AUSTRALIAN FRONTIERS OF SCIENCE, 2005
Walter and Eliza Hall Institute of Medical Research, Melbourne, 12-13 April
Exploring the origin of the universe
Dr Elisabetta Barberio, School of Physics, University of Melbourne
Introduction by Professor Geoff Taylor (Session chair) High energy particle physics is the study of the most fundamental particles we know of in nature, and their interactions. This session will have a talk from Elisabetta Barberio about where we are right now, what is interesting and where the goals are in particle physics, and another from Lyle Winton, on one of the aspects of an application of particle physics.
Particle physics is based on accelerator laboratories. It is big science, using very expensive equipment. It involves necessarily very strong collaboration with hundreds and, nowadays, thousands of physicists on one experiment. So it is a different way of doing things from many of the sciences that you have heard from today and yesterday. The collaborations are extremely big, but within collaborations there is plenty of room for people to find particular projects.
The Australian program is very strong – it has gained considerable strength over the last decade or so – with fairly major experiments and major participation at CERN, in Geneva, and at KEK, the national laboratory in Japan. We are looking towards the next generation of experiments at what is now called the International Linear Collider. The time scale for such things extends into decades, so we are looking already at a machine which we don’t expect to run until 2015.
‘High energy’ comes in because we want to get down to very fine scales, and energy is related to the wavelength at which you are sensitive to things. Einstein would also tell you that high energy, then, is related to creating mass you can create mass from energy so in going to high energy we are creating very heavy particles as well. In fact, as Elisabetta will show, this leads to a discussion of the very early universe, where we know that the average energy for particles was extremely high. So, in fact, the study of the very small now is telling us in great detail what happened in the first fraction of a second after the Big Bang. That is the subject that Elisabetta will be covering.
This is not just a knowledge-based region of science. In fact, the costs are high enough that no politician would bet his life on just finding out what happened 10 to the minus seconds after the Big Bang. Governments all over the world know very well that there is not just the training, which is a very important aspect of particle physics, but also spin-off. You heard yesterday a couple of the direct applications, very direct examples of spin-off – positron emission tomography and the synchrotron. These things would not have existed without particle physics. That is just a statement of fact.
You would all be very used to using the internet. The World Wide Web was generated by particle physics back in the early 1980s, or even the late ’70s, to handle these sorts of collaborations. The World Wide Web now is much broader than particle physics, and Lyle is going to talk to us about the extension of the World Wide Web into what is called the Grid, another direct application which is also finding very great application in other areas of science.
We will hear first from Elisabetta Barberio. Elisabetta is a lecturer at the University of Melbourne now. She has been in Australia for a little over a year. She has a very strong international reputation in experimental particle physics, with programs at CERN, in Geneva. She has worked in Europe, the US and now Australia; she is also involved in the Japanese program. Elisabetta will tell us about particle physics and how it relates to our view of the universe.
I am going to explain to you a
little bit about what we do, the motivation for it, and how we started.
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When we speak about particle physics, people always think about the elementary constituents of our matter. During the last century, ideas of the elementary constituents of matter have changed. At the beginning people believed it was the atom, but later they discovered that the atom was composed of particles that are electrons and there were nuclei. And eventually people discovered that the nuclei were made of smaller particles. And these particles were the quark.
To probe what was really the elementary constituents of matter, we needed to build ‘accelerators’ of particles, taking particles and smashing them together to see what is inside. From doing that, it turned out that we could reproduce the energy density and the condition that existed in the early universe. In fact, when we speak about what is the elementary particle, we have a zoology, we have so many particles that existed at the beginning but do not exist any more. They had a role, and they had a role in shaping the universe as it is now.
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Particle physicists are a sort of people that create time machines to go back to what happened at the beginning, soon after the Big Bang. The quark and lepton domain is where particle physicists are, in the region of 10-10, 10-6 seconds, soon after the Big Bang. And here at three minutes, where it is written nucleo-synthesis, is when we stop and astrophysics starts. So we are a sort of astrophysics that tries to see a little bit beyond where they stop and they cannot see any more, whatever telescope they have.
Shown here is the experiment I have been working on for many years. Some of these experiments are very complicated and they run for a long time, because we really need to take advantage of time. We have been exploring the time soon after the Big Bang, which was a very fundamental and important discovery – more than a discovery, it made the theory that we have to explain what happened there on very solid ground. However, we want to go a little bit further to see why we had the Big Bang and why we are here. That is the experiment that we are building and that is going into operation in 2007.
