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Gamma rayMail (armour)MeasurementBecherwerkMorningPhysicistParticleMeeting/Interview
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Ground (electricity)YearWoodturningBahnelementMeeting/Interview
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Transcript: English(auto-generated)
00:16
Good morning. I am very happy to be here today.
00:21
This is the third time I come to Landau. 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.
00:42
And second, and it 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.
01:07
People often say high energy physics is very expensive and you may be a lot of people and it is not clear what you get out of it.
01:22
Now, what you get out of high energy physics, and this to me, is to have a basic understanding what is the building block of nature. We have been looking for a building block of nature
01:41
for thousands of years. A few thousand years ago, people think earth, air, and gold are the fundamental elements. At the turn of the century, we have the periodic table and we view the building block of nature
02:00
as hundreds of those elements. And then, electrons and protons were discovered. At that time, we knew the building block of nature has two particles, electron and proton. Subsequently, the positron was discovered,
02:22
muon was discovered, pion was discovered, and a host of elementary particles were discovered. And then our concept changed again. We view the building block of nature a hundred to two hundred elementary particles. In the early 70s, through the work of Murray Gell-Mann,
02:44
George Zweig, and others, we have the quark model. And then we view the building block of nature as from three quarks. From 1974 on, the building block of nature
03:01
has maybe five or six quarks with its corresponding leptons. And so, what is the truth? It really is a function of time, of our understanding of the structure of protons.
03:25
Please switch off the light and I can now talk in the dark. One of the fundamental building blocks of nature is a proton. In the 20s, we view it as a small object
03:42
in the heart of a hydrogen atom. In the 50s, we view it as a large object with pi mesons in its vicinity. In the 60s, mainly through the work of Professor Hofstadter and others, we view it as a large object with a structure.
04:01
It's denser at the center than at the edges. In the 70s, we view it as containing many small point-like objects, protons, or quarks. Nowadays, there are even people who speculate that it may be unstable.
04:22
At this moment, we view the work is made out of quarks. So far, five have been discovered. U, D, S, and C, and B. Leptons, electrons, mu, and tau,
04:42
and two neutrinos associated with electrons and mu. This, at this moment, we view it as the building block of nature. The forces among the elementary particles are three kinds. There's a strong force, the force between quarks,
05:01
that's transmitted by gluons. There's a weak force, follow the very important work of Weinberg and Salaam and others. We view the weak forces transmitted by charge and neutral currents. An example of electron-positon collision at very high energy of close to 100 GeV,
05:22
GK2 muons will be dominated by the transmission of the Z0 particle. We also have electromagnetic forces, the force between two electrons as transmitted by photons. Now you can ask some experimental questions.
05:42
The first question you can ask on strong interaction, how many quarks exist? And you think, at this moment, in an elementary particle, we have a U, you have a D, you have a S, you have a C, and a B,
06:02
and where's the sixth one, seventh one, eighth one? Currently, from the work in Hamburg, at Dege, we know the sixth quark, if we call it T, if the charge is two-thirds, it must have a mass larger than 22 billion electron volt. Second question you can ask, what is the size of a quark?
06:24
The current limit is the size of a quark is less than 10 to the minus 16 centimeter, one part in a thousand the size of a nucleus. And there are some more detailed questions you can ask, such as what are the properties of gluons?
06:42
The difference between gluons and photons, the most striking difference is shown in the following. A photon, since a charge conjugation minus one, cannot decay into a pair of photons. A gluon, charge conjugation is not a good quantum number, can decay into a pair of gluons.
07:03
So such a three-gluon vertex does exist, and it will be a characteristic of so-called quantum chromodynamics. And such a thing has really not been identified conclusively. It will be very important in our understanding of strong interactions.
