Ri Discourse: Jon Butterworth - High energy physics at the LHC

[Music] [Applause] [Applause] thank you very much this is the view that I get um twice a week um it's the CERN Chadron collider um they haven't actually painted this yellow thing on the ground but the um but this is mon Blanc um this is Geneva airport which is usually where I'm headed away from or to at the time Lake Geneva and um this is my experiment the atas experiment so I'm going to try and I mean it's very easy I'm assuming since you're all here you're at least mildly interested in the LC and I've probably heard of it um but I'm going to it's and it's very easy with the LC to go to do the wow GW um lots of super conducting magnets lots of money lots of people um and lots of and and it's very hard sometimes to connect that with the the actual science that made all these people really put that much effort into doing it and exactly how they do it now I can't cover all that in obviously in an hour but I'm really going to try and rather than wow you with how amazing it is and it is amazing but I I'm I'm going to actually try and explain some of it but before I do I do want to say something about why the thing is so big and and why you know why we need to do go to such great lens to do the science that I'm going to describe to you so um to set the scene then this is the CERN lab it was um set up in the 50s um I've I've been told this is not firsthand evidence that um that it was set up to kind of revitalize European science when when a lot of European science have gone East and the rest have gone west um and it's done quite well it's the one place in the world now where you can really do this energy Frontier physics and that hasn't been an easy path that's been a a lot of commitment from successive um European governments many successive European governments what we're doing at the moment is this is a 27 km tunnel underground which is full of superconducting cryogenics and um and very very high power magnets um and it's accelerating protons um in in opposite directions and colliding them head on in two um well in in four places actually but in two that I'm going to talk about tonight so the the atlas experiment here and the CMS experiment here are essentially if you like you go to all this effort to collide proton head on they are the cameras that tell you what happens when you Collide them um this is actually about coincidentally the it's the tunnel itself is the diameter of about the northern line but the the the diameter of the of the ring so that's the diameter of the ring is more like the circle line coincidentally UCL built a big chunk of this and Imperial built a big chunk of this so this is you can see that kensington's a little depopulated whereas Bloomsbury is where it's all at um so there are also two other experiments there's a lot more science you can do with a facility like this um Alice and lhcb I'm not going to talk about them all tonight but I just want to let you know that I'm going to obviously necessar necessarily focus on the bits of science I'm doing at this machine but it's a fairly general purpose facility facility there's a lot you can do and all around here is the infrastructure of what what goes into injecting these protons into the thing in the first place um this dotted line here is the border between Switzerland and France so you can see the Swiss did a smart thing they claimed atas and the main kit and then they made all the rest in France which is good pleased the French French Farmers no end um and one of the question you can't do a lecture in this theater um I think without um without doing a demo so given that this is such a high-tech experiment um one of the biggest in the world I thought I'd have a really high-tech demo so so why is why is the LHC so big well the thing that limits the energy that we can get the protons up to is the magnetic equivalent of the strength of this string so if I didn't know what I was doing this string might break and hit one of and the bow would hit one of you or me um centripedal Force it's not rocket science right but we the important the difficult thing with this is not um it's not was that a rocket scientist in the audience I'm sorry about that they the um the the uh or maybe it was just a cliche Hunter um the the point is actually surprisingly with the LHC it's not getting the protons up to the energy that's a tricky thing it's actually making them come around again and hit each other so the reason you need the big circumference and the reason you need very powerful magnets is actually to bend them it's Newton right it will they will go in a straight line they're very keen on going on a straight line when they're traveling at the speed with 99 some large number of nines a speed of light um so in order to the the trick is actually keeping this pencil like beam but pencil is not it's a human hair actually keeping it in this tunnel and making sure it hits itself head on when it comes back I mean people are chuffed when they talk about the the channel tunnel meeting in the middle imagine what it's like trying to make two human hairs 27 km me in the middle and Collide um and that and the the real technological limitation and thing that's driven a lot of technological development in this accelerator is actually getting the the powerful bending magnets up there okay so but I don't do accelerat physics I look at the results of it so that's all I'm going to say about that for now um the stage on which this is being played I hope is um yes excellent this is that working now no it's not just give me one second it would be easier if this works in the end um is this so this is the standard model of particle physics as we know it um or most of it um which in our arrogance we would say is most of physics as we know it in fact um and that it's most it's not all um so what we have here is is the building blocks in nature if you like these are the things that as far as we know are not made of anything else they are just what they are um this is the electron the nutrino that goes with it neutrinos aren't going to feature much in this talk but you know that they're quite media friendly these days um up and down quarks are what makes up the proton and the neutron so they um this this is most of matter is made up of this and then on the green side here these bons these are the things that carry forces so we know there are fundamental forces in nature there's the electromagnetic force which is responsible for these bright lights shining in my face um there's the uh which is the photon carried by the photon there's a strong nuclear force which is the reason the up and the down Quark stick inside the proton and the reason that protons neutrons stick inside Atomic nuclei and then there's the weak Force which I'm going to talk quite a lot about here which is in a way the least obvious force in nature but in fact it's the one that makes the sun work for instance it's involved in beta Decay um it can turn um it's the only one that can flip things between these Generations in fact it can turn up and down quals into electrons and neutrinos so those those are as far as we know the fundamental forces of nature these are the fundamental particles for some reason these are repeated three times um I'm going to spend most of my time talking about Electro symmetry breaking as it says in the title um so I'm not going to talk about why these are repeated three times lhcb another experiment on on the LHC is doing some work on that I would not like you we're particle physics is quite good at at making big questions that we have to address and it's correct that we do but one should remember that they're not the only questions and and whatever we find about the higs is not going to tell us why there are three generations here although the higs is if it's there responsible for Mass so it does tell us the difference between these or how that happens because these are just heavier they seem like slightly heavier copies of the previous ones so that's the standard model of particle physics now I said that was everything all the fundamental stuff though I'm sure many of you have spotted the absence of gravity which would be a bit of a shame if we didn't have gravity it's probably the first one we discovered and it's the last one we've managed to explain and it doesn't feature in the standard model and it's very unlikely the LHC will tell us much about it although it may it's a long shot um and the other things that's missing here is actually the higs B on and I'm going to