Samuel Ting (1985) - Search for the Fundamental Building Blocks of Nature

Good morning. I am very happy to be here today. This is the third time I come to Lindau. I come here often for two reasons. First, this is a good opportunity to meet with young physicists and to discuss with them their interest in physics. And second and it's to me equally important is to have an opportunity to meet with other laureates. What I would like to do today is to give you a feeling what is high energy experimental physics about. People often say high energy physics is very expensive and involve a lot of people. And it is not clear what you get out of it. Now what you get out of high energy physics, at least to me, is to have a basic understanding what is the building block of nature. We have been looking for building block of nature for thousands of years, a few thousand years ago people think earth, air, gold as the fundamental elements. At the turn of the century we had the periodic table and we viewed as building blocks of nature as the 100 or so elements. And then electrons and protons were discovered, at that time we viewed the building block of nature as two particles, electron and proton. But subsequently positrons were discovered, muon were discovered, pion were discovered and a host of elementary particles were discovered. And then our concept changed again. We view the building block of nature 100 to 200 elementary particles. In the early '70s through the work of Murray Gell-Mann, George Zweig and others we have the quark model. And then we view the building block of nature as from 3 quarks. From '74 on we view the building block of nature as maybe 5 or 6 quarks with these corresponding leptons. So what is the truth, really it's a function of time. To start with I would give you example of our understanding of the structure of proton. Please switch off the lights and I can now talk in the dark. The fundamental building block of nature is a proton. In the '20s we viewed it as a small object in the heart of hydrogen atoms. In the '50s we viewed it as a large object with pi meson in its vicinity. In the '60s mainly through the work of Professor Hofstadter and others we viewed it as a large object with a structure. It's denser at the centre than at the edges. In the '70s we viewed it as containing many smaller ball-like objects, protons or quarks. Nowadays there are even people who speculate that it may be unstable. At this moment we view the world is made out of quarks, so far 5 have been discovered, u, d, s and c and b. And leptons: electron, mu and tau and 2 neutrinos associated with electron and mu. At this moment we view it as the building block of nature. The forces among the elementary particles are 3 kinds, there's the strong force, the force between quarks that are transmitted by gluons. There's a weak force following the very important work of Weinberg and Salam and others. We view the weak forces transmitted by charged and neutral currents. Example of electron-positron collision at very high energy of close to 100 GeV, indicates the muons would be dominated by the transmission of the Z-zero particle. We also have electromagnetic forces, the force between 2 electrons as transmitted by photons. Now you can ask some experimental questions. The first question you can ask on strong interaction, how many quarks exist? Are you saying at this moment in elementary particle you have u, you have d, you have s, you have a c and a b. Where is the 6th one, 7th one, 8th one? Currently from the work in Hamburg, at DESY, we know the 6th quark, we called it t, you can charge it +2/3, it must have a mass larger than 22 billion electron Volts. The second question you can ask, what is the size of a quark? The current limit for the size of a quark is less than 10^-16 centimetre. One part in 1,000 the size of a nucleus. And there are some more detailed questions you can ask such as, what are the properties of gluons? The difference between gluons and photons, the most striking difference is shown in the following. A photon since the charge conjugation -1, cannot decay into a pair of photons. A gluon, charge conjugation is not a good quantum number, can decay into a pair of gluons, so such as 3-gluon vertex does exist and would be a characteristic of so-called quantum chromodynamics. And such a thing has really not been identified conclusively. It would be very important, your understanding of strong interactions. Some questions, experimental questions, you can ask on electromagnetic interactions, are also very obvious. The first is, how many kinds of heavy electrons exist? We know a high energy photon go to electron pair when the energy is high enough and go to a mu pair and when the energy is even higher it goes to a tau pair which has a mass about 2 GeV. When you have a 100 GeV photon, how many heavier electrons exist? The current limit in Hamburg shows it must