Samuel Ting (1979) - Light Rays, Massive Light Rays, and new Particles in Nature

Good morning. I have been in Germany since 1964, so it’s been many years. However, despite my effort I have not been able to master the German language. And so, rather than speaking Chinese, I will give my lecture today in English. I will try to also speak very slowly, so my distinguished former professor, Professor Rabi will be awake. May I have the first slide please. Most of the phenomena in the universe can be characterised by a distance parameter. Where you have a distance larger than from 10 to the 18 to 10 to the 24 metres, we have cosmological phenomena. Where you have distances of 10 to the minus 3 to 10 to the minus 6 metres, we have biological phenomena. Where you go to distances of 10 to the minus 9 to 10 to the minus 12 we have atomic physics. And when you go to a distance less than 10 to the minus 15 centimetres we have the nuclear phenomena. Next slide please. Now this is a picture of the Milky Way which shows the physics of distances of 10 to the 20 to 24 metres. Next slide. At large distances most of the phenomena are covered by gravity. Its chief cosmic role is binding planets, stars and galaxies. It is extremely weak in atoms but very strong in collapsed stars. It is infinite in range, it acts on everything. We assume it is carried by gravitons. Next slide please. When you go to the other extreme, distances of 10 to the minus 15 metres or smaller, we have the nuclear phenomena. This is a bubble chamber picture of a high energy particle hitting an atomic nucleus and producing 100’s of other particles. Next slide please. In the nucleus basically there are 3 kinds of forces. The predominant one is the strong nuclear force, known as strong interaction. The chief cosmic role, binding atomic nuclei, burning in stars. It’s about 100 times stronger than the electric force, it is very short in range, it´s 10 to the minus 13 centimetres. It acts on quarks, so far, only an assumption. And we know it is carried by the mesons. Next slide. Besides strong force, there is then the weak force. Its chief cosmic role is alternating basic particles, namely change protons into neutrons. Its strength is very weak, it´s 1 part in 100 billions of electric force. It is very short in range, 10 to the minus 15 centimetres. It acts on all basic particles, quarks and electrons. We assume it is carried by an intermedaite vector boson, W+, W- or Z0, which I will describe in some detail later on. Next slide please. The third force is the most common experienced one, is the electric force. Its chief cosmic role is binding atoms, creating magnetism. It´s strong in atoms, weak over cosmic distances, because matter is neutral. It is infinite in range, it acts on all charged particles and is carried by light rays or photons. Next slide please. Now, we experience light and its interaction in matter almost constantly. In daily life, when the energy of the light, its order of the electronvolt, passing of the electric current, it generates light. In atomic world, when energy is order of 100 MeV to 100 eV, light is a transmitting force between the electrons and nucleons. The applications are x-ray in medicine and in industry. In the nuclear world, when energy is a billion electronvolt, light wave, because it is stable and because it’s neutral, is a very useful tool for penetrating into the nucleus. Next slide please. The forces in the electric charge, can be expressed as function of the distances. When the distance is order of a centimetre, you have 1/r² law, the Coulomb law. Where you have a distance of 10 to the minus 8 centimetres, you have atomic physics. When you go to the distances of 10 to the minus 13 centimetres or smaller, you have phenomena like light ray interact with light ray, known as quantum electrodynamics. Next slide please. Now, quantum electrodynamics is the study of light and electric charge. Experimentally there are 2 methods to check quantum electrodynamics. The first method is to do it at very large distances but with very high accuracy. The measurement of the muon g minus 2 over 2, done at CERN, has the number 001165915 plus minus parts in 10 million and agrees exactly with the theory. And this I believe is the most accurate measurement in particle physics. And comparison between experiment and theory is accurate to a part in 10 million. There is another method of testing quantum electrodynamics experimentally, and that is to probe deeply into the electric charge, namely at small distances, but in this way you can only do it to approximately 5% accuracy. You do this by using the uncertainty principle delta p * delta x = h, and so you if you give the electron a momentum or a kick of 1 billion electronvolt, you go to a distance of 10 to the minus 14 centimetres or even smaller. Next slide please. The first experiment of this kind, performed at the Deutsches Elektronen-Synchrotron in Hamburg in 1966, consists of using a 6 billion electronvolt light, produce a 3 billion electronvolt electron, 3 billion electronvolt positron, a pair, in the field of the Coulomb nucleus with a momentum transfer to the electron about 1 billion electronvolt. Experimental difficulties for all this type of experiment are two kind. The first is the electron positron yield is very small, because it´s alpha2, because it´s one virtual photon, 1 real photon. And that means, whatever detector you need, you need a large acceptance and very high incident flux of 10 to the 11 gamma rays per second. The second difficulty is, there’s always more strong nuclear particles are produced like the Pis, compared to the electrons. In fact, the ratio of photon go to electron pair versus Pi pair is 1 part in 10 to the 5th. And that means, 1% experiment must have a rejection of Pi*Pi by a factor of 10 million. Next slide please. What is the experimental method to identify electrons? When you have a gamma ray or a proton interact with a nuclear charge, you produce many, many particles. There are pions, kaons, protons and electrons. Pion has a mass 140, K has a mass 500, proton has a mass about billion electronvolt. The electron has a mass of only 0.5 MeV. But electrons are produced 1 part in 10 to the 5th of all the other strong interaction particles. The methods to identify electrons are well known. The first is, the charge and momentum are measured by its bending in a magnetic field. In the magnetic field, positive particle going one way, negative particle going the other way. The lower the momentum, the more the bending. The second: the electron has the smallest mass, therefore for given momentum you have the highest velocity, therefore by properly use of gas as a radiator, you will yield Cherenkov light, the other particle behaviour will not give Cherenkov light. The third is: the electron, also because of its small mass, will enter into a piece of dense material like a glass, loose all its energy into light, and the amount of light is proportional to the electron energy. And therefore, if you have a device, collect an amount of light, you know its proportional to the energy of the electron. Next slide please. So the experiment carried out in ’66 has the following arrangement: With a gamma ray of 10 to the 11th per second on the target, produce the electron positron pair, you have counters in here, behind magnet and therefore measures trajectory, therefore measures momentum. And there the Cherenkov counters measure velocity, knowing the velocity and momentum, therefore identifies mass and identifies the electron. And then you have a shower counter here which measures the total pulse height, which is proportional to the momentum of the electron and therefore is further identification, that it is indeed an electron. Next slide please. This is an arrangement where an intense electron beam goes through and these are Cherenkov counters behind here and shower counters located at the end. Next slide please. What is plotted in here is the ratio of the measurement of the electron positron yield, compared with the predictions of quantum electrodynamics, where the photon goes to the electron positron pair. Versus a mass of the electron positron pair, which is equal to the square root of 2, times the momentum transfer for symmetric pairs. One means complete in agreement with quantum electrodynamics. At a mass slightly below 800 MeV, you see a deviation from the predictions of quantum electrodynamics. This observations of a deviation from quantum electrodynamics come from the fact that