which is not as obvious as gravity I grant you but I'm going to try and explain to you why that that's important so this is the Atlas detector this is one of those big um I said digital cameras is the best analogy they're there to just record every you go to all this effort to make these proton scalid you really want to know what happens when they do and these machines are there to record everything that happens when they do so um what can I tell you about the atlas it's big these are European standard sized people here and there um it's um it's got It's kind of I guess the if I had to say what is the Atlas detect it's a cylindrical onion in in several ways if you see what happens here is the um beams come in here and they Collide in the middle and you'll notice is this kind of structure different Technologies surrounding the beam there are layers of this of of this onion of a detector and they Collide in the middle the beams Collide in the middle here and you've got successively different Technologies doing different jobs so right in the middle here there's um uh tracking detectors so they're they're semiconductors and we like semiconductors because as the name implies they're nearly conductors so there are a lot of electrons in there in the conduction band that it doesn't take much energy to excite them and to produce um an electron and a hole and then if you've got a voltage over that you can read out that where they were created and then you can work out that something went by and that something happened so and it's very nice we used to use things on my previous experiment we had um ionization CH drift chamber which relied on similar principle but it relied on electrons hit on charge particles hitting atoms and ionizing them and then reading them out trouble to make an ion takes a lot of energy and it would perturb the particle so you'd measure it but you'd have perturbed it quite a lot with this you get a lot of electron whole pairs very cheaply with very little energy so it's a really nice compromise it moves us along on the the kind of measuring it without disturbing it kind of path so we get very precise measurements here and that's very important because some of the particles we produce Decay within 100 microns of being produced and if you can extrapolate the tracks that from the Decay back into the beam line and pinpoint that vertex it gives you a lot more clues as to what's actually going on in these collisions when we when they Collide so that's there and in addition to that we have around it a solenoidal magnetic field very strong um which will which will bends the part bends charge particles and so of course the high energy ones are straight the lower energy ones are bent and we can tell them mum from that um which is all great except that we know nothing about neutral particles at this stage and there are a lot of those produced so Pi zeros for instance are produced usually abundantly in these collisions Pi Z is Quark antiquark pair little hron decays the photons it has no charge um doesn't register at all in any of this and um and there are other other culprits like that so around that we have the thing which is a called a calorimeter and again as the name suggests calorimeter calories it measures energy um it's basically very dense it's mostly liquid argon and it stops everything and as as the things stop they slow down and they radiate photons as they slow down count the number of photons you know the amount of energy that went in so that measures even the neutral particles even photons Pi zos neutrons will get stopped in that um and that's I've basically explained to you most of the atlas detector and I've only talked about this bit this these big wheels and all this blue stuff and these These are these are toroidal magnets around the edge are all for one kind of particle which is the muon which is this one the heavy version of the electron but the muan is really special and it's worth putting all that effort in so what happens with the muan is because it's a charge particle so we do measure it here but um it it's a heavy version of the electron it doesn't feel the strong force so it doesn't really interact with the nucleus in the atoms it's also very heavy so it punches through the electrons in in the liquid argon from but like as other candy floss it just goes straight through so it it's but also they're quite rare they're often a sign that something interesting happen if you create a muon in one of your collisions then that's a sign that something they don't appear much in nature they appear in cosmic rays and things but they they don't appear much in nature they're not in your average Collision so we want to know when they happen so that's what all this effort is about and really it's just they punch through the calorimeter they're just more tracking detectors done a bit more cheaply because we can't throw all the technology that we have here on such a big space to cover for them but they measure the momentum of the muons very precisely so that's that and the one thing that we miss in all that which we can't really do anything about is neutrinos so if we if you have um billions of neutrinos and you build an enormous detector under a mountain somewhere you'll see some of them but we've going to produce them one at a time we have just no chance of seeing them so the only way we can see the nutrio that's why we have to surround the Collision point because we can only tell if we create the nutrino by the fact that we buy momentum conservation so if we see a load of stuff going in One Direction and it's not apparently balanced then there's probably a nutrino went there because it could be something else it could be D matter it could be something new but we we kind of know how many neutrinos to expect and we look for anomalies there that sounds a bit flaky maybe but that's actually why po um propose them in the first place beta Decay is a similar situation where you you have um neutrinos produced and you can see something leaves the the the Decay um but you can't see the thing and that's why he post he postulated them that's how we detect their their presence even though we don't actually see them directly but you know neutrinos clearly have been seen since and they exist so we know and we know how many of them there should be and we can look for if there's more of them apparently and then it could be something else so that's Atlas and that's what Atlas looked like before it was uh before it was filled these are the these these orange stripy things are those pidal magnets on the outside this is some guy who helped build it um who I don't know this is from about 2005 live I realized that the LHC was a bit special when I saw this in the center fold of L of FHM apparently not not that I buy FHM but someone said hey have you seen FHM anyway that was in there and and it's you know it's great that people are interested and it's great that it gets these places even if it does look like a James Bond villain's Lair sometimes but there we are this is um this is a collision event so I show this for a few reasons it's quite a boring event in many ways um but it gives you an idea of what's going on first I say that this event display was written by UCL in particular by Nicos constantinus is in the audience somewhere there he is good and and his and his students and post dos so that's UCL contribution but the the point of showing this picture really is that um well first of all you see the components in action that I was showing you before so you see the tracks here these are these are it's a digital representation of what we've seen of those semiconductor readouts right um so the extrapolate back to Collision vertex this is the calorimeter around the outside this is those big muon chain usually compressed to fit them into the picture to be honest there's nothing very interesting happening in this event in fact it's a sneaky event it's the first Collision event that we actually got the highest energy in the world so this 2.3 36 T that's 2 point that's 2,360 billion electron volts of energy in the collision and an electron volt is the energy kinetic energy in electron picks up um by being accelerated through 1 volt of potential so 12vt battery gives you 12 electron volts this is this is two 2,360 billion electron volts there was a machine in the tatron in uh Chicago which was the highest energy machine in the world was running just under two TV and this was the moment we overtook it it's not where we were supposed to be we' gone higher than that now but we