be larger than 22 GeV. Next question you can ask, what is the size of an electron? The current limit is again less than 10^-16 centimetre. That's also true for the tau and for the mu. Tau of course has twice the mass of proton but its size is one part in 1,000 of the proton. Next question you can ask, are there excited electrons? We know a pion together with a nucleon goes to N*. Is there an excited electron which can go to electron plus a photon? The current limit is, if such thing exists, its mass must be larger than 70 GeV. Then some experimental question you can ask on weak interactions are the following. The first is, how many kinds of Z-zeros and W's exist? From the work of Weinberg and Salam shows if you use a standard model, mass of Z-zero should be 94 GeV and that has been discovered. Experimentalists can ask the question, is this the only one, could there be more Z's and more doublets? Let me remind you in 1940s when pi meson was discovered most of the physicists thought we have understood everything. And subsequent today, quite a few particles very similar to pion were found. Second question you can ask is, how many kinds of neutrinos exist? We know that leptons, the electron has its own neutrino, mu has its own neutrino, whether tau has its own neutrino or not we have not found the tau neutrino and with more leptons, whether there will be a corresponding number of neutrinos. Another very important question is, do Higgs particle exist which are responsible for the origin of masses? To answer some of these questions, the largest accelerator in the world is now under construction in Geneva Switzerland, let's first define, this is the border between France and Switzerland and this is the city of Geneva and this is the accelerator which has a circumference of 27 kilometres. It's buried under the ground, between 500 metre to about 1 kilometre. There electrons and positrons are accelerated to a centre of mass energy, initially at 100 GeV, finally at 200 GeV. At four intersection regions, number 2, number 4, number 6, number 8, electron positron collide. And during this collision, experiments were set up to answer some of these questions. A particular experiment I want to discuss with you today is experiment in area number 2, an experiment which I am involved in. I want to go over a little bit of the nature of this experiment to give you a feeling what high energy physics is about and what's its purpose. This is the detector of this experiment, its buried 50 metres underground, electron-positron collision occurring here. There is a very precise device known as a vertex chamber which measures decay of elementary particles, surrounded with a device known as electromagnetic detector which measures photon and electron with very high precision. It's a special new kind of crystal Bismuth germanate, otherwise known as BGO, and then with 400 tons of uranium hadron colorimeter. What this device does, is to absorb all the pions, kaons and hadrons and measure its total energy, the coordinate of the energy. What is left are the muons which should be measured very precisely in a magnetic field of 5 kilogauss provided by 1,000 tons of aluminium coil with a return yoke of 8,000 tons. For comparison this is a standard physicist. This experiment is the first large scale collaboration with some physicists from the United States, Soviet Union and the People's Republic of China. Unfortunately it involves a lot of people, involves a lot of people not because one wants to, but because the complexity of this experiment. From the United States they are from MIT, from Harvard, from North Easton, from Yale, from Princeton, Rutgers, Johns Hopkins, Carnegie Mellon, Ohio State, Oklahoma, Michigan, Caltech and Hawaii, about 120 physicists. From Soviet Union, from the State Committee for Utilisation of Atomic Energy, 40 very good physicists working with us, and from the Chinese Academy of Sciences and from the Ministry of Education 40 students. And all the Swiss universities, ETH Zurich, Geneva, Lucerne, are working on this experiment. Physicists from France, from Italy, from Spain, from India, a very good group from Aachen and from Siegen are working with us and from DDR, from Holland, from Hungary and from Sweden, about 150 physicists. Looking from this I think I could not resist to make an observation, it has been easy for me to obtain a collaboration between United States and Soviet Union and between Soviet Union and China, than to have all the Swiss work together, seems to be very difficult. Now with such experiment it's not only physics idea, only instrumentation, you encounter some logistic problems. So these are maps of physicists involved in this experiment. And the total cost is somewhere between 120 to 150 million Swiss francs. What is important besides the physicists and the financial resources are the engineers and technicians who are involved in building