The quandary of the quark

good evening you've all gone so quiet that I have the impression that perhaps we should start although it's just a moment or two before half bust my name is Julia Higgins and I'm foreign secretary and one of the vice presidents of the Royal Society and it falls to me tonight as the chairman of the committee that selects the rosalind Franklin Prize winner to have the pleasure of introducing this year's lecturer and the prize lecture before I say a little bit about professor Christine Davis this year's winner I'd like to tell you a little bit about the history of the prize it's now three years old Christine is the third Prize winner we've had a chemist we've had a somebody at the interface between biochemistry and biology and now in the Einstein year very suitably we have a physicist the prize was is funded by the Department of Trade and Industry and it was the brainchild of Patricia Hewitt and she wanted something that recognized the importance of women in science engineering and technology but the prize is not specified to be given to a woman it's specified to be given to a brilliant scientist developing out of mid-career the person where they're just really bursting over the horizon and becoming world famous and that the piece that is about women in science is that each proposal for a prize winner must contain something that the prize winner themselves is going to do to help encourage support and get women into science engineering and technology and as a consequence of that very often the people proposed are women but by no means exclusively while I remember Christine tells me that what she's going to do with the part of her prize money that is she's spending on helping women in science is to develop pictures that can be given to children school children but not necessarily in school about the science that she's going to talk to us about and particularly in a way that she feels it will be interesting to girls because often the nuclear and particle physics is told in the whiz-bang crash sort of mode which excites perhaps more young boys and she wants to bring out the beautiful symmetry and the underlying laws of physics which she thinks will attract girls more into physics and we certainly need girls attracted into physics now let me tell you a little bit about Christine herself she's been based in the department of physics and astronomy at the University of Glasgow since 1986 but she was a student and did a PhD in Cambridge she's worked in CERN that is the European Organization for Nuclear Research which is based in Geneva she spent two years in Cornell University and she's also worked in Ohio State University in the University of California at Santa Barbara a place that I too have spent a few months very beautiful place so she's a very well traveled physicist she's the theme leader for particle physics in the Scottish universities physics Alliance that is a grouping of the physics departments in the number of Scottish universities to form a critical mass of physics in Scotland and she's also head of the particle physics theory group and a member of the departmental management team in the department at Glasgow University she's a fellow of the institute of physics and also of the Royal Society of Edinburgh what she's going to talk to us tonight about is her own research which as I say is in the area of theoretical physics and I'm really looking forward to hearing about it having read the documents in her proposal the title of the talk has the very intriguing named the quandary of the quark and some of you if you've seen the leaflet will have read the synopsis on the back so I didn't need to say any more but to introduce Christine and from this year's Roslyn Franklin Prize lecturer okay thank thank you very much I'm very grateful to the Royal Society for this award and grateful for the opportunity to give this talk here tonight and thank you all for coming to hear me so my title is the quandary of the quark I'm theoretical particle physicist and one of the aims of particle physics is to find the ultimate building blocks of matter the quark is the deepest level that we've reached so far and yet in fact quarks are quite tricky to deal with both theoretically and experimentally and I'm going to tell you something about that tonight so one of the big questions that we want to answer is what is everything made of I'm trying to get my laser pointer to work if you look out at the night sky you see stars stretching away into the distance and you would like to know what is all of this stuff out there made of and linked to that question of course is how did the universe evolve and how did it reach the present state in which we exist embarrassingly you may know that actually most of the material of the universe we can't even see the stars that you see out there make up a very small fraction of the universe and most of it we actually can't see and we have no idea what it is that tells us that we need theories beyond our current understanding of particle physics to explain that and it's going to be a tricky problem that will take many years to solve and it will need to be attacked from many different directions but one direction that should bear fruit is to actually understand in more detail and more precisely the material that we actually can see when we have a deeper understanding of particle physics that will have to explain both the material that we can't see and the material that we can see and what we would expect to be true is that if we can understand the material we can see at a very precise level we will find discrepancies there that will give us an in to this deeper theory that will eventually explain the material that we also can't see now the material that we can see we believe is made of the same stuff that we have here on earth and that exists inside our solar system so the material that we can see we can study in experiments on earth and of course we have been doing that for hundreds of years and we've reached a level of understanding about that that I'm going to try and explain to you tonight the new thing that we've been able to do in the last few years is to actually simulate numerically this material in much more detail on supercomputers that's the area that I'm working in and I'll try and tell you something about that area at the end of my lecture tonight so it helps to begin this to actually think about some of the distance scales that are involved if we think about the earth the earth is ten thousand kilometers across so if we express that in meters that's a one followed by seven zeros or ten to the seven meters we're somehow familiar with that distance scale if we go down from that distance scale by a huge factor 10 to the 7 we come to a distant scale that we are very familiar with and that's the human distance scale of about a meter so roughly speaking humans are about a meter big this is a picture of rosalind Franklin that we are honoring tonight if we go down by another factor of 10 to the seventh so from here to here and then another factor of 10 to the 7 we come to the scale of 1 10 millionth of a meter that's sometimes called a tenth of a micrometer or a micron or 10 to the minus 7 meters at that level we can see the molecules of which everything is made and that includes the picture here shows you the molecules of DNA but carry our genetic inheritance and that rosalind Franklin did so much work to uncover the structure of these molecules are made of millions of atoms so atoms of substances such as carbon hydrogen oxygen and so on and if we go down by another factor of 10 to the 7 we reach the size scale of the nuclei of these atoms so that's now 10 to the minus 14 meters so that's 100 million millionths of a meter is that as far as we've got no because we know that the atomic nucleus is made of particles called protons and neutrons we're going to discuss a bit more about this in a minute and they the size of those is another factor of 10 smaller than this distance scale so see how far we've come so far we go down another factor of 10 we'll come to the objects here the protons and neutrons that make up the atomic nucleus and both I'm going to discuss tonight is going inside the proton and neutron and inside there are particles called quarks that we're going to talk about and is there something inside the quartz currently we don't know but it seems reasonable to imagine that in this scenario which we're going deeper and