were quite pleased to overtake it what's even more funny is that we weren't supposed to have this event the LC were doing some tests and they didn't tell us they were doing this but we guessed and uh some of our students and and some of the people involved in it were in there and the detector was turned on we were desperately looking at these events you can tell it's a it's a cheat because I said there was a magnetic field and none of those tracks are bent we didn't turn the magnetic field on even and we weren't ready really but we thought well I'll just take an event anyway so that was it that was the first event this is much more typical of what we see now this is at 7 TV which is a factor of five bigger than what we saw before um what's happening is that the the beams are colliding in the plane in this represents there are three pictures of the same event um in this picture the beams are colliding in the plane of the board in the plane of the screen and and you see this stuff throwing off which I'll explain in a minute in this picture the the beam is along here and the one proton comes in there and in there and the Collide and stuff flies out and here it's like if you kind of split the cylinder and rolled it laid it flat and these towers here are the amount of energy in these clumps that you see these yellow blobs in the calorimeters that are measuring so it's a way of know us getting some idea of what's going on what you're seeing here is that we've coll Ed two protons together now we we don't really Collide protons at the LHC because the energies are so high the proton is such an enormous thing on the scale of things you can see at our energies that really we're colliding gluons and quarks together the gluon the proton is made up of gluons and quarks and we Collide gluons and quarks together and they hit each other and they bounce off and they head off and and you would expect to see them maybe gluons and quarks don't actually um appear in the detector and this is because the strong nuclear force is a force electromagnetism as you pull two opposite charges away the force goes down gets weaker in the strong nuclear force it's more like an elastic band they they the force gets stronger as you pull them apart so as very inside the proton they behave like the free until they get to the edge of the proton kind of thing and and as they as they fly away from each other there's a huge potential energy builds up between them and as that happens then it becomes at some point energetically more favorable to create more quarks and to have instead of quarks free to have two hadrons flying away from each other instead there's an equation that explains that I'll show it in a minute but the you just imagine it's huge potential energy in the elastic band and suddenly the band snaps if you like and the quarks and gluons are the ends of the band so what you end up with is a spray of these hyrons made of quarks and gluons and that's what all these tracks are here there's a whole but but the but the direction that the average energy of these things goes in is the direction that the original Quark or the gluon went in and these are called Jets we we reconstruct them that way so that's what we're doing at the moment we can produce other things other than just scatter quars and gluons and I'll say something about those in a minute but this is a kind of typical is interesting event that we see and in fact we're now colliding many protons and together at the same time so these are all these are n different proton I don't know about nine in this event different sets of protons collided within it's all happening on a kind of 50 nond time scale we've got to try and unpick what happened in all this 50 NCS is interesting light speed of light is about 1 foot per nanc right if I'm allowed to use feet as a unit um they they uh so 50 NCS the experiment is bigger than 50 ft right so we've got one event coming while the other one's still leaving the event leaving it or and that's even in the bunch Crossings but in within every bunch Crossing we've got many events as well so there's a lot of this is why the detectors have to be very precise and where the Computing challenges are just ridiculous to actually work out what goes on in these collisions but what happened in this one is basically these are all boring and this one's interesting because it's got two muons in it this is actually a zed boson one of the Carri of the weak force that was produced in that Collision indicator to muons and this is why we put a lot of effort into these muon detectors because that's how we pick out this kind of stuff from this kind of stuff so we we were interested in everything at some level but we have a lot of examples of these ones and very much fewer of these so we're we're interested in in those kind of events and a lot of the art of this stuff is actually going through this mass of data that the lxc can produce and finding out what you can learn from it of course it's science that's what you do but we have it coming in at kind of um nine events every 50 NCS or something we have to kind of do some of that online and make sure that by doing it in our Electronics we're not missing something that we would wish we'd saved the last if you build your your trigger your it's called the trigger this thing that makes sure you save the right events if you build your um your trigger electronics and everything perfectly so you only save what you want to save if you do it wrong you will only ever see what you expected to see and that's there's no point doing the experiment in that case so you have to keep your eyes open these are our electronic eyes okay I said I'd show an equation that explain that quack thing this is it um so we have a lot of energy in the um in the uh in in this potential between the quarks that that allows us to create new quarks according to eals mc² if you have enough e then even though C squ is a big number you can make new stuff and that tells you also why it's one of the reasons I'm going to go that through three reasons why it's interesting doing energy frony of physics in general so one of them is this that if there's new stuff out there there are new fundamental degrees of freedom in nature there are new particles for instance the higs bone but maybe other stuff if you want to see them you have this potential wel to climb you have to get you have to create them so if you if you want to create a new particle with some Mass you need c^ s times that mass energy in order to create it and see it so at some level we're just exploring right we we know that the energy that the Universe was has very high energy areas in it we know that the energy the the physics of this high energy impacts on our on our understanding of the universe we like to this is just a way of looking we're kind of looking under Stones if you like to see what's there this is a frontier of nature there are other Frontiers but this is one of them for sure okay so that's one reason why we want High energies another reason why we want High energies I think this is where you get my bad cartoon yes excellent um so I keep I keep thinking I might upgrade this but I never do um the the way I prefer actually than the equals mc² is to think about this this is my preferred justification for the kind of science I'm doing is that we want to study nature at the shortest distances we want to look and look this is a long history of looking you know down from from matter to see what it's made of to atoms to see what's inside the atom see what's inside the nucleus to see what's inside the proton and the neutron and and it's been a very fruitful um field of endeavor and in order to look at very short distances you need very short wavelengths so this is why you know if we this is why if you like we one of the reasons we see in the optical wavelengths at hundreds of nomers because if we used for instance R radar to see then it would be useless radar is great for seeing a ship or a plane because it's got wavelengths of a meter but if you want to see a doorway or your friend then it's not going to be much use um Optical frequencies are much better for that and they can go a lot more fine than that if you want to start studying surface science and really looking at the details or looking at proteins or something you need things like x-rays or electron microscopes or something because they've got shorter wavelengths we don't even want to do that we want to look right inside the proton we want it maybe even look inside the quark and see what's going