such a detector. From the Institute of Ceramics in China, there are 200 technicians and then from Aachen, from Holland, from CERN, from Switzerland, from France and from ITEP 300 technicians, total about 700 engineers and technicians are involved. The contribution from ITEP, from Soviet Union is fairly large. Involves about 20 million Swiss francs and 300 technicians involved in the construction of a 400 ton uranium colorimeter and 3.5 tons of very high purity germanium oxide for BGO and provide 7,000 tons of low carbon steel for the magnets and with equal amount of contribution from the Unites States, Department of Energy and from the rest of the European countries. Now the question you want to ask, how do you design such a detector, what is the criteria you use to design such a detector? It is important to realise in designing a detector of this type involves a lot of people and a long time constant, you had better not designed a detector based on one person's model and one person's theory. Because it is easy for a theoretical physicist to create a new theory and it's much harder for experimentalists to change his detector. So let me report to you on the design consideration. The first thing we decided is, there are many elementary particles you can measure. Whether you can measure all the hadrons or you can measure hadrons plus electrons and what we have decided to do is to concentrate on three particles, photon, electron and mu. We measure them very precisely with a momentum resolution of 1% up to the mass of 50 GeV. What is the justification, the justification is basically intuitive one and by making the observation in the last 30 years or so, some of the most important discoveries in elementary particle physics were done by experiments measuring photon, electron and mu. The work I have done measuring the J particle was made possible by observing a peak, a 3.1 GeV with a detector measure electron pair with a mass resolution of 1 part in 1,000. The discovery of the b quark was made possible by measure mu pair with a mass resolution of 2%. The discovery of the various transition states on charmonium was possible because the detector using sodium iodide has a very good resolution. The discovery of Z-zero by Professor Rubia was done on electron pair and the discovery of W+, W- were done by measuring large momentum transfer muons. Now what you do with hadrons, with pion, kaons and so forth? Hadrons in this high energy tend to come in a bundle, like jets. And so what we do is not to measure them individually, but measure them collectively with a very good resolution of about 0.45% versus square of V. An experiment in 1979 carried out in Hamburg and the discovery of gluon was done with such a simple technique. Theoretically when you have an electron-positron collide you produce a quark which fragments into a jet, anti-quark fragment into a jet, gluon fragment into a jet. And if you measure the total energy, you will see a 3-jet-pattern. And therefore it was not necessary to measure individual particles but measure them collectively. Those are then the two design considerations for doing such experiment. The experiment now is under construction, let me show you a few transparencies how these things are done. And these are 2 large holes where the detector will be lowered into it and the experimental hole is buried underneath. The hole size is 23 metres across. It's 50 metres underground. To build a magnet with the size of this lecture hall is a very simple job. What you do is to build them with, the same way as you construct a house, except you use more steel and less concrete. And so what we would do is just first pour the concrete and then put long 40 metre, 1.2 metre by 10 centimetres, 220 pieces of bars of different shape as a return yoke and the power pieces again are large pieces at the end. The coils are made out of aluminium, the first aluminium pieces and then you use an electron gun to weld aluminium pieces together. After you weld them together you will make them into half turns like this, you have a special crane, you take them up and you store them outside. Of course during this time you make the necessary checks on current and on cooling. More than half of these are already finished. These are about half of the coils. That will be finished in end of 1988 and to start experimentation in the beginning of 1989. So besides the coil, the inside part round the major element is muon detector. To provide a mass resolution of 1% for a muon pair, a mass of 100 GeV means a 45 GeV muon would bend 3.7 millimetres. To have a mass resolution 1% means the alignment for this detector, the mechanical alignment, the resolution for the temperature and the supporting stand must be in the order of 30 microns. And this is not an easy thing because the device is rather big and this is about 12 metre by 12 metre. And you want to know this to 30 microns. And