deeper into matter that there would be something inside them we haven't reached it yet but again that would be part of this deeper understanding of particle physics that we're after and again it seems reasonable to assume that a way into this deeper theory about what could be inside walks will be to understand quarks themselves much more accurately so let's discuss atoms everything around us and in us and that includes the building of the Royal Society is made of atoms this gives me the chance to bring up the point that Julia mentioned that this year is the world year of physics and it's the hundredth anniversary of the annus mirabilis in which Einstein wrote some of his most famous papers so his picture of Albert Einstein and one of the papers he wrote in 1905 was on Brownian motion and this was a paper that actually disgusts a very direct demonstration of the existence of atoms and what happens in Brownian motions you take tiny specks of material and put them into a liquid for example and they scooted out as they're buffeted by the atoms and molecules in the liquid and Einstein explained the motion of those specks that you can see under a microscope and that was one of the most direct ways of looking at atoms that convince people finally that everything was made of atoms now around around the same time people were trying to uncover the structure of the atom and we now know that the Aten contains electrons protons and neutrons the electrons have negative electric charge the protons have positive electric charge and neutrons are rather like protons but they have no electric charge one of the experiments that was done shortly after this was by Ernest Rutherford and he fired particles at atoms and found that some of them came back directly and from this he inferred that the atom had a structure in which the protons and neutrons were in the time-poor the nucleus at the centre of the atom this takes up a tiny tiny part of the whole volume of the Aten and the rest of the volume has these electrons in now the electrons have a negative electric charge protons have a positive electric charge and the electrons are whizzing around the nucleus held there by the electric attraction between the negative electrons and the positive protons and a picture developed in which the electrons are sort of moving around the nucleus in these orbits that looked a bit like a solar system it was quite an attractive picture at that time however it's not correct and we'll discuss that further in a minute but one point I just want to make here is that another very basic thing that we don't understand is why the charge on the electron should be exactly opposite to the charge on the proton these are charges exactly cancel so that an atom a neutral atom actually has no electric charge and we'll come back to that point later but to explain that again is another point that we need a deeper understanding of particle physics for so I said that this this picture in which the electrons orbit in these circles is not really a very good one we now know at this this small distance scale inside atoms that we're talking about quantum physics comes into play and actually the behavior in quantum physics is very counterintuitive it's not at all what we're used to in the everyday world and the electrons instead of going around in these circles here actually have more wave-like properties and effectively they're sort of everywhere at once with some probability so they're sort of smeared out in the atom like this so this is a better picture of the atom really although it's not nearly as clear and this quantum physics is very counterintuitive and yet we know that it's true because we've done lots of experiments and the reason one of the reasons why we can do so many good experiments when we study electrons is that it's easy to obtain electrons to study easy to obtain free electrons now you've probably at some point done this party trick yourself in which you blow up a balloon and you rub your hair with it and then you find that your hair is attracted to the balloon and what you've done just by the simple process of rubbing is to actually transfer electrons from your hair to the balloon or the other way around one of the things becomes negatively charged the other becomes positively charged and they attract each other so you can easily yourself transfer electrons and so electrons leave their atoms with without very much provocation and we use this of course in the whoops sorry in in the electrical wiring of our houses we have free electrons running around all the wires and intellivision tubes you have free electrons and we can actually use electrons to make images so electron microscope we actually use electrons to make images of very small objects this is a pond grain I think so electrons are easy to obtain and to study and in fact in particle physics experiments they show up very readily as well because they're charged particles they leave tracks in particle physics detectors and one of the early particle physics detectors that was used was a so-called cloud chamber and in that an electron or any chanse particle would leave a track and if you put a magnetic a magnet near the cloud chamber the path of the electron would actually be curved and would spiral in like this and from making measurements on these tracks you can actually work out what the electric charge is which I'll give the symbol e to and the electrons mass so we're able to do lots of work on electrons because of this feature sorry I haven't quite got the hang of this point so electrons and electrical interactions are actually something you have a lot of intuition about I said earlier that if you put a magnet near a moving charged particle it would his path would be deflected this shows you that electric and magnetic fields are deeply related to each other closely related and in fact we give the word electromagnetism to the sort of joint theory of electricity and magnetism and this was another thing that Einstein gave us some insight into in another 1905 paper on his theory of special relativity because he showed something that looks like an electric force to one observer if you have another observer moving relative to that first observer will look partly like a magnetic force so he showed us how electric and magnetic forces transform into each other and this is another feature of the fact that there they aren't deeply related and that was another thing he did in 1905 now you learn a lot at school about electric charges and you learn this little mantra like charges repel unlike charges attract so you learn that charges electrical charges come in either positive or negative charges and if you have two positive charges they repel each other if you have a positive charge and a negative charge they are attracted to each other and in fact you can think about that repulsion or attraction as that one of the charges has an electric field associated with it spreads out from the particle and the other charge interacts with that field and again we have a lot of intuition about these fields in fact from everyday life and so you would have both electric fields and magnetic fields because these fields actually spread out over large distances that we can experience in the everyday world and you experience the magnetic field and that the strength of the magnetic field and long distance nature but every time you put a fridge magnet on the fridge you know the way it jumps the last little bit onto the fridge and I demonstrate it with with these two little magnets here you see when you get to about that distance which is a you know a distance of a centimeter sort of distance we can experience in everyday world they jump together and you can actually feel that force so electric and magnetic fields are long range and we know about them from the everyday world another thing that we have a lot of everyday experience about is and this will see that this is connected is light and other things like light x-rays radio waves etc and these are all forms the way in which they're connected to what I've previously been talking about is that these are all forms of electromagnetic radiation traditionally we think of them as waves I mean certainly radio waves you think of as waves and the thing that's waving then this sort of sketch here the thing that's going up and down the wave goes along are electric and magnetic fields so these are electromagnetic waves they interact with charged particles