on so we need the shortest wavelength ever produced anywhere in any experiment um what the trouble so that's fine but the trouble I always get here is without doing quantum mechanics how do you know that wavelength has got anything to do with energy and the best analogy I've so those of you who are physicists or chemists will will know that blue light is higher energy phot blue photons are higher energy than red photons because they have a have a shorter wavelength but the classical analogy is I always pick is the is the double bass string so if you have a double bass string um or a bass guitar string charie is that all right that's my bass player over there Basse guitar a bass string right big fat string and you um you pluck a note then of course you get a deep note it's quite easy to make a noise with it if you half the string length you get an octave higher and and it's a little actually a little harder to to get a note out of it if you imagine taking a big fat bass string making it say 2 cm long and trying to excite the vibration in that make a note you'd have to hit it extremely hard right you have to put a lot more energy in to excite any kind of vibration in that string that you could ever hear that's a classical analogy but it actually is what it's a very good analogy for what's going on in quantum mechanics that that to excite short wavelength things you need huge amounts of energy so that's what we're trying to that's why we need the energy we want to study nature we want to look inside the Quark see what's going on there the third justification which is probably my least favorite because it's kind of historical but given where I am I should go with it it's um the it's the energy density of the universe you hear the lxc described as the Big Bang machine which was a bit embarrassing when it broke the first when we but um but but it you also see that you know we we you hear that we're recreating the conditions at the first just after the big bang and stuff well of course we're not really right I mean the the universe had all the energy in one place then and we don't have all the energy in Geneva believe me you would know that if you've been out in Geneva um but the the um what we do know is that this is this is the physicist Universe right this is a spherical cow or whatever this is the universe now um represented in less dimensions of course um and we know it's expanding we know there's a certain amount of energy in it so we know that in the past it was smaller and there was the same amount of energy in it so as you go backwards everything is getting more energy more dense the energy is getting more dense energy density is basically temperature everything's moving faster all the particles in the universe are moving faster they're bouncing off each other faster um and so there's this really high energy density at some point things are bouncing off each other so fast atoms are bouncing off each other so fast they cannot remain atoms they ionize it becomes a plasma the electrons are just knocked off in just your average Collision that's fine we can do that in Labs we know that happens that must have happen then at some point even the nucleus doesn't stick together protons and neutrons can't hang together because they're getting hit off each other so much and at some point even protons and neutrons don't hang together and you have quarks and gluons all the time and you go back and back and you say do we understand the physics of this stage of this stage of this stage at some point there was there will be a point in this regression back to the big Bank where every particle every quo gluon which is what would be around then would be colliding at the typical energy the typical energy of a collision would be the typical energy of a collision at the LHC so we're taking a very few very lucky quarks and gluons and banging them together with the energies that every single quark and gluon would have been experiencing in the a few Mill micro but 10 to the minus 34 actually I I think after the big bang in the end um second after the big bang so well it's between 10 to- 34 and 10- 12 I think we got a little paranoid about this because I don't know I was on Horizon lately and I got the age of the dinosaurs Wrong by three orders of magnitude so I now I I really don't I really don't want to get the age of the W and the Zed both on Wrong by the same amount but it's something like it was between 10us 34 and 10us 18 I think is is kind of where we're probing at the LHC so we in a sense there is some truth in this that we are actually looking at the physics which is at the moment unusual though fundamental physics but it was the only physics at that time after the big bank and it it's clearly has something to say about how the universe evolved to where it is now and this is the picture I got just to remind me so the 10us 34 you can see here this is us apparently um this is where the atoms break up this is where the quarks and gluons break the the protons and neutrons breakup and this bit where the W's and Zeds appear is where the LHC is is probing beyond that the cosmologists think they can say something about some of it no they don't actually the cosmologists the cosmic microwave background is about here and we don't really understand the physics back of that although there are cosmological theories that that can that that at least have some plausible explanation but this is where we're actually doing doing the science now um so yeah 10^ Theus 34 10- 10 I was I was okay three orders of magnitude between friends anyway when you're talking about 10 to Theus 34 okay so that's why we built the thing that's why we went to all this effort I'm now going to try and explain um and I've also told you something about how we detect what went on right and in the you know semiconductors and the magnetic fields and things calorimeter I want to now try and tell you the kind of plots that we're we're pouring over every day I'm trying to see if we can see something in them and what it means when we say we see something in these pots so to do that I start with a fan diagram this is actually a Sakely disguised equation um because F and diagrams are absolutely beautiful in that they um they map on completely to the quantum field Theory you get them much easier to understand they're so easy to understand that actually a lot of physicists only understand the world in terms of Fame and diagrams and they sometimes take that a little bit too far but um but I also I get told I should I meant to do this I got told off today when I showed this to some students I should draw my time axis on this time is going in this direction so what you have here is an anti- electron a positron um meeting an electron they anate there's a photon produced and then they Decay again to the same thing again okay this is what actually was going on in the tunnel that the lxc is in now but the um there there was a machine called LEP in there which is colliding electrons and anti- electrons um before that shut down in 2000 in the year 2000 so we could build the LHC so I'm going to talk a little bit about what we learned from that but one of the things that's interesting in this is if you remember that equation yes that equation equals mc^2 and then remember energy con obervation and there's a problem here so this the energy has to be conserved so the energy here has to be the same as the energy here has to be the same as the energy here okay now we collided these electron this electron and positron at horrendous energies 90 GV it was when La turned on um and uh so the 90 GV here 90 GV here 90 GV here there's a problem the photon has no Mass so how can the energy be 90 GV if m is zero equals mc^2 if this is zero what's going on so the the concept you have to that we have here is this concept of virtual particles and this is really this is kind of technical concept but important concept that I want to get across so we said the forces are carried by particles okay they're carried by the exchange of these particles is what mediates forces um the the the thing that is though in quantum mechanics all you ever measure it's like young slit experiment or something you you can't measure which slit the thing went through without destroying the effect all you can measure is where you started from where you ended up and everything in between is some just a way of calculating what what might have happened and these particle this photon is a way of calculating what might have happened this is actually what I mean when I say physicists