This time that, for us, corresponds to energy has not been chosen by chance, just because we can go a little bit further. It has been chosen because particular things happen in this particular regime that can explain one of the fundamental puzzles we have had up to now.
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I will show you here a little bit of our zoology of the particles that we know exist. The matter particles that we know exist are divided into two types. We have the quark type, the one that includes, for example, the proton or the neutron. And after that we have the lepton type, particles like the electron. And each kind of type always goes in pairs. The electron is always associated with a neutrino; the neutrino has no charge and the electron has a charge. And I have three families of each. These two other lepton types of particle are similar to the electron; the only difference is that they are much fatter, they have more mass. It is similar with the families of the quark type.
However, if I go and look at what is the particles that really compose the matter that we know, it is just the quark family. It is just the quark composed of the proton and neutron that comprise the atom, and the electron and the neutrino. The other particles in this table do not exist any more in our world. They existed at the beginning of the universe but now they don’t. They don’t play a role.
However, why they existed at the beginning, why they were so important and why three, we don’t know. This is one of the things that we are trying to address with this new machine.
We have a theory to know why they disappeared at a certain point and we ended up only with the particles in the first column on this slide. And the reason is that they lived for a very short time.
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Another part that is quite important to know about what happened at the beginning of the Big Bang is to understand the real nature of the force. Four fundamental forces govern all of nature. One is the gravity that everybody knows. There is also electromagnetism, which again everybody knows – it governs the electricity and everything we know, but it has also quite a bit to do at the level of our atom. We have the strong force that is responsible for the protons, even if they have the same charge, being together in the nucleus of the atom. And after that we have the weak force, which is a force of which the only manifestation that we have right now in our world is due to radioactivity. When an element is radioactive, this is the responsibility of the nucleus.
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When we speak about force in our world, in the particle physics world, forces are carried by other particles. They have a transfer. They communicate. Matter particles communicate between themselves through carriers, through the force particles. For example, in the case of the photons, this is the particle that carries the electromagnetic interaction.
In our world, also, we measure everything in electron-volts. This is the conversion to joule. Just to remind you a little bit: the order of magnitude of a normal battery that you put somewhere is just one electron-volt.
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Our fundamental forces – I will not discuss at the moment the gravitational force, because it is too weak with respect to the other forces at our microscopic level – are mediated, we say are transferred, by particles. The electromagnetism, the current that you see everywhere, is mediated, transferred, by the photon. The weak force is mediated by two particles that are extremely heavy, 80 GeV and 91 GeV (giga-electron-volts). So you can imagine how much heavier they are with respect to a battery. Just to make a comparison with particles, or with the matter that everybody knows, this particle is as heavy as a silver atom. So we have a fundamental particle that is as heavy as an atom.
Later we have the strong force that kept together the quarks and the protons inside the nuclei, and this is mediated by other particles that have mass. (Zero mass is the default.) The intensity of the force changes. The strong force is the strongest that we know, the weak intensity is much less than the electromagnetic interaction.
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However, even if these forces are so different, we do believe that in reality they are just a low energy manifestation – a what is happening today manifestation – of a single force that just in the condition of the universe today seems to act differently. In fact, during the years, if we go back to the time of the Big Bang, the first ramification is that we realise that magnetism and electricity are in reality the same force. It is called electromagnetism. And here you see the Maxwell equations were put together.
In the 1960s physicists also realised – and they got the Nobel Prize for it – that the weak interaction that had seemed so strange because it is responsible for the decaying of nuclei, and the electromagnetism that is responsible for our light, are indeed the same force. So we believe that most probably, if we go back far enough in time, maybe we can put the strong force together with the weak force that we call now electroweak. And maybe even the gravity can go back.
So in these new experiments we are really are choosing where to go, because we want to study whether it is true that we can put these kinds of things together. However, to do this we need to have a different thinking from what we have had up to now.
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How the electroweak unification works – just to start on one unification: in 1976 Glashow, Salam and Weinberg proposed that the weak forces and the electromagnetic forces were indeed the same. That means that if I go a very short distance, 10-18 metres, the strength of the weak force, which you saw was so weak at our scale, is exactly the same as the electromagnetic. But as soon as you go just a little distance further, 10-15 metres, where it is typical of a quark or a proton, the weak force is much, much smaller than the electromagnetic. And that this is true has been verified to a precision of 10-3, or one part per million, by my previous experiments.
