nothing that predicts what a top quad or a bottom quark or Trump Park should weigh but once you've said what they weigh, then you've got a direct prediction for what the interaction strength of the Higgs should be, and then it's just a question of going and doing the same sort of calculation I would have done for scattering 2 electrons off each other, or something, I put in an interaction, strength and outcome the different decay rates.
STFC-RAL-Pickavance R22: Don't look at the Warwick exam from
STFC-RAL-Pickavance R22: where we forgot that there's a factor 3 for decaying into colored objects
STFC-RAL-Pickavance R22: when we set the question. So the quarks come in red, green, and blue versions. So you have to triple the rates for quarks. There's 3 copies of each.
STFC-RAL-Pickavance R22: Okay, I think everyone's needing a break. So thanks everybody. And
STFC-RAL-Pickavance R22: enjoy the rest of your time here.