sometimes take these things too seriously because there often no no physically well- defined way of saying did this fan diagram occur or this one often they both might have done its quantum mechanics you don't know all you all you know is if you put all the right ones together you'll get the right answer so these are virtual particles the forces are carried by them they don't have to have the correct mass is the thing they they they that Photon does not have to have zero Mass so and it it this is a way of you can derive this from Quantum field Theory you can actually derive Heisenberg's uncertainty principle from principle from Quantum field Theory but it's good to think of this as a manifestation of that if you like and that this thing is is more likely to happen if that Photon can have the correct mass and as it moves away from the correct Mass it becomes increasingly less likely to happen or decreasingly likely to happen and there's the the formula that governs that is actually this it's not that tricky a formula this q^ s the q^ S there is the um the the mass it has to have in order to conserve energy with via eal mc^2 the m is the mass it really ought to have if it's a proper Photon which is zero actually in this case but I'm going to show the ones are not zero in a minute so and and this is one over that so when those are close together this the way I've ritten it would be Infinity now it's not quite Infinity but you see a huge spike it gets very large um and if you move as you move away from it the probability of that happening drops so for the photon in this case the energy is 91 GV this is one of those collisions at Le um the mass what does it have to be well it has to be 91 GV over c^2 and why so why do we do that well the reason we did that is there's another particle there there a z BOS on which is one of the these characters of the weak Force which has a mass of 91 GV over c^2 and so what you should see if you if you say tune up if you Collide en electrons and positons at higher and higher and higher energies you should see the probability of them of this happening of them colliding and annihilating like that and having something in between falling As you move away from the photon but then as you get near the Zed that one of mq^ s - m^2 would start getting bigger again for the Zed and we would see it go up again and that's indeed what you see this is the data these are the data um this is the total cross-section which if you're not familiar with cross-section you can just think of the number of times it happens or the the pro the event rate the probability if you like of it happening and you see this is this is the mass of the particle that has to be in the middle the center of mass energy of the thing and as you move away from the photon which is a zero Mass it drops and drops and drops and drops and then it rises up again and this is 90 GV this is where the Zed is this bump is that 1 over q^ s - m s bump it's not quite infinite as I said there are other terms that come in but it's very large it's really you wouldn't miss this right this is not so esoteric I have to say I I got I'll show you in a minute I I just find this remarkable it's such an unnatural looking shape for something to happen like that but it does and and you see also there are things like so this is when it decays to um hadrons to quarks this is when it decays to muons same thing this curve that doesn't have the bump in is when the E plus minus give you two photons and the Zed doesn't have any charge so it doesn't couple to photons there's no diagram you can draw that has a two photons and a zed in it like that so there's no bump so we really know what's going on there we can also see it at Atlas so this is data from the LHC now and this bump here is the same bump there's several things to notice here first of all I said we're really colliding quarks and gluons and you may be surprised to know that the proton has antiquarks in it as well it does it's got three if you it's most basic it's three quarks but they're swapping gluons in quantum mechanics they're swapping gluons between each other all the time and they're are quar antiquite pairs in there so you can collide an quarks and quarks at the LHC they can anate they can give you a z or a photon and it can decate to e plus e minus and this is one of those um a measurement of that this is the Zed bump and one of the things to notice here excuse me one of the things to notice here is that the energy reach is much much higher than this right this kind of dies away about the Z the later experiments went a little higher but this goes really high and this is one of those plots we're frantically looking at all the time to see if another bump shows up if we got another Zed or we got something else going on so far not these are some people's dreams as to what might happen they're always just a bit beyond where the data stops but you know it's the way it goes um but we this is an example of kind of this scan above high energ is what you're looking for when you might see something new when we say we're looking for physics beyond the standard model we don't expect any bumps in this but you know we'd love to see one so back to the standard model then this is the the DI the thing I showed before the kind of little zoo of particles is up and down Quirk the electron the nutrino and the heavier partners and then the force carriers neglecting gravity so the whole point of what I'm going to try and tell you now is why not just looking above in that of course it will be interesting to look at new phys look for new physics higher energies than anyone's ever looked before we're looking at the universe around us so carefully so so you know we're breaking new ground it's a frontier of knowledge of course it will but is it really really worth all this eort all the you know these scientists 6,000 of them whatever could have been doing other science is this why is this the most important question to ask what I want to try and tell you now is that this is there's a really special we're not just looking into new territory we know there's a question that is really important that we will be able to answer now those of you who are researchers know that that is a really precious thing a well-framed question that you know you can get an answer to not a lot of good science I mean a lot of good science doesn't come that way a lot of good science is serendipitous that you did something you didn't think was going to be interesting it turned out it was the most interesting thing you ever did but if you're going to invest this much of your career and this much money you want to know at least you've Advanced knowledge at the end of it you can't just do it on spec and I'm going to try and explain to you why the energy reach of the lxc is really special in in that sense now so and it's connected with these guys with the photon and the Zed and the W now they're they're the two forces the weak force and the electromagnetic force and this is a another one of these scattering cross-section plots like the the Le one that I showed you with the bump in this comes from an experiment in Hamburg that I did my uh PhD on in fact and what this experiment did was collided protons and electrons together um which is they don't annihilate they're completely different particles it's almost like a big electron microscope studying the inside of the proton and it did a lot of that it really taught us a lot about how one of the reasons we can do science at the LHC with confidence now is that we understand how quarks and gluons are distributed inside the proton so what what you have here You' got a blue line and the red line um ignore all this stuff down here for now just look at these what what on the axis here is the rate the probability the event rate if you like of what happens and it's a logarithmic scale so it's different from the one before it's really going up very quickly and everyday energies are down here and this is what happens when you you bounce the um an electron off a proton and it bounces off and you see an electron bounce off which is kind of what you'd expect okay and everyday energies that happens a lot that's the dominant thing as you go up in energy it drops and the reason for that we know now because it's because the photon you exchange has to carry a huge amount of energy and as it carries more energy its mass has to go further and further away from where its mass ought to be so it becomes more and more virtual less and less likely to happen and it drops on a logarithmic