07:21
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 goes to electron pair, when the energy is higher enough, and goes to mu pair, and when the energy is even higher, it goes to tau pair,
07:43
which is 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,
08:01
so what is the size of an electron? The current limit is, again, less than 10 to the minus 16 cm. That's also true for the tau and for the mu. Tau, of course, has a twice massive proton, but its size is one part in a thousandth of the proton.
08:21
Next question you can ask, are there excited electrons? We know a pion together with a nucleon goes to n star. 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.
08:45
And some experimental questions you can ask on weak interactions are the following. The first is, how many kinds of zeros and w's exist? From the work of Weinberg and Salaam, so if you use the standard model, mass of zeros should be 94 GeV,
09:03
and that's been discovered. Experimentalists can ask the question, is this the only one? Could there be more z's and more w's? Let me remind you, in the 1940s, when pion was discovered, most of the physicists thought we have understood everything.
09:20
And subsequently, 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 the lepton's electron has its own neutrino, mu has its own neutrino, whether tau has its own neutrino or not, we have not found a tau neutrino,
09:41
and with more leptons, whether there will be corresponding number of neutrinos. Another very important question is, do Higgs particles 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.
10:06
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 is a circumference of 27 kilometers. It's buried under the ground, between 500 meters to about 1 kilometer.
10:26
There, electrons and positrons are accelerated to a center of mass energy, initially at 100 GeV, finally at 200 GeV. At four intersection regions, number two, number four, number six,
10:44
number eight, 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 two,
11:03
the 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 its purpose. This is the detector of this experiment.
11:22
It's buried 50 meters underground. Electron-positon collision occurred in here. There is a very precise device known as a vertex chamber, which measures decay of an elementary particle. Surrounded with a device known as electromagnetic detector,
11:46
which measures photon and electron with very high precision, it's a special new kind of crystal, bismuth germanite, otherwise known as BGO. And then, with 400 tons of uranium-hydron colorimeter,
12:05
what this device does is to absorb all the pions, kaions, and hydrons and measure its total energy and the coordinate of the energy. What is left are the muons, which we measured very precisely in a magnetic field of five kilogauss provided by a thousand tons of aluminum coil
12:27
with a return yoke of 8,000 tons. For comparison, this is a standard physicist. This experiment is the first large-scale collaboration between physicists from the United States,
12:43
Soviet Union, and the People's Republic of China. Unfortunately, it involves a lot of people. It involves a lot of people not because one wants to, but because of the complexity of this experiment. From the United States, we are from MIT, from Harvard, from Northeastern, from Yale,
13:05
from Princeton, Rutgers, Johns Hopkins, Carnegie Mellon, Ohio State, Oklahoma, Michigan, Carnegie Mellon, Caltech, and Hawaii, about 120 physicists. From Soviet Union, from the State Committee for Utilization of Atomic Energy,
13:24
the 40 very good physicists working with us. And from the Chinese Academy of Science, from the Ministry of Education, 40 students. And all the Swiss universities, ETH Zurich, Geneva, Lausanne, are working this experiment.
13:43
Physicists from France, from Italy, from Spain, from India, a very good group. From Aachen and from Sagan are working with us. And then from DDR, from Holland, from Hungary, and from Sweden, about 150 physicists.
14:02
Working from this, I think I cannot 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, and then to have all the Swiss work together.
14:22
Seems to be very difficult. Now, with such experiment, it's not only physics idea, only instrumentation. Then you encounter some logistic problems. So these are maps of physicists involved in this experiment,
14:41
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.
15:03
And then from Aachen, from Holland, from CERN, from Switzerland, from France, and from ETAP, 300 technicians. Total, about 700 engineers and technicians are involved. The contribution from ETAP, from Soviet Union, is fairly large.
15:23
ETAP involves about 20 million Swiss francs and 300 technicians, involving the construction of a 400-ton uranium calorimeter, and three and a half tons of very high-purity germanium oxide for BGO, and provides 7,000 tons of low-carbon steel for the magnet.
15:45
And with equal amount of contribution from United 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?