there are quite a few of them and this was worked out at MIT and these are some of the chambers, this is the inner chamber, the middle chamber, the outer chamber, the wires are going through here and you can see the electronics and then the cabling system. A question which is very important for precise measurement is the calibration of your detector, without a calibration of course you will not know where you are. Calibration for muons are provided by N2 laser, which is a laser in here. And then there's a guide for the laser into a movable mirror which on command flips the laser into many positions and then goes through the chamber with the position sensitive diode simulates infinite momentum muons. Without such a thing you cannot really precede. And this shows 1 middle chamber, lower chamber, upper chamber and here is the guide for the laser to go through, for the N2 laser to go through. When the chamber is finished, we fire the laser and see what resolution we will get. With 1,000 shots of laser we measure a straight line to 50 microns. That means for giving start that delta pi/pi ...(inaudible, 23.40) 3,800 micron, which for an individual shot with 50 microns means delta pi/pi, so 1.3% ,which means delta m/m is better than 1%. For 1,000 shots a centre is known as 50 versus squared is 1.6 microns, that means the centre is now known to 1.6 micron even though the distance is order of 12 metres by 12 metres. Beside the muon chamber, inside is a 4-pi-hadron calorimeter which provides the energy resolution of 50 versus squared V, also measured the collective information in jets to two degrees and also enable you to track the muons go through the detector. This large hadron calorimeter is being constructed, involves the First Institute of Physics in Aachen and in Soviet Union. What is the principle of hadron calorimeter? What you need to do is to put very dense material, let the hadron go through and let it interact, loss of its energy, you measure the total energy. And so the construction with 144 elements of uranium plates sandwiched with detectors which measures the charged particles. It's divided into 9 rings, each ring is divided again into sectors, in total 144 sectors, 16 sectors in each ring. Here is the construction map how the hadron calorimeter and the electromagnetic calorimeter are being built. The uranium plate is made somewhere deep inside Soviet Union and then the support rings again made in Soviet Union. The raw material for germanium oxide is near the Black Sea and they're all shipped to Moscow and then go to Switzerland. These are one of the 144 uranium plates and this uranium is of very high quality, very flat, to correct size, 60 pieces together made to hadron. Between the uranium you have ... (inaudible 26.15) at ITEP, where this calorimeter is now being constructed. The next item when you go from outside to inside is a device to identify the electron. The resolution, image resolution of the electron is done with this new crystal, BGO. The pi rejection from this BGO has been measured to be between 10^-3 to 10^-4. When you identify the electron or measure photon, one of the very important things is to reject pi0 to two-photon, a so-called ... (inaudible 26.49) pair. And that you do by the vertex chamber, which we will call TEC, which measures the opening angle. The raw material for this crystal, as I have said before, comes from Soviet Union. The people who are very good in growing this crystal, is not in Western Europe or not in United States but is in the People's Republic of China in the Institute of Ceramics in Shanghai. For whatever reason I have discovered the shortest way between Soviet Union and China is via Switzerland. So we have then set up a factory in Shanghai involving about 200 physicists under Professor Ying, a very well-known crystallographer involving 11 research staff, 20 engineers, 26 technicians, 92 workers and 50 administrative people, probably party members. And these are some of the very good crystal growers in Shanghai Institute of Ceramics which are making the 12,000 pieces, close to 10 pounds of germanium oxide for us. And these are some of the crystals. Its 24 centimetres long, 2 centimetres by 2 centimetres at one end, 3 centimetres by 3 centimetres at the other end. And we have carried out a world-wide competition and they are the ones in Shanghai produce the best crystals. When the crystals arrive we put them into the beam, measure its response and this is the Van-der-Graaff, 4 MeV Van-der-Graaff accelerator located in Lyon, produced from radioactive capture process, 20 MeV gamma rays and then we measure the response in this box which we located BGO crystals. And this shows when you have a photon and 20 MeV you have a resolution of 7.7%, this is a low energy Compton scattering which you have to reject. And from the centre to the upper peak you see a 27.7. What does this mean? This means when