scale it drops very quickly okay the the interesting thing or the the other interesting thing on this plot though is this red line the red line is the weak force and that's when you bounce a proton an an electron off a proton and you don't get an electron back you don't see an electron bounce off what you see or don't see rather is a neutrino so what's happened there is that instead of swapping a photon in this diagram you swapped a w boson which is one of the weak Force carriers and the W boson is charged so it carries the charge of the electron it turns the electron into a neutrino now we don't actually see the the neutrino as I explained on Atlas but we can tell that it was there because we see the Quark that that the electron bounced off fly off in the other direction and we we know something must have gone in this other way otherwise we've given up on momentum conservation so as I said po decided that it was neutrinos rather than momentum conservation that had to go a long time before so we can measure this process charge current called interactions where a w is exchanged the weak Force so we have the weak force and the electromagnetic force and you'll notice though that as you go up in energy way this is everyday life if you like down here as you go up and up in energy the weak Force isn't falling very quickly if at all at first and the reason for that is that the W even in everyday life this is is is a long way from its mass the W wants to have a mass of 80 GV in everyday life we have nowhere near enough energy to make any W's that are anywhere close so it's it's very you know it's very unlikely to happen that's why it's called the weak force it turns out the weak Force doesn't happen very often because the W and the Zed are heavy and the photon the electromagnetism happens more because the photon is light that's not a pun actually the photon isn't as heavy the um the so so as they as they go up in energy here you see that they come together and this is the electro unification scale if you like or I prefer to call it the electro symmetry breaking scale because I think backwards so what actually what's going on is that there's a symmetry between these forces up here they they've got roughly the same strength they're qualitatively very similar objects and at some level as as you go down in energy that symmetry is broken and they they break apart and you have a completely different weak force from from electromagnetic force now but in the early universe or at the LH see they're the same or they're very similar anyway there are differences but they're mixed up together in that way so this is this is this point Electro unification here this is and this is a special scale in nature and the the the important thing to point out is that this is not Theory I mean the C the lines on there a theory but the points of data we know this happens this is not a fantasy of some particle physicist there's a special energy scale in nature which we're doing physics above for the first time at the LHC and we want to know why these this symmetry how the Symmetry is broken we know it's broken by the mass in fact but we want to know where that mass comes from how what's the mechanism what's going on there now Electro symmetry breaking is is a really fundamental concept and it appears in a lot of physics and in fact Peter higs who who uh who is whose name is on the higs B on and who first proposes the mechanism that we think is behind this at the moment um is um got a lot of his inspiration from condensed metaphysics where this stuff happens so I have two fancy demonstrations um hi so one of them is pretty low Tech and I will do this while we get ready for the first one so although the lowake one might be more dangerous amazingly given what happened earlier but so this is the wine bottle the answer to the universe is in this wine bottle there's I don't know if you can see it do I have to I should bring it in front of the the screen so there's a a wooden marble in the wine bottle it's quite important that it's not glass um this is this is the everyday Universe it's an empty wine bottle with a marble in the bottom if you imagine going back in time or going up to higher energies then you shake the the bottle up the the marble can be anywhere around the axis of the bottle but as it cools down it has to choose okay so the idea is that there's there's a completely symmetric situ situation there's a complete symmetry around the center of the axis down the middle of the bottle no preferred direction if I shake it again the marble will come down in a different direction won't you marble yes there you go so okay so what you have here is is a simp syry in nature which is the Symmetry around the middle of the bottle which is is not broken by gravity there's nothing to break it really but because of the bump in the bottom of the bottle there oops as the bottle cools down as the marble loses its energy loses the kinetic energy that I give it by shaking it it has to choose it has to break the Symmetry and now this this is no longer a symmetric situation there's a special Direction the mble is on one side that that's the analogy of what's going on in the higs mechanism okay okay there's but there's a much better and closer physical analogy to this which we'll show you in a minute but I there's a diagram of it here which is in a magnet which we've got here it's niobium is that right NE neodium sorry need a condens metaphysicist sorry um we we're going to heat this up this is something that is a magnet okay at the moment if we heat it up it stops being a magnet so you see that at the moment it's a magnet this is the magnet magnetic sensor showing it as a field now we heat it up and we just have the slide back while he Heats it up again for a minute so when it's heated up we're going to go to this situation all the little magnetic dipoles on the on in the material are um are jigging around because of the Heat and there's no preferred Direction and Maxwell's equations thanks to Faraday are you we know that they're um they're symmetric there's no special Direction in nature I mean there is on the earth there North and South Pole but we know why that is but there's no real special Direction the whole of Maxwell's equations are completely spherical symmetric and these are just doing that under Maxwell's equations but as they cool down a couple of them will notice that they have slightly lower energy if they line up together and as the temperature goes down there will no longer be enough energy to move them away again so they'll stay there and then two of them are aligned another one will align and in the end you they will choose a direction and they will all line up together and you will end up with a magnet and that's what's going on here so this magnet we've heated up will hopefully if this works become become become non-magnetic magnetic fields gone right so no it's not moving so and that's because we've jiggled all those little dipoles that were lined up they've been heated up they've all jiggled around that's the analogy of the early universe or the LHC if you like going to higher energies where this symmetry is restored there's there's no no special direction there's no magnetic dipole going on in this thing there's a complete symmetry as it cools down we don't put it near anything fancy and this will come back if we just leave it to one side for a minute and I so what what I haven't done is explained why this this idea of symmetry breaking is important and that is rather difficult to do um why we think there's some symmetry so you can see that there's some symmetry here I mean really what I've told you is the forces look the same that is a symmetry but if you just kind of break it by hand and put in the mass of the W and the Zed the whole Theory falls apart this is sort of been um Nobel prizes awarded for proving this that you you cannot have a predictive Quantum field f without this symmetry being built into the theory yet we know in nature this symmetry isn't there so this whole business of spontaneously broken symmetry is is a Dodge to get around that it's a way of saying yeah the wine bottle symmetric gravity is symmetric the whole potential is symmetric but the ground state when I leave the wine bottle alone is not symmetric anymore and we're in the ground state we're in the low energy State fundamentally symmetric forces lead to a fundamentally a an asymmetric ground state and it looks like as we cool down again retreated the magnetic fields come back again see so though it won't be