16:01
What is the criteria you use to design such a detector? It is important to realize, in designing a detector of this type, involves a lot of people and a long-time constant, you have better not design a detector based on one person's model and one person's theory,
16:25
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 considerations.
16:42
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,
17:01
but 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 an intuitive one, and by making the observation in the last 30 years or so,
17:21
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 at 3.1 GeV with a detector measure electron pair
17:41
with a mass resolution of one part in a thousand. The discovery of the B core was made possible by measure mu pair with a mass resolution of 2%. The discovery of the various transition state and harmonium was possible because the detector using sodium iodide has a very good resolution.
18:03
The discovery of Z zero by Professor Rubia was done on electron pair, and the discovery of W plus, W minus was done by measuring large momentum transfer muons. Now what you do with hadrons, with pion, kaions, and so forth,
18:24
hadrons in this high energy tends to come in a bundle like GX, 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 root of E.
18:43
An experiment in 1979 carried out in Hamburg on the discovery of gluon was done with such a simple technique. Theoretically, when you have an electron-positon collide, you produce a quark which fragments into a jet, anti-quark fragments into a jet, gluon fragments into a jet, and if you measure the total energy,
19:04
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 experiments. The experiment now is under construction.
19:22
Let me show you a few transparencies, how these things are done, and these are two large holes where the detector will be lower into it, and the experimental hole is buried underneath. The hole size is 23 meters across.
19:40
It's 50 meters underground. To build a magnet 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.
20:03
And so what we will do is just first pour the concrete and then put long, 14 meters by 1.2 meters by 10 centimeters, 220 pieces of bars of different shapes as a return, yoke, and the pour pieces, again, are large pieces at the end.
20:23
The coil are made out of aluminum. The first, aluminum pieces, and then you use an electron gun to weld aluminum pieces together. After you weld them together, you will make them into half turns, like this,
20:42
and 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 this are finished. This is half, about half of the coil.
21:02
That will be finished in the end of 1988, and to start experimentation and beginning of 1989. So beside the iron, the coil, the inside part, one of the major elements is muon detector. To provide a mass resolution of 1% for a muon pair, a mass of 100 GeV,
21:31
means a 45 GeV muon would bend 3.7 millimeters. To have a mass resolution of 1% means the alignment for this detector
21:45
the mechanical alignment, the resolution for the chamber itself, and the supporting stand must be order of 30 microns. And this is not an easy thing, because the device is rather big, and this is about 12 meters by 12 meters.
22:00
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. And this is the inner chamber, the middle chamber, the outer chamber. The wires are going through here, and you can see the electronics,
22:23
and then the cabling system. 16 such pieces are now under construction. A question which is very important for precise measurement is a calibration of your detector. Without a calibration, of course, you will not know where you are.
22:44
Calibration for muons are provided by N2 laser, which has 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,
23:01
and then goes through the chamber with a position-sensitive diode simulating infinite momentum muons. Without such a thing, you cannot really perceive. And this shows one middle chamber, lower chamber, upper chamber, and here is the guide for the laser to go through,
23:23
for the N2 laser to go through. When the chamber is finished, we fire the laser and see what resolution we will get. With a thousand shots of laser, we measure a straight line to 50 microns. That means, for a given shot, the delta P over P,
23:43
since the total suggestions is 3,800 microns, each individual shot is 50 microns. That means delta P over P is 1.3%, which means delta M over M is better than 1%. For a thousand shots, a centroid known as 50 versus square root of N
24:01
is 1.6 microns. That means the center is now known to 1.6 microns, even though the distance is the order of 12 meters by 12 meters. Beside the muon chamber, inside is a 4-pi hydron calorimeter, which provides an energy resolution of 50 versus square root of E,
24:23
but also measures the collective information on jets to 2 degrees, and also enable you to track the muons to go through the detector. This large hydron calorimeter is being constructed in both the first institute of physics in Aachen and in Soviet Union.