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Why is this true? Let’s understand why. The reason is that indeed the weak force and the electromagnetic force have the same strength. However, the carriers of the force are these particles. But while the photon has no mass, the other particle, the mediator of the weak particle, is a very heavy particle, the W and the Z. And this is giving the huge difference through the fact that, since it is very, very large, its range is very, very small. It is a fact that the heavier the particle, the lower is the range that it can travel. And so the strength of interaction of a force depends on the mass of the carrier of the force.
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We understood this, but there is one thing that we cannot explain and is a big puzzle: why do these particles have mass? Why does the photon have no mass and these other particles which are really mediating the same strength have a huge mass? They are as heavy as a golden or a silver atom. We don’t know. This is something we don’t know. We have an hypothesis, introduced by Peter Higgs, that there is a sort of Higgs field, like an electric field, that is generated by a particle in the same way as an electric field is generated by a charge, and this particle is the Higgs.
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How do these things create a mass? Well, suppose you were a gathering of scientists chattering quietly. And at a certain point there is a well-known scientist coming into the room. People start chatting with him and try to speak with him. So what happens? A lot of people gather around this scientist and it will be very much more difficult for him to go through the room. So this physicist is acquiring a mass. And this is more or less the mechanism I have. As soon as the Higgs comes in, there is something that happens, a clubbing together, so the particles acquire mass.
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We haven’t yet found this particle. There are limits. We know that for our theory to work with electromagnetism, unification must be below one tera-electron-volt, 1012 electron-volts. And we know that it cannot be below this either. So we need to build the machine that we are building now, the Large Hadron Collider.
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However, there is another point. First I should explain that when we speak about energy scaling going up, or going closer in the problem, it means also going back in time, closer to the Big Bang. If we use the electroweak unification theory as it is now, when we go back we see that the strong force doesn’t cross – doesn’t unify – any more with electroweak and electromagnetic. And as physicists we find that a little bit uncomfortable.
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So a revolutionary idea arose. We divide particles into mass particles and force particles. However, suppose we introduce a symmetry in which a mass particle can transform itself, can have a partner that is a force particle, and vice versa. Then things change a little bit. We introduce a symmetry in which, to our particle that we know, we have an associated ‘shadow’ particle that has the same characteristics as a force particle. Then for each force particle there is a shadow particle that has the characteristics of the mass particle.
This symmetry is called supersymmetry. What happens if we introduce that?
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Well, something very nice happens in something very nice: at a certain point, if we really do the calculations correctly, the strong force can unify. If we go back close in time, they meet together. And the point when it is changing from the difference, when something is happening to show us that indeed this is happening, is exactly at the energy of the machine we are going to build.
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The other thing we really like about this supersymmetry is related to astrophysics, to astronomy. It is called dark matter. We know from observation, from the fact that weak matter held the galaxy together, that most of the mass of the universe is probably not ordinary matter. We cannot see it, but we know it is there. There are some astronomical objects as candidates, like a black hole, a neutron star, a dwarf. However, they are not enough and may not have the characteristics that we are really looking for.
We particle physicists have many candidates, and one of the supersymmetric partners may be a candidate. It has exactly the characteristics of something that we call dark matter – a matter, something that is in the universe, that corresponds to almost 70 per cent of the universe but cannot see it. So reproducing in our time machine this kind of supersymmetric particle may also allow us to study in the lab something that composes most of our universe and we have no means to study elsewhere.
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So how do we produce this condition? The only way we know at the moment is to accelerate a proton beam to 7 TeV. Just to give you another analogy to the battery, it is 7012 electron-volts, which means that, to create this energy, if I associate a battery with each star of our galaxy I will need seven times our galaxy to produce this kind of energy. That just gives you a little bit of an idea.
And what we do is to smash the protons together, and the quarks inside the protons will start smashing together. And since energy equals mc2 – Einstein’s equation – I can produce mass, and I can produce these particles that existed at the beginning of the universe, and study how they interact between them.
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How do we accelerate? We accelerate protons in a ring. This ring exists already; this was an older machine that accelerated electrons to do precision measurements. We are in Geneva at CERN, in Switzerland. This picture is of a tunnel of 27 kilometres circumference that exists 100 metres underground.
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This photograph shows what you see above the ground, including the laboratory. We have dismantled this machine that we had before, this big ring, and we are going to fill it up with superconductive magnets to accelerate protons so they will smash together.
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This slide shows what it looks like: this is our tunnel and this is our machine. We need to fill this with an awful a lot of protons if we really want to observe the matter of interest.