in the same direction that it was before necessarily it's just they've chosen another Direction and they said okay the atoms were all lined up we jiggle them around so they weren't which is the analogy of the early universe as they cool down again they they passed the cury temperature and they they all line up again and and that's so spontaneously broken symmetry is actually crystallization is also spontaneous to Broken symmetry you have a completely symmetric liquid as you cool it down it will crystallize and suddenly choose his special Direction crystalline directions you so it's a very common feature in nature and we think based on that thank you very much we think based on our I didn't believe they could do that it was great there a we think based on our mathematics that this this idea of spontaneously broken symmetry we know it's an emergent phenomenon it happens we just saw it happen twice with with the bottle maybe less convincing but with that it's pretty good the the um we know it happens in nature in Collective phenomena it seems that it might be really fundamentally behind what's going on why why mass is there at all not just why not just how does collective systems have matter behave but why they have mass in the first place and that's the higs B on so the the postulate is that and this this um I gave talk similar to this to some students A- level students today and the question at the end of it isn't this a bit like The Ether and it's true it is the the higs field fills the universe it's responsible for it's the bottom of the wine bottle it's the higs potential um fills the universe and it gives everything MTH and all the math and all the maths works out how do you satisfy yourself this is really true well you go look for experimental evidence and the only way to detect a field that fills the whole universe is to excite that field excite an um excite a wave in that field if you like vibrate the field so I mean you could imagine if you didn't believe that there was any air in this electri theater for instance um how would one piece of evidence that tells you that there is actually some air in here is that you can hear me there there are sound waves traveling from me to you that if you if you lived in a without the material to detect air one way of detecting it would be to shake it to make a noise and say okay we there are waves traveling across this thing very like The Ether right we thought thought there was um waves we thought there was something that that um electromagnetism had to travel in it turns out it's the electromagnetic field not The Ether but nevertheless the idea the way of detecting an electromagnetic field if it's just constant everywhere there's no way of detecting it you have to excite an electromagnetic wave you have to have a photon that shows you the higs Bon is the um is the reason is is the experimental that would be if it exists will be the experimental evidence that this field is there so you if something's everywhere it's very hard to detect you need a lot of energy to excite a field now think about the the the double Bas string again we need a lot of energy to excite a vibration in it what we're trying to do is excite vibrations in this field that fills the whole universe the higs field and that those vibrations are the higs B on so in quantum mechanics waves and particles are pretty much the same thing so the this this particle the higs particle the higs boson is basically a vibration in the higs field that is the thing is the bottom of the wine bottle it's the thing that gives that breaks this Symmetry and gives the W and the Zed mass and doesn't give the photon Mass because that's what the the ground state of nature is as you come down and it's but it's the ideas we know these ideas work in everyday physics um what's fascinating now is they might actually be very deeply fundamentally built into the nature of the universe even at the smallest scales we can look at it so that's the kind of grand scale view of what we're doing I I want to um I mean to be honest any competent LHC physicist could have given the talk I've just given you there are 3,000 people working on my experiment we're not daed we put a lot of time into this a lot of effort um we know why we're doing it and this is one of the main reasons why it's not the only one but it's one of the main reasons why I want to take a little detail just to show you something that I've done myself with my student just for fun so which is related to this as well so what we have here is is something again I'm trying to give you an idea of how you really do science with this and trying to connect the the the gwiz stuff with the with the actual data with what you actually do dayto day so this is one of the ways a standard model higs might appear in Atlas so this would be a quark and it should be an antiquark annihilating here give you a z BOS on Zed BOS on is very heavy so it's actually relatively likely to kick off a higs this this there's a number associated with this vertex here which is quite large it's to do with the mass of the of the Zed so it's it's quite likely to happen so you can get this is one of the ways we might be making higs bons right now well not now because we stopped for Christmas still but until December and starting up again in March um making um higs B on at the Lou CH drun aider there's a problem with with finding these though because the higs is um it it's responsible for Mass so the the heavier something is the more likely the higs is to Decay into it which is know sensible logical it's one of the things we need to that's kind of um fingerprint of the higs we need to see that actually but the problem is for a very for the range of mass where the higs might be now as we know now we didn't know before but we do now then the thing it's most likely to Decay into is a pair of quarks it's the the B Quark the beauty Quark or the bottom Quark depending how romantic you are um to BB bar because it's the heavy the top quack is the heaviest quack but it's so heavy the higs can't Decay to it but the B Quark it can Decay to so it will do and the problem with that is that as I showed you here we got quarks all over the place at the LHC and quarks don't show up as beautiful little objects that you can tag in in the in the uh in the detector they mixed up with glue ones there are lots of up quarks down quarks strange quarks charm quarks it's very hard to find them pick them out so it's it's not we might be creating higgses and just missing them because they're decaying to stuff that we can't pick out it's a needle in a Hast stack but the needle's kind of strong call and bendy it's it's not really working so so so we we we have trouble with that now that that's a little pessimistic because the b-quarks are a little special in that they do decay in the end to something else but in the end is maybe 100 microns from when they were created and so there is a way of maybe detecting that there was a beark in there that's why we had that Precision silicon detector that I mentioned before for instance you can see that that not all your particles came from the same place some of them came from a few hundred microns away that the signature that maybe a b Quark was there so there is information we can extract but even given that there were a lot of B quarks created just from gluons colliding so there's a huge background to this so the Zed is useful this helps us pick it out if the Z decays to muons that's also useful it's a sign something interesting happened but even given that Zeds plus gluons which go to B quarks was was a killer so people had kind of given up on this so in 1999 which is roughly when I joined the experiment actually um where we preparing the experiment the studies have said this is probably not worth looking for okay there are other ways we can find the higs at the LHC and I'll talk about them in the M but this is probably not worth looking for because you'll never pick it out from the background so we had an idea me this is me and my student at UCL and he's a friend of mine he's a theorist in Paris and his student so well okay we'll give up on most of those events but what about we have so much energy at the LHC that sometimes the higs is moving really quickly and then instead of having one jet of stuff with a B in it and another jet of stuff with an antib in it you you have them both together and they're are both inside so all that stuff is inside this Jet and that's probably why we're missing a lot of these golden events they're the kind of highest energy