24:46
What is the principle of hydron calorimeter? What you need to do is to put very dense material, let the hydron go through and let it interact, allow for its energy, and you essentially measure the total energy. And so, they are constructed with 144 elements
25:04
of uranium plates sandwiched with detectors which measure the charged particles. It's divided into nine rings. Each ring is divided again into sectors, a total of 144 sectors, 16 sectors in each ring.
25:22
And here, then, is a construction map of how the hydron calorimeter and the electromagnetic calorimeter are being built. The uranium plate is made somewhere deep inside Soviet Union, and then the support rings, again, are made in Soviet Union.
25:43
The raw material for germanium oxide is near the Black Sea, and they all ship to Moscow, and then goes to Switzerland. These are one of the 144 uranium plates,
26:00
and this uranium is very high quality, very flat to the correct size, 60 pieces together, made of hydron. Between the uranium, you have up at ETAP where this calorimeter is now being constructed. The next item, when you go from outside to inside,
26:23
is a device to identify electrons. The energy resolution of the electron is done with this new crystal, BGO. The pi e rejection from this BGO will be measured to be between 10 to the minus 3 to 10 to the minus 4. When you identify an electron or a major photon,
26:44
one of the very important things is to reject pi 0 to 2 photons, and so-called Dallas pairs, and that you do it by the vertex chamber, which we call TEC, which measures the opening angle. The raw material for this crystal, as I've said before,
27:03
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 it's in the People's Republic of China and the Institute of Ceramics in Shanghai.
27:22
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 Yin,
27:43
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
28:04
in Shanghai Institute of Ceramics, which are making 12,000 pieces close to 10 tons of germanium oxide for us. And these are some of the crystals.
28:20
It's 24 centimeters long, 2 centimeters by 2 centimeters in one end, 3 centimeters by 3 centimeters at the other end. And we have carried out a worldwide competition, and the other ones in Shanghai produced the best crystals. When the crystals arrive, we put them into the beam,
28:42
measure its response. And this is a Van de Graaff 4MeV Van de Graaff accelerator located in Lyon, produced from video active capture process, 20 MeV gamma rays, and then we measure the response in this box, which will locate the BGO crystals.
29:02
And this shows, when you have a photon of 20 MeV, you have a resolution of 7.7%. This is a low-energy confidence gathering which you have to reject. And from the center to the upper peak, you see a 27.7. What does this mean?
29:21
This means when you deal with large quantities, with large quantities of sodium iodide, like the crystal ball experiment, or large quantity of BGO, you obtain the following comparison. In terms of full weight half maximum, these are the measurements of BGO,
29:43
and this is a measurement of sodium iodide. For individual crystals, I think sodium iodide resolution is better. But when you put them together, they are somewhat compatible resolution. The difference is BGO is non-hydroscopic, so you can handle with your hands,
30:00
and it's denser, so more compact together. When you go from outside to inside, beside the muon chamber hydrogen 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,
30:22
from Swiss Institute for Nuclear Reactor Research and from University of Geneva, from Zeegan, and from DDR, and also from ETH theory. The principle, like all these things, is very simple,
30:41
and can be visualized 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 put gas in, when a particle goes through, it loses energy by ionization, and so you have a cluster of electrons,
31:01
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,
31:21
we're putting a grid, reduce the drift region velocity by a factor of 10, and the amplification region, you keep the original velocity. Because your velocity now is by a factor of 10 lower, your electronics have enough time to identify not only the first arrival,
31:42
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 of the particle in this chamber, and therefore a very good resolution.
32:01
Of course, to obtain a good resolution, your detector, the mechanical part of your detector, has to have a compatible resolution. And this is a model, a full-scale model, that's made in Zurich, 30 centimeter by 30 centimeter, with the principle which I just described. When you put it into an experimental test beam in Hamburg,
32:26
you will see, at the mean-flip length of one centimeter, if it's two atmospheres, it's 30 micron resolution. If it's one atmosphere, it's 30 micron resolution. If it's two atmospheres, it's 25 micron resolution. This is a very large device,
32:41
and it's not very easy. In fact, I think this 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.