like electrons and the thing that's waving is the electric and magnetic fields and again we have a lot of experience of those in the in the everyday world I mean that's how we see is by seeing light waves now the interaction of the electromagnetic radiation with the charged particles was very important to my story and one of the key sort of milestones on that path is again due to Einstein it was discovered that there was an effect by which if you shone light electromagnetic radiation onto atoms electrons would be released they would take energy from the electromagnetic radiation and they would use that to escape from the atom people studied the behavior of this the energies of the electrons were released and so on and I'm Stein in a 1905 paper on this event called the photoelectric effect showed us that actually it was important again down at this quantum level where we're working to think of light as packets of energy rather than waves and these packets of energy are called photons and in fact this was the work for which you got the Nobel Prize in 1921 so that's an important factor in the story that I'm building up to another point that again Einstein told us that is linked back to his paper on special relativity is that mass is just simply a form of energy and so this tells us that packets of energy like photons can actually create particles and here I've got an explicit picture of this that's taken from another type of particle detector a bubble chamber and if you can see that a photon is created here at this point see now you can't see the photon because the photon has no electric charge but it travels from C to D and then it produces charged particles which you can see and because it has no electric charge it has to create two particles one of each charge so positive and the negative charge that creates an electron and the electrons antiparticle called the positron which has positive electric charge and those two particles they're their paths spiral in opposite directions because they have opposite electric charge in the magnetic field that's been put on this bubble chamber so this is a very explicit demonstration of how energy can create particles okay so those are various factors that will come into my story so now we can move on to thinking about now we've put some of the ingredients together an actual quantum theory of the electromagnetic force and this theory exists it's been very accurately tested and it's called quantum electrodynamics or QED and this is a way of thinking about the electromagnetic force slightly different from the way that we think about it in everyday world but this is appropriate to the quantum level is that the force between two charged particles actually should be thought of as the exchange of a photon so the exchange of this photon which if you like carries the electromagnetic force between the two particles this isn't a particularly good analogy won't stand up to a lot of questioning but it's quite an attractive one so just think about two particles passing a football back and forward and that is an interaction effectively between them now in the theory of quantum electrodynamics the way we draw this is on the right here in which we have charged particles that come in and they're drawn with these straight lines and they exchange a photon which is this curly line between them so this would be the basic interaction of QED but because this is a quantum theory and because as I told you earlier energy can create matter you can actually write down more complicated interactions than this which you have to take account of in the theory and so one of them would be this interaction here in which the photon that you're exchanging actually temporarily splits into a positron and an electron and then they go back into a photon and interact with the other charge and of course you can draw more complicated things even the miss however one of the nice features of quantum electrodynamics that you can make use of when you're studying this theory is the fact that the electric charge is actually rather small you know that when you have learned this at school when you learn about the interaction of charged particles that the strength of the interaction goes like the charge on one particle multiplied by the charge on the other this means that if we're talking about the basic unit of electric charge which is the electron basically that is the particle that carries that the smallest quantum of electric charge then the strength would be the square of the electric charge on electron fees squared and another term for that is to talk about the coupling constant of QED or the fine-structure constant and give that the symbol alpha it's basically a squared and in the units in which we use in particle physics this is usually quoted as a rather small number so seven thousandth so every time you write down one of these interactions you have to put effectively an either so in this interaction is that the probability of this interaction would be proportional to the square of your alpha and when you write down more complicated interactions you have to put more powers of alpha in there and because alpha is small these more complicated interactions are less and less likely so if you want to do calculations in QED you actually don't have to write down very many diagrams of interactions before they quickly become negligible and you don't have to calculate them this is the thing that means that calculations in QED can be done by by calculating effectively the probability of these diagrams and you can get very accurate answer from the theory and you can test it very accurately and so QED is basically the best theory we have it's been tested to a few parts per million and again another reason why we can actually test it so well is that photons and are also easily obtained as three particles we know that every time we switch on a light three photons come through this room that's how you're seeing me standing here talking to you and also when you look out into space and this is the picture that we showed at the beginning and in fact this isn't just any space picture this is a picture taken by the Hubble Space Telescope and it's part of the Deep Field project so this picture is looking billions of light years into the depths of space and the photons that you're seeing here have traveled all the way across space to us and obviously they would need to be free effectively they would not be need to be interacting for them to make it all that way so that's quantum electrodynamics let me just have a little aside here talk about an important symmetry in physics and symmetry is a very important feature of physics at a fundamental level and what we mean by symmetry is that you can change something and yet the thing looks the same so if you take a circle for example that's quite a pleasing object to the eye and one of the reasons is that you can rotate a circle by any amount and it looks exactly the same it's very symmetrical now a square on the other hand is not so symmetrical because I have to rotate that by 90 degrees to get it looking the same to you symmetry has important consequences and so for example one of the things that rosalind Franklin knew very well when she was doing her x-ray diffraction work on the structure of the DNA is that this X shape in her DNA in her x-ray diffraction pictures indicated that DNA had a helical structure so this sort of very basic feature of the helix showed up in this x-ray diffraction pattern now in particle physics we actually deal not only with symmetries in in real space and time such as the one described here but we also talked a lot about symmetries in internal or hidden spaces they're sort of rather theoretical spaces if you like but nevertheless the symmetries as part of those spaces are very important and quantum electrodynamics has a very important symmetry it's called a local gauge symmetry I'm not really going to explain what this is but I just wanted to tell you that this is very important symmetries basically the symmetry of a circle in this internal space so it's a very attractive symmetry and it's one of the things that makes quantum electrodynamics such a beautiful Theory it constrains it to be very well-behaved and a very attractive theory to work with so people knew that QED was a very good theory some sense and so then the question arose is it possible to make a theory like this for the other forces operating inside the atomic