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We send protons around and later we have different points where these protons collide and we have our apparatus to study what is happening. This experiment, ATLAS, is what we are dealing with.
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Now, how does it happen? We have these protons colliding. They start colliding, they arrive in bunches, all together, because we need a lot of protons. Each proton will collide and will produce a new particle, for example a Higgs particle, if there is enough energy to produce the mass. Those kinds of particles, as I say, don’t leave very much. The only thing we can see is their decay products, and their decay products, when they disintegrate, are ordinary matter. So we know how to see them and how to study them.
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This is how we will make our particle. We will produce electrons and see what the electrons look like. Our apparatus is like an onion, with different kinds of layers. We have an inner part in which we will try to see charged particles, which leave a sign, and then we have another kind of detector called calorimeters, which will try to measure the energy of these particles. And so we will try to absorb all the energy to stop them.
Then we can do the same with photons. Photons will just leave energy and no tracks. Protons and neutrons will do the same, will leave something in the calorimeter, but the protons will be seen in the inner tracker and this calorimeter, and the neutrons only in the calorimeter. And then we will have particles like the muons that escape, they just leave a track everywhere, in this case in the detector. And the neutrinos are not seen by anybody.
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Since the energies that we are talking about are very large, we need a very large apparatus to collect everything we need. This is our ‘beast’, as I call it – this is our detector. It is a detector that is 22 metres high, 44 metres long and 7000 tonnes heavy. (The photograph shows the size of a human being with respect to this beast.) The collisions will happen in one area, particles will come out further along and then we’ll try to put together all the information in order to study them.
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To build this apparatus, as Geoff Taylor said before, is a worldwide enterprise. We say that on the ATLAS collaboration the sun never sets. There are 34 countries, 151 institutions, about 2000 collaborations.
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Australians are doing quite an important part of this detector – that is, the inner part, where we see the track of the charged particles. It is a silicon detector.
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Our component is very small. We are going to build this huge beast and it is down to a very small component, just a few centimetres. It is a silicon component. And here you see our spokespersons who came and visited our lab.
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There is another complication in trying to see the Higgs. When we have these collisions, many other events happen. And the probability that one of these events produces a Higgs is one over 1012. So it is very, very rare to see the Higgs.
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And the challenge is also debt. This is a very complicated event. We need to produce 10,000 billion events to produce one particle of the Higgs, for example, the event that we are interested in. And afterwards we need to dig it out from this mess. So this is really the problem that we have, and this has produced a lot of technical challenges.
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For example, because the experiment is big we are pushing these events every 25 nanoseconds. The particle travels at the speed of light. But in these 25 nanoseconds the particle travels only seven metres. So while the particle tries to go out of the detector and do what it should do, another particle comes in. So we must synchronise all the other parts to be sure that everything is kept in account.
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You see here a little bit about the numbers that we are dealing with. Every 25 nanoseconds we will be taking out 40 MHz of information. Suppose that every person on earth – women, men, infants – does 20 telephone calls at the same time; that is the rate at which we need to drive to our detector in 25 nanoseconds. The amount of information we have is terabits of data flow, and in each event where we must dig out a Higgs we have 100 particles per bunch crossing – it is a huge thing – so we must have a power reduction to find out what events are really there.
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We have the means to do that. I have no time to go into detail, but this is something I have been involved in the design of what we call the trigger, how to select events early enough that you won’t get this megaflux of data. There is no way that we know of, or no existing technology, by which you can just record 40 MHz of information. So we need to dig out and to find out what we need.
We decided to do different stages. We took first a decision to do the first screening, then we do a second screening, and then we have the third screening. So for the first screening we have only three microseconds; that is enough. For the second screening we can do much better, we have milliseconds. And for the last screening we can go at the level of seconds. The idea is to go from this mass of information of 100 Hz, which is something we can do. And we have some technical challenges there.
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Here is the state of the art for our beast. Here is the cavern where he will go, 100 metres underground. This is part of the detector that is getting built – the first coil of the part is something like 10 or 15 metres. This is the status in December. There is a web page where you can have a camera and they show the status of the construction. And on a platform here are some people, so you can see how huge it is and how it will fill up the cavern.
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The operation of this experiment is expected to be in 2007. This is a very exciting time for particle physicists, because the current theory rises mainly from experiments and we have tested it to a very, very accurate level. However, many questions need to be answered, and this machine can answer and shed light on the beginning of the universe. Whatever is the outcome, even if we don’t see anything of the particle we expect, we will have terrific information. It means that we didn’t understand anything.