33:00
And this shows what happens in one burst, you have three particles go through the chamber. When two 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.
33:22
With so many physicists involved, the first thing you have to do is to make sure people analyze the data and communicate with each other, and to have established their own computer net from all the physicists in the United States, and all the physicists in Europe, from different countries,
33:41
and they are linked together. Now you will ask, what is the difference between this detector and the three other detectors now that's been built at CERN, and two similar detectors built in California, where they have a 50 Gb linac collide
34:00
with a 50 Gb linac. So I want them to discuss a little bit of the unique physics with this experiment, which we call AR3, not covered by three other lab and two single-pass collider detectors. All these other five detectors are very good detectors involving very advanced technology,
34:22
but mostly concentrated on identifying pion, kion, and protons, and hadrons. All these different measures, electron, muon, and photons. So let me give you three examples. First, at energy of 100 Gb.
34:42
At 100 Gb, because the BGO crystal has such a good resolution, if you have an excited toponium state, order of 70 Gb, which then decays like a toponium state to a single 200 mV photon, plus a p state decays to many hadrons,
35:02
your crystal can identify the single photon at a clear peak. And what happens when the accelerator, let's say, goes to 180 Gb? 180 Gb, let's say, give you an example, 165 Gb for one year of experimentation.
35:23
If you do an experiment of electron, positron, goes to Z0 plus Higgs particle, which is responsible for the origin of masses and of which the decay property is not known, what you can do is to measure the Z0 decays to mu plus or minus,
35:41
or E plus and minus, and therefore identify the missing mass peak of the H particle. Because a good resolution for muon and electron, you identify 20 events for muon if the Higgs is 50 Gb, and 43 events for electron because acceptance is larger for electrons,
36:01
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 a particle with 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 a missing mass technique.
36:24
There are some plans. In Hamburg, engine has turned to use the left tunnel to higher energy to do a proton, anti-proton, or even proton, proton collider. If such a thing is visualized,
36:41
for example, you can put a 5 TeV anti-proton or 5 TeV proton collision. In such a collision, this detector, without modification, will provide the following properties. For a particle of mass 1 TeV, when it decays to electron-positon pair,
37:02
you will provide a mass resolution of half a percent. For this 1 TeV 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
37:20
of this 1 TeV particle. There are many theories now saying the next mass scale is about 1 TeV. For hydrogen gas, 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 Z0,
37:42
a mass of PP bar collision at 10 TeV, and then if you produce a Z0 or mass 1 TeV, you will get hundreds of thousands of events, and clearly it can be identified. Now, if you would view it based on our understanding of theory
38:01
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 two fundamental principles. One is gauge invariance, another is symmetry breaking. Gauge invariance leads to quantum electrodynamics,
38:22
quantum chromodynamics, the theory of Weinberg and Salaam electroweak theory, and this has been tested by many experiments. G-2 experiment, electron-positon to mu pair, gluon jet, neutrino scattering, muon scattering to discover Z and W.
38:43
Indeed, all the experiments are done up to now, at CERN, in Chicago, at SLAC, at Brookhaven, all involved in tests of QED, QCD, and electroweak. Clearly, with new accelerators, you can look for more quarks, more Zeros, more Ws,
39:02
study three gluon vertex, and clearly more tests are necessary, and it's 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.
39:23
So far, there's really no experimental test. There are many predictions, Higgs particles, supersymmetric particles, technions, technicolor particles. I'm sure by the time this detector is finished, and there will be many, many more predictions. By measuring, by designing a detector,
39:43
measuring photons, electrons, muons, precisely, the main aim is 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 thereby try to understand the origin
40:01
of masses of all particles. Thank you.