nucleus because the answer that question is going to be yes and I want to describe how that happens but it's true that all of the forces of nature that we understand now in particle physics are all based on this kind of symmetry so it's a very deep and important one so let's return to the atomic nucleus and here it is so the atomic nucleus is made of positively charged protons and neutrons packed together in a tiny volume so the obvious question is what on earth holds it together because these protons must be repelling each other electrically and you would think that the nucleus would fall to pieces because of this so obviously what you need is a strong short-range force to hold the nucleus together and very unimaginative Li it was called the strong force and I'm afraid that's the name we're stuck with the name of the second question that you can ask is what is inside the protons and neutrons and particle physicists take a rather aggressive attitude to these things and the idea was to smash the particles together and see what came out and in fact what we'll find is that the strong force operates inside the protons and neutrons and actually what's holding the protons and neutrons together it's just a leftover feature of the strong force inside these particles now when we talk about smashing things together see what comes out one one feature that you should know is that if you want to look at very very small distances so we want to go inside the protons and neutrons to very small distances we need very high energies and the higher the energy we can reach the smaller the distance we can look at and that's basically related to the wave nature of these articles at this fundamental level because if something is a wave then the features that you can resolve depend on the wavelength so to resolve very fine features you need a very short wavelength and high-energy particles have very short wavelengths so you need high energies to probe short distances and the way that you accelerate particles up to high energies is to use electromagnetism actually so you're accelerating charged particles and in order to get up to very high energies you need huge accelerators so to get to short distances you need to have a huge very large distance Excel later now the one that you probably heard about is the one that's under the French Swiss border near Geneva called CERN and CERN has been operating for many years and lots of very good experiments have been done there and so I wanted to show you something about the experiments that are going to be done there so these are actually much higher energy than anything that comes into the story I'm telling you but it will give you a flavor of what's going to happen so here's a picture of the CERN accelerator 27 kilometers round it's Underground so you can't you can't actually see it from the air so it's just marked on this map here and in 2007 the new accelerator called the Large Hadron Collider will start up and the energy of that accelerator is enough to make 14,000 protons that's a huge energy much higher than we've ever reached before and I'm going to show you a little animation of a collision happening at the Atlas experiment which is going to be part of this Large Hadron Collider so what's going to happen in the Large Hadron Collider is that protons are going to be provided together this enormous energy and first are you have to you have to produce the protons and then you have to speed them up in these subsidiary rings and then you let them out into the main ring in two opposite directions get the providing beams and you let them go around there for a while and then in a minute you'll see them acts collide with each other so this is now close up of the petition inside the Atlas detector and you see that huge number of particles are going to come out and this again is a feature of these much earlier experiments lots of particles came out when particles were collided together at very high energy and they produce signals in various parts of this enormous particle physics detector that you have to pick up the signal so obviously the early experiments weren't as impressive this is but nevertheless they produced all sorts of particles many many particles and these were particles like the proton nor'easter had similarities to the proton and they would given the generic name but hold zoo of these things would produce so it's very confusing picture and then it was noticed that they could be put into patterns and patterns our dead giveaway that these things have substructure and we know this from early work on atoms because we know that when the when Mendeleev came up with his periodic table he ordered the elements according to their chemical properties and he produced this nice table with repeating pattern in it and basically what this table does as you go across is counts the number of electrons that there are in the atom and corresponding number of protons in the nucleus and that affects the chemical properties and that's why can be arranged in this table atoms are quite simple though in the sense that they only I mean you're basically just counting the electrons and they don't have that much they don't have that many subunits to them what was found when people started to make patterns out of hadrons was that there were several different kinds of patterns you could make if you put particles of similar mass together you could make this group called an octet and you could make this other group called a Dekker plate and there were several other groups of this kind there wasn't one simple group and so this tells you their substructure and the name of the particles making the substructure was we're called quarks so this is where quarks first appear in our story and it's a nice quirky name for a change that has actually stuck in the field but the fact that there are several of these different patterns tells you that there are quarks of several different types here not just simple making up an A tonight of electron proton Neutron and in fact although we're not going to discuss this there are actually more types of electron so this periodic table is a rather incomplete picture even even of this but we're not going to get into that another experiment that was done was actually to look at how high-energy electrons were scattered from protons and the idea is that whoops sorry you you applied basically a high-energy electron and a proton together and if you have a high enough energy then the proton breaks up into all these Hadron so you get a collision of the kind that I'm telling you about and that tells you that the electron has been able to see inside the proton basically but if you want to study how deep inside the proton the electron was seeing all you need to do is actually look at the energy and the angle of scatter of the electron and by looking at this you can come up with a variable that's called Q squared and that's the thing that's plot along the x axis here and Q squared basically tells you the distance inside the proton that you're looking so going along the x axis this way goes to smaller and smaller distances what the y axis plots modulo some some kinematic factors is the probability of this scattering happening and this again I'm showing you an up-to-date picture this was obtained at the Daisy accelerating near Hamburg so this is much better than the original data that was obtained at in California at the Stanford Linear Accelerator Center but nevertheless that data itself showed the picture and there lots of different line Civet if you concentrate on the ones at the bottom the the point is that these lines are actually very flat results are very independent of the distance and this basically tells you that this scattering is occurring from substructure in the proton the quarks in other words which have no size because there's no scale dependence in these pictures and they're behaving as if they're free particles basically the fact that it's not exactly flattered because they're not exactly free but that's a detail that we aren't going to discuss so basically what we found now is that we look smaller and smaller distances we're thinking here about a strong force over a short range that's holding the protons and neutrons together but what we found is that we've actually go to smaller and smaller distances this force is actually not as strong as we thought we've actually found that is quite weak and this was very very difficult to understand and