you deal with large quantities, with large quantities of sodium iodide, like the crystal ball experiment, or large quantities of BGO, you obtain the following comparison. In terms of full-width half-maximum these are the measurement of BGO and this is the measurement of sodium iodise. For individual crystals I think sodium iodise resolution is better, but when you put them together they are somewhat compatible resolution. The difference is, BGO is non-hygroscopic so you can handle with your hand and is denser and more compact together. When you go from outside to inside, beside the muon chamber hadron calorimeter BGO crystal and finally at the end you measure the particle vertex and the vertex is done with many physicists from Aachen, from CERN, from Swiss Institute for Nuclear Reactor Research and University of Geneva, from Siegen and from DDR and also from ETH Zurich. The principle, like all these things, is very simple and can be visualised in the following way. In an ordinary particle detector you have a ground, you have a negative high voltage, you have an electric field. And so you could guess when a particle goes through, it loses energy by ionisation and so you have a cluster of electrons which then drifts into your anode. Normally the first arrival gives the signal because the electronics is not fast enough. And therefore you have a very large fluctuation. To obtain high precision, what we have decided to do is in this detector we're putting a grid. Reduce the drift region velocity by a factor of 10. In the amplification region you keep the original velocity. Because your velocity now is by a factor of 10 lower so your electronics have enough time to identify not only the first arrival but the second arrival, the third arrival, the fourth arrival. So you have a complete history of all the clusters. With this then you will be able to get all the information of the history of passing through the particle in this chamber and therefore a very good resolution. Of course to obtain a good resolution your detector, the mechanical part of your detector has to have a compatible resolution. And this is the model, a full scale model that is made in Zurich, When you put it into an experimental test beam in Hamburg you will see at the mean length drift of about one cm, if it's 2 atmosphere, it's 30 micron resolution, if it's 1 atmosphere, 30 micron resolution, if it's 2 atmosphere, it's 25 micron resolution. These are very large device and is not very easy, in fact I think it is the first time people have done that with such a large device to obtain a 25 to 30 micron resolution. What is more important is this device has a property of simultaneously identifying many particles. And this shows what happens in one burst, you have 3 particles go through the chamber. When 2 of them are separated by 230 microns you clearly can distinguish them. So much for the detector, the next item I would like to discuss is computers. With so many physicists involved, the first thing you have to do is to make sure people analyse the data and communicate with each other. And so have to establish their own computer net from all the physicists in the United States and all the physicists in Europe, from different countries and they are linked together. Now you will ask, what is the difference between this detector and the 3 other detectors now that's been built at CERN and 2 similar detectors built in California where they have a 50 GeV LINAC collide with a 50 GeV LINAC. So I want then to discuss a little bit of the unique physics which this experiment, which we call area 3, is not covered by three other labs and two single path collider detectors. All these other 5 detectors are very good detectors involving very advanced technology but mostly concentrate on identified pion, trion and protons, hadrons or these different measures electron, muon and photons. So let me give you three examples. First, at energy of 100 GeV: which then decays like a charmonium state to a single 200 mu photon, plus a P state decays to many hadrons. Your crystal can identify the single photon as a clear peak. Now what happen when the accelerator let's say go to 180 GeV? if you do experiment of electron positron, go through Z-zero + Higgs particle which is responsible for the original masses and of which the decay property is not known. What you can do is to measure the Z-zero decays to mu+ mu- or e+ e- and therefore identify the missing mass peak of the h particle. Because the good resolution for muon and electron you identify 20 events for muon, if the Higgs is 50 GeV and 43 events for the electron because acceptance is larger for electrons, you can clearly see sharp peaks. This is a very important example because you are able by measuring mu pair and electron pair to look for particles which property you really do not know, therefore there's no way for you to design a detector to identify