events we should they should be the easiest ones to see but the all everything is stuck inside this mess here and we're just missing it because of that so we came up with an idea of how you can interrogate that mess and find out that there were two bees in there and that maybe they came from a higs and so to cut to the chase this is a simulation plot of that and again another reason for showing this apart from kind of vanity because it's one of my own plots and I want to give you some kind of personal view on the lxc rather than just the generic lxc talk but also this bump here is one you've seen many times before this is unfortunately not real data but it might be in a few years um this is this is simulation but this this green bump here is one you've seen before it's about 90 GV it's the Zed BOS on decaying to a b and an antib in exactly that kind of diagram so you can have this diagram instead of a higs here you can have other things going on and you can have two Zeds or a w and a zed coming off so that's there it should be there we know that that exists so we better see it if this technique works at all we'll see it um but this blue bump is what a higs would look like if it had 120 GV mass and you see that it shows up really quite well and this is this is now one of the techniques that we're using to look for the LHC um that was developed at UCL and is you know we're quite proud of it so it was a bit of a vanity detour but I wanted to show you something that was me rather than just the whole LHC this is what the whole LHC is doing this is what we're doing right now um that that Channel with the B and the B Bar it's very nice it it's a great thing to look at but it's not going to be the first place we see it if it exists first place we see it bizarrely is if it if it couples to photons which is a bit weird because photons don't have any Mass but they're they're easy to spot and the way it couples to photons it has a little Loop via a top quar so don't need to worry about it but we we might see it this bump here is what a lot of the excitement was about in December it might be that in this spectrum that that there's a little bump there which is the sign of the higs at about 120 GV going to photons this bump here is a simulation of a higo 130 GV but this one obviously isn't there if it's there at all it's about 126 so we're looking for photons that may be produced from the from the higs we're also looking for Zeds that can come from the higs and the Zeds go to leptons and it's a very clean Channel and it doesn't happen very often but if there's a higs at about 125 GV you should see a few more events here than you expect and indeed we do not many you can see the size of the erab bars there I wouldn't take this to the this is why we're not saying we discovered it but it's you know it's suggestive and you can churn this through your statistical analysis and see what it tells you and this is the kind of limit plot that we make so this is the higs mass along the bottom here this dotted line here is the one is the expectation if there is a standard model higs at that mass along that line okay so and and then this dotted line here that's dipping around is where you would what our expected sensitivity is given the amount of data we have and the the cleverness of our detector so where this line crosses it it tells you well here we wouldn't have expected to have seen a higs if it's above about 530 GV we wouldn't have expected to see it yet with the data we've got we're not sensitive to the standard model higs at that point however we would have expected to see it if it's if it's below that all the way down to about here which is about 123 GV that whole range the LHC should have given us brand new information about the the higs bow on and whether there is one or not okay we could have made that plot without building the LC that's a simulation plot that tells you what we expect we've done the experiment now and the experiment is the the black line and you can see that actually we don't exclude the higs in this region and we might have expected to the significance isn't great these bands are too Sigma it's not that great but it's suggestive um maybe it may just be statistics as you go down there indeed we do exclude it we know there's no higs over all this range maybe here something over all this range no higs the the solid line is below the dotted line then here suddenly what happens it shoots up it goes way higher and um it comes down again and we didn't even expect to exclude the higs here but we actually have even down tells you so you learn a lot from this plot you can go mad looking at these plots if you're not careful but it's what one of the things it tells you is statistics are um beware of Statistics these these lines are moving up and down every time we make this plot again everything moves up and down so you got to take the errors seriously nevertheless if there was a higs we should have excluded did it in this region and we haven't and in fact we see something bordering on on suggestive that there might be something going on there um and that's this bump again at about 125 now I've been very biased and I've only shown you the atlas data there is another experiment you should get someone else to talk about the lxc here as well some point um and they've got results which in the end if you put them with ours say that there's a tiny little window left for the higs to be but there is some evidence that it may be there now scientifically I would probably not call it evidence cuz usually want three sigma before we call it evidence and it's not three sigma yet but we should have the window isn't as small the window for the higs boson Mass where it might be it's been shrinking and shrinking and over the last year the LHC has wiped out huge amounts of real estate as to where the higs ought to might be but it stopped shrinking now and and we're we're in the end game now we're really we will know very soon so the the I focused on the HS I said you know what the how well the standard model what we'll learn from the L8 is more than just the the higs the the U we we'll know when we're learning how well the standard model Works above the electric symmetry breaking scale in fact that's what I do most of my time I'm not directly working on the Hig search I'm working on measuring what happens in proton proton Collision when we do them at these energies and whether our predictions actually work for a whole range of of processes we will learn why or something about at least why the W and the Zed have mass and the photon doesn't where the mass comes from there are also a whole host of things new part par that may show up super symmetry is another one which is postulated to explain some of the things but that we have up there some of the issues that are in that little diagram of particles you know the higs isn't the whole story there's there's other things we'd like to know about it um it's even possible that there are extra New Dimensions of space many black holes if those are true we will will start learning something about gravity I wouldn't bet on it especially not now but it's a bit of a long shot but it is a an idea that you know we're really doing physics in a region where we we actually don't know um we're we're as I say essentially doing physics in a fundamentally qualitatively new regime it's it's the region where these two forces look the same whereas all the physics to date they don't look anything like each other and as um Ralph Hoyer director General CERN who's a very very conservative um cautious person said to the media in October last year I think by this time next year I'll be able to bring you either the higs bows on or the message that it doesn't exist I hope that in this talk I've managed to persuade you that's a very important question to answer the question answering the question is important it's not finding the higs or not that's important it's not whether it the higs is a higs is not it's not like there it's a failure if there is a higs or a failure if there is no higs it's the fact that this will be cross the our list of important questions that we need to know the answer to about nature it's something to do with Electro symmetry breaking so watch out for the next update [Applause]

Channel: The Royal Institution

Views: 6,515

Keywords: Physics, CERN, LHC, Large Hadron Collider, Particles, Universe, Higgs, Boson, Energy, Ri, Royal Institution, Atoms, High Energy Physics

Id: 9CWNROmhplY

Length: 61min 40sec (3700 seconds)

Published: Tue Feb 07 2012