so the question is what theory can possibly explain this and QED is completely hopeless I told you already that that QED gives you long-range forces so do very hopeful to start with but there's another feature of QED that makes it impossible to explain this behavior and let me just briefly describe what that is I mentioned earlier that we could discuss QED in terms of the electric charge on electron and then I talked about the QED coupling constant or the fine-structure constant now that was a lie there when I told you that that thing was constant because it isn't the electric charge actually depends on the distance away that you are when you measure it and you may be familiar with this feature when you put electrically charged particles into materials if they're made of molecules which have positive electric charge at one end and a negative electric charge at the other or molecules that can be made like that this is called a polarization phenomena the material polarizers and the molecules sort of line up around the charge that you put in and they screen it scored screening what this means is that if you try to measure the charge some distance away you get a smaller value than the charge that you know you put in now in fact exactly this polarization and screening effect just occurs in free space in the theory of QED and it comes about through diagrams of this kind that I showed you in which your photon splits into electron-positron pair which goes back into a photon these this pair here causes polarization of the vacuum and screening of the charge and what this means is that if you try to measure the electric charge or correspondingly alpha it decreases as R increases or in other words the rate of change of alpha with R is negative and so that means that if you go two short distances alpha the coupling the interaction is getting stronger and stronger and that's exactly the opposite from what we want so the question is could we come up with a theory that has the opposite behavior and I'm going to describe to you such a theory and it's called quantum chromodynamics or QCD and it's basically a generalization of QED so it works in a very similar way it has this beautiful symmetry underlying it and yet in the detail it there it's details that cause it to behave completely differently so including the basic particle that we're talking about if the electrons we've discussed and in quantum chromodynamics we're talking about quarks and as we describe charged electrically charged particles can have positive or negative electric charge that's the only possibility the electron has negative electric charge just by convention and it's antiparticle then has the opposite positive electric charge now quarks also have electric charge but in QCD they carry a new charge called color charge and that comes in three types and we call them we call the types red green or blue it's nothing to do with real color in the everyday world it's just another one of these quirky particle physics names that's used and the anti particles have the opposite charge that anti quarks have anti color so they'd have anti blue anti red or anti green charge and I'm going to use the symbol G for the magnitude of this color charge and that's again a universal thing just like E is universal for charged particles now in QED we explained that there was a particle essentially carried the electromagnetic force between charged particles and that was the photon in QCD the particle that does the same thing is called the gluon in QED i represented the photon with this curly line i'm going to represent the gluon like this and a very critical feature of QCD is that although the photon had no electric charge the gluon does have a color charge and it's required by this symmetry of the theory to have this color charge in fact it carries both a colour and an anti color charge simultaneously and this is critical now the simple QED interaction that we wrote down before look like this in which charged particles exchanged a photon the QCD interaction then looks like this in which quarks exchange a colored glue on and they change color and doing that but this is the simplest thing that we can write down and we can also write down more complicated things and if you do then the calculations and these were done in 1974 by David gross David Pollitz and Frank Moore check you discover that QCD has the complete opposite behavior to QED and this earned them the Nobel Prize last year in 2004 so the critical thing here is the additional interaction that you can have in QCD so you have just as in QED you have this kind of screening diagram in which a quark produces a gluon that makes a quark antiquark pair that goes back into a gluon that will screen the color charge but in addition you have this diagram because gluons have this color charge and now a blue on it can split into two gluons that goes back into a balloon and that actually effectively spreads out the charge it's an anti screening and this diagram wins out over this one it turns out and what this means is that the color charge or the equivalent of the fine-structure constant that we now call the strong coupling constant alpha s actually decreases as R decreases so as we go to smaller and smaller distances the interaction becomes weaker and weaker and this is what these people found and this is exactly the behavior we need for QCD to be the theory of the strong force what this tells you is that quarks then can behave as if they're free at very small distances and this is called asymptotic freedom and it's only asymptotic it's not real freedom because in fact the flipside of asymptotic freedom is that the strong force at large distances actually becomes strong enough to confine quarks so that they can never escape and they have to stay inside bound states these are the things that we called hadrons these are the particles that we see in our particle detectors and in hadrons the overall color charge then is zero so it's a bit like an atom which has an overall electrical charge of zero but we've seen how easy it is to split up an atom you can't free a quark from a hedron and the key point here is the way that these gluons have color charged and they can split into other gluons so that if you try to pull two clamps apart effectively a spray of gluons develops between them and confine them and we actually use the terminology of a string for this so you can think about an analogy to this let me see if I can get in storm of a particle on a string this is a tennis ball on a piece of string and it's not a terribly good analogy but let's just go with it for the moment when the string is quite slack you can actually hit the ball and it behaves the visit as if it's free at that point but when the tension in the string cuts in it actually can't escape now that analogy only goes so far because you don't expect this string to break or if it did you would lose the ball whereas when you try to take two quarks apart what happens as you try to separate as you've got to put more and more energy in to the field between them to the wrong field and eventually that gluon field does break but when it does it creates new particles so that actually you just end up with more hydrants you never can actually pull the quarks out as free particles so what you see when you do an experiment is that for example this is an experiment at CERN from some years ago you made a quark and an antiquark briefly right at the very very tiny distance of the interaction but as they separate out they produce a spray of hadrons each so you get a sort of visual idea that there were quarks here but you don't see the quarks you just see these head drums coming out from the interaction so what do we know about quarks now we know that quarks come in six different types or flavors up down strange charm bottom and top now quarks as well are little spinning particles electrons are as well and what this means is you can put them into lots of different configurations to make hand drums you can either have their spins pointing in the same direction or the opposite direction and you can put them in lots of different configurations in space and what this means is is is that they're actually a huge number of hadrons if you're a particle physicist you carry around a book with you which tells you all about the masses and properties of these particles and there are over a hundred particles in there two ones that are we talk about a lot so so these