this. So you have to use the missing mass technique. There are some plans in Hamburg and at CERN to use the left tunnel to higher energies to do a proton-anti-proton or even proton-proton collide. If such a thing is visualised, for example you can put a 5 GeV anti-proton and 5 GeV proton collision. In such a collision this detector, without modification will provide the following properties. For a particle of mass 1 GeV when you decay to electron-positron pair you will provide a mass resolution of ½%. For this 1 GeV particle to decay to a mu pair you will provide a mass resolution of 10%. Except this time, because the precision of the muon detector, you can measure the charge asymmetry, therefore locate what is the original property of this 1 GeV particle. There are many theories now, saying the next mass scale is about one TeV. For hadron jets again, you can measure mass to 3%. Indeed we have carried out some study already to see if you have a pp bar, produce a heavier Z-zero, a mass of pp bar collision at 10 GeV. And then if you produce a Z-zero, at maximum GeV, you will get 100s of 1,000s of events and clearly can be identified. Now if you would view it based on our understanding of theory, to view the purpose of this detector, you can view it in the following way. Our current theory on elementary particle physics is based on 2 fundamental principles. One is gauge invariance, another is symmetry breaking. Gauge invariance leads to quantum electrodynamics, quantum chromodynamics, the theory of Weinberg and Salam, electroweak theory and this has been tested by many experiments: T-2 experiment, electron-positron to mu pair, gluon jet, neutrino scheduling, muon scheduling, the discovery of z and w. Indeed all the experiments that's done up to now at CERN, in Chicago, at SLAC, at Brook Haven, all involve tests of QED, QCD and electro weak. Clearly with new accelerator you can look for more quarks, more Z-zeros, more Ws, study three-gluon vertex. And clearly more tests are necessary and is very important. But what is more important is understanding, at least to me, understanding of symmetry breaking. Symmetry breaking is thought to be responsible for masses of all elementary particles. So far there is really no experimental tests. There are many predictions, Higgs particles, supersymmetric particles, technions, technicolour particles. I'm sure by the time this detector is finished and there will be many, many more predictions. By designing a detector measuring photons, electrons, muons precisely the main aim is to try to find by missing mass technique any of these particles which we don't have to know its property, you do the missing mass technique and therefore try to understand the origin of masses of all particles. Thank you. Applause.

Samuel Ting (1985)

Search for the Fundamental Building Blocks of Nature

Samuel Ting (1985)

Search for the Fundamental Building Blocks of Nature

Comment

For his third lecture at one of the Lindau Meetings, Samuel Ting had chosen a title which he in principle would keep as a running title for a total of three lectures, 1985, 1988 and 1991: “Search for the Fundamental Building Blocks of Nature”. This title, as Ting explains in his introduction, is the driving force of his continued work in high energy elementary particle physics. The main reward of the costly experiments he performs is a better understanding of these building blocks. Before the quark model appeared, there were hundreds of particles in what looked like the periodic table of elements. Then the quark model brought this number down with a factor of about ten and brought with it something very similar to the understanding of the periodic table of elements through Rutherfords discovery of the atomic nucleus and Bohr’s model of the atom. In a pedagogic way, Ting follows our view of the proton from the small object of the 1920’s, through the large object of the 1950’s, the large object with structure of the 1960’s to the large object built of point-like quarks of the 1970’s. As he points out, in 1985 there were already on-going experiments to determine if the proton can decay. After listing some other open questions, e.g., how many different kinds of heavy electrons and neutrinos exist, Ting then moves on to his main theme this year, the construction of the new 27 km accelerator ring LEP at CERN and its detectors. In this ring, electrons would circulate one way and positrons the other. In certain places the two beams were brought to collide and Ting himself is involved in a collaboration building one of the huge underground detectors at such a collision point. The collaboration consists of an international team of several hundred physicists and technicians from all over the world. Ting spends considerable time on the design of such a detector and even goes into some technical detail.

Anders Bárány

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