headphones come in two different types basically all these the ones that we know about do haeseong's and baryons and in amazon you have a quark and an antiquark and so one has color and the other has anti color and they cancel and amazon that we talk about a lot of supply um that's made of an up quark and an anti down quark for example baryons actually have three quarks in there and because the three quarks have the three different colors that also cancels out the color charge and a very well known berry on one that we've already been discussing is the proton that's actually made of two up quarks and a down quark now but as i save the whole book full of these particles some of them are live for very very short time your heart being done you don't see them in particle detectors others live for a relatively longer time it's still only a fraction of second and you do see them in a particle detector but all this whole this whole physics of this little book is all predictable from QCD if we could only solve the theory of QCD but hadrons are actually very complicated objects in so inside a proton here we have the three quarks but it lives in a very complicated soup interacting with gluons and the p ones are producing quark antiquark pairs and so on and it's a strong force in there at that the the size that is actually confining these quarks into the hydrants and we know that these effects aren't quite complicated because example if we if we look at these two different particles they're made of basically the same class the proton has an additional up quark but they have their masses actually differ by a factor of seven so there's a lot of QCD happening in there to create that mass difference that we should be able to understand if we could solve the theory so so the big question the quandary of the quark for my title is how can we study quarks without being able to see them and in fact you can study them quite a lot just by looking at those jets and sprays of hadrons that i was talking about earlier and a lot of that work has been done and has been very successful but how do we actually connect quark properties directly to this suit of hadrons and to do that we actually have to solve QCD completely and to do that we have we work as I'll show you how to solve it numerically now I've written down here the basic equation of QCD not because I expect you to take anything much from this except how how beautiful it is how compact how short a little equation this is the equation for these so-called Lagrangian of QCD and it can be written in this very compact form and in fact if you wrote down QED it would look the same as this but as I say there are these deep differences hiding in this equation there which give you this very different behavior in QCD and QED now this equation describes the quantum fields of the quarks and gluons in four dimensional space and time that's the field of which these things are operating and the parameters of this theory are the color charge of the quarks and the quant mass now this M here is the quant mass now these parameters actually are not intrinsic to the theory I mean they the QCD doesn't actually know what these masses are they come from this deeper theory which we haven't yet reached and in fact this quite mass here comes from the interaction of the quarks with the Higgs field that's one of the particles that we believe exists that we haven't yet found we hope we'll find at the Large Hadron Collider so we don't understand where these parameters come from that is part of this deeper theory but of course one of the ways of getting at this deeper theory is to ask what actually what values do these parameters actually take and in order to work out what these parameters take we have to solve QCD because if you were to ask in fact it's quite difficult even to define the mass of a quark if you can't actually isolate a quark away at what do you mean by the mass of a quark and in fact if you think about it really the only way to define it is as the mass parameter that I have to put into the theory in order to get the right mass for the hadrons that I see in my particle detector so if we can actually solve this theory for hydra masses and properties then we can work out what parameter we have to put in here to get the right answers and in fact we only need to get one answer right to fix this parameter and then everything else is a prediction QCD though is a strongly coupled nonlinear system so to actually solve it is very difficult and as I explained to you there are we know that the binding effects of QCD are very significant because we can have particles that apparently have similar quite content with very different masses so how do we go about solving this well in fact is very similar to the way that people solve other equations in physics and on a computer and that is if you think about solving equations that actually operate in continuous space-time so a good example is for example weather prediction you know that continuous space has an infinite number of points so you'd affect it you in principle you have an infinite number of equations and you have to solve them everywhere you can't do that so what you do is you take a block of space in this case and you split it up into a grid of points like this and that's effectively a lattice it looks just like the crystal lattice so we talk about a lattice in that scenario and then you just solve the equations at the points on the lattice so that you produce your infinite problem to a finite number of equations that you have to solve now of course you have to be careful here because you have to make sure that the block of space that you're talking about is big enough to contain the area of interest so it has got to contain enough weather to make the calculation meaningful in the weather case and the spacing of the grid has got to be small enough that it can resolve all the physics that you're interested in and those of you in the audience most of you here I think tonight are old enough to remember the great storm of 1987 which are the metal first singularly failed to predict properly and caused a lot of damage in South East England and at the time one of these statements at the time was that their weather prediction models were not fine enough to resolve this storm coming up the English Channel so we do the same thing in lattice QCD now we're working in four dimensions of space and time and we make a grid or a lattice and so why it's called lattice QCD and then we do the calculations on this lattice and in fact we we generate a lot of random numbers to do these calculations so they're called Monte Carlo calculations and you get very complicated set of gluon fields we generate long fields that we we describe as snapshots of the vacuum and then for example if we want to make a mess on we put a quark and an antiquark on these fields and we count interactionist the Maison moves in time and it settles down into its lowest energy configuration which is basically the metal that we're after and then we can work out the mesons mass and its properties but this is a very complicated calculation to do and this little demo shows you that the bloom fields on the last have a lot of structure there's a lot of things happening there but we can take account of that in our computers it's numerically extremely costly process and the main reason is because of problems with quarks and the basic problem I'm not going to spend very much time on this is just because quark fields because they spin in the way that they do means that they can't be represented by normal numbers on a computer when you normally take two numbers you know 1 & 2 if you multiply 1 by 2 you get the same answer as if you multiply 2 by 1 doesn't matter what order you do the calculation for quite fields that's not true so you actually have to determine what the effect of required fields is on the glowing fields and that becomes numerically extremely costly and in fact the most costly part of the calculation is including this screening effect that we discussed and this is very important to include that in order to get the accuracy that we're after and recently so this of course takes a lot of people and a lot of people's work and we build up into quite large collaborations to do these calculations and recently two collaborations have come together I'm a member of this HP QCD collaboration which is a US UK Canadian collaboration and we've linked up with the mill collaboration and for the first time we've been able to include realistic screening effects in these calculations and working on supercomputers we've managed to calculate a number of hadron masses now as I told you have to fix the parameters of the theory the quant matters in the quiet charge by using some hydraulic masses but once you've done that then your theory is entirely fixed and you don't have any other freedom so then you ask the question do we get the right answers for other things and this is the plot that we made in which we calculated a whole range of other things coming from all sorts of different headphones out of this book and what's actually plotted is the lattice QCD calculation result divided by the experimental so the answer should be one for all of these things and you can see on the right here when we include these screening effects properly we do get one with very small errors all the calculations because screening effects are so numerically difficult actually worked without including the screening effects and then you can see the answers all over the place and you get quite large errors these calculations include work on all sorts of different hydrants as I said there's the something to do with the pi PI on the PI meson up here the Omega burial made of strange quarks and these particles at the bottom these Absalon particles are made of bottom and anti bottom quarks so we covered a huge range of the physics of QCD and we get the right answer for everything that enables us then to proceed to other calculations and one of the things that we were able to do last year was actually for the first time to predict the mass of a particle that hadn't really been seen yet and this particle was a piece of C Messam made of a bottom quark and a charm anti quark and it had been seen roughly in the past but with as mass was only determined with very big errors and likewise we done lattice QCD calculations in the past but we had very big errors on the mass that we calculated because we weren't including these screening effects so last year got very exciting because the experimentalist these reps are working at the Fermilab accelerating in Chicago and include people from Glasgow they told us that they would actually be able to get the mass of this particle very accurately and so a big race was on for us to get the theoretical prediction out first and we just managed it and both now both the theoretical prediction and the experimental result have tiny errors I mean you can't actually see them on this plot and they do agree so that was a big boost for understanding of how to do these QCD calculations numerically let me just say a few final words about some of the things that we are after and that I mentioned back at the beginning so now we can actually ask what are the mass of the quarks because we have to actually work out what the masses are parameters of the theory from our calculations and this is what we've managed to do for the up the down the stranger charm and the bottom quarks but one of the things that you will notice this is in units of the proton mass here is that the up and down quarks that make up the proton actually have masses which are very much smaller than the proton so that brings me back to the question right back at the beginning that we asked when we look out at the universe what we see at the stars and they're burning hydrogen basically so what you're what you're seeing when you look out there are nuclei of hydrogen protons that's what makes up the visible matter of the universe and in fact that visible matter is not the masses of the quarks that make up the proton but the QCD binding energy that makes up the proton proton is mainly its mass mainly comes from QCD binding energy so that's what you're seeing when you look up at the out of the visible universe and that's quite a weird thought I I think quick word about the future of this now that we can do these more accurate calculations in QCD on a computer where where is this field hitting one of the things that I'm looking at particular are focusing on particles made of bottom quarks and in fact looking at the weak force of nature haven't mentioned the weak force at all but this is the force such as responsible for radioactive decay of nuclei but in fact what radioactive decay is which a nucleus changes from one type to another and emits an electron and the particle known as an anti neutrino it's actually a neutron inside the nucleus changing into a proton and that is actually a down quark inside the neutron changing into an up quark so that the edgren becomes a proton and exactly the same thing happens inside these metals that contain bottom quarks bottom quark can actually change into a charm quark or an up quark and the weak interactions of quarks and here's a picture from experiment showing you exactly this happening they have very interesting property they break a lot of very fundamental symmetries and in particular they can tell the difference between matter and antimatter I told you matter and antimatter actually very similar but there are subtle differences there as far as the weak interaction goes and by studying how quarks behave we hope to be able to get a handle on that symmetry breaking which will again be handled in to a deeper theory and yet if we want to look at how quarks behave they are inevitably bound up into these hand drums we've said there's nothing they can do about that they're confined and so we've actually got to calculate this process in lattice QCD and when we have a precise calculation in lattice QCD combined with precise experiments that are going on around the world at the moment we'll be able to understand whether there's the standard model is consistent in this respect or not and we're hoping not and we hope that that will give us an in again to this deeper theory that we're after that's all I would like to say I just want to thank you all for coming and thanks to my collaborators Christene that was a tour de force taking us from the first physics we learned that school right through to the cutting edge Christine said she's happy to answer questions or try to answer questions if there are any questions I won't keep people for very long with of any questions please over there there's a microphone coming from behind you I would like to have a comment to Europe marvelous the presentation of the medals at the Swiss and German scientists find out that in plasma state when a very high hot are very strong magnetic field existing and the quarks became essence and in the plasma state the heavy Muslims these heavy quarks are melted away but the very light the up and down quarks and melons the PI meson etc surviving and after they are making again a recombination and became once again protons and neutrons and my question is there the anti quark disappear after these recommendations I think you're asking about how QCD behaves at very high temperatures and densities and a lot of work is being done on that at the moment and in particular new experiments are starting after Brookhaven laboratory in the USA and it's thought that a new state of matter will exist if you can make a large enough volume of space-time at a high enough temperature and that the quarks may then take up a sort of plasma state in in that situation and there will be various signals of that that haven't unambiguously been seen yet but one of them will include looking at how the light had drones and the heavy hand rooms coming from this plasma and I think we have to wait for the experimental results to see what comes out but it is one of the things that you can also study in lattice QCD and there are a lot of people working on lattice QCD calculations appropriate to those kind of scenarios to make sure there's a question here we got the microphone regard these lattice calculations presumably the smaller your lattice the easier the calculation or the curricular calculation but the less reliable they are yes how did you know that for example that you had enough certainty in theoretical calculation that is you were rushed to do the calculation as I understand so you were at the limit of what you you could achieve with their present calculate calculations but how do you know about the uncertainty in the calculation we know the experiments sort of have got that set up but what about the theory itself well obviously that's a very important feature and one of the things that I yes actually things disappeared I'm showing you my slide here because it one of the things I I skipped over because I was running out of time yeah is that obviously when you do these calculations you have to make sure just as in the weather case that your box is large enough to contain Headroom you're looking at so say a proton so I told you a proton was about a femtometer across so your box needs to be something like two to three femtometers across and your spacing must be small enough to capture all the physics you're interested in and you might guess that that would be around a tenth of affirming obviously if you make this box bigger or you make the spacing smaller then you have many more points in your life this cost of the calculation goes up as you say but we've been doing these calculations for many many years now and we've developed our theoretical tools that enable us to estimate how much of a mistake we think we're making in terms of both the discretization error for the error that's coming from the fact that a grid and the errors that come from the finite volume that we're using so we we do believe that these results have have got a good error on them and we can also actually when you do these calculations you make boxes with several different spacings and you do the calculation you check to get the same answer and you can also take boxes of different size and check that you get the same answer so there's lots of checks that you can do in the same way that the experimentalists perform checks and tests of the systematic errors estimates of the systematic errors and so on so I believe we're in we're in good shape but of course you know the price of good physics is eternal vigilance so you have to keep keep tracking and keep doing the calculations as you've said much is known about the properties of the many different particles their energy must spin etcetera but is anything known about what the particles really are what is inside of them we talked many many particles but do we know what these things really are yeah well I hope I've convinced you of the real existence of quarks I can't show you what want because I can't I can't get one in the way that I can an electron to study and that is one of the problems with this whole area but you know you do experimental tests you do theoretical tests with a good theory now it agrees with experiment and so on and these things should eventually demonstrate up to us that quarks are real things because everything that we do that assumes that quarks are real things gives us an answer the degrees with experiment and that and that's you know for some level that's all all you can do and eventually you have to believe in the reality of quarks just because of that but but you're as far as we know anyway you will never see a free quark that you can actually you know put on some scales it's the quickest quark name from James Joyce's Ulysses I work yes in fact I I must admit to not having read Ulysses I haven't checked that out but I believe that's okay sir the question I have is to do with a previous question really about what is actually there and the understanding of I know we have to use a particular kind of language when we say that inside the proton there are these three quarks and gluons and so on but if are you actually trying to say that as far as experimental science shows that quarks are fundamental particles in a way that cannot be broken down into anything else and if that is the case how does that connect to string theory and the idea of something in a much much finer level than anything that the quarks are and does one kind of science have anything to say to the other or one branch of it rather to do with quarks have anything to say to string theory which is not as we can't demonstrate by experiment and vice-versa I assume theory would be an example of this deeper theory that we're after and as I told you back at the beginning what we hope to do is by studying our current theories of particle physics testing them very precisely we will find discrepancies that will lead us to this deeper theory and that deeper theory might eventually lead to string theory or it might not we don't know so at the current level at which we are operating as far as we can tell quarks are fundamental particles but as I said back at the beginning there all sorts of questions we have about the matter we can't see in the universe about why electron and proton electrical charges cancel so completely so there are lots of things that tell us that fundamental you know that fundamental physics is our understanding of fundamental physics isn't complete and this is why people are trying to develop things like string theory and they're basing string theory on a lot of very beautiful fundamental symmetries of the kind that I was telling you about but there is no experimental test of string theory there are experimental tests all this picture of quarks and so we have to look to the future if all week we can see whether the deeper theory that comes eventually out of experiment and theory leading the direction of string theory or not if you stop there bring this one here that's great so if you have a question here in the back have we any idea what is in the space between electrons and protons what is in the space between electrons and protons I mean as I showed you like back at the beginning you know it's not really a question that you can ask exactly where the electron is so it's got to go a long way back through this whoops the electron is somehow everywhere at once inside an atom and so you can't actually say an electron is here and a proton is here what's between them it's not a question that you can ask and this is one of the things about the quantum world that makes it so counterintuitive because we're used to certainty about objects you know it's here and I put it here and it stays there but particles don't behave like that down in the quantum world and there are some things some questions you can ask and some that you can't ask and the one that you asked is one that you can't answers apparently if you're trying to break the gluon bond within a proton you end up with a Nissan do you know which is the new quark the one in the proton or the one in the Nissan if you try to separate a quark and enter got something if you try to pull any quarks together any quarks apart know what I'm sorry about it ah let's go through to this point if you if you try to separate the quark and an antiquark as I say we can think of it as a we talk about a string developing between them it so if you like it's a flux tube of gluons that develops between them and when you've stretched that enough so that has the energy to create new quarks it will do now typically the quarks that you would you so you would create a typically our County Clark pair when you do that and then the two sides would separate and Ramez on here and a mess on there now typically the quarks that you would tend to create most easily when you do this are the quarks which have the lightest mass and the ones I showed you with the lightest mass of the up and down quarks so often what would happen is that after down quarks would be created here and then the mess on that you have here that of this quark and antiquark would fall apart into two different mares ons in which an up quark is paired say with the anti quark here and the anti up is paired with this quant that would be a typical scenario but of course different things can happen and do yeah yes modified I'm going to draw a close here Christine I think as I say a second also done done wonderful journey and into a wonderful world and he remains for me to have the pleasure of presenting her with her Rosalind Franklin's certificate and her medal I'm wishing her a wonderful year as this year's rosalind Franklin Prize me I'm thanking her for a wonderful lecture thank you all for coming

Channel: The Royal Society

Views: 78,350

Rating: 4.5963855 out of 5

Keywords: Rosalind Franklin Prize Lecture, Christine Davies, University of Glasgow, Physics (Idea), fundamental particles, universe, Quark (Idea), Royal Society, Julia Higgins, Einstein Year, physics, department of trade and industry, DTI, Patricia Hewitt, scientist, mid-career scientist, women in science, science for women, lectures for children, children lectures, laws of physics, energy, electrons, type of electron, proton, high energy electron, collision

Id: SVC5d2CfEOU

Length: 72min 42sec (4362 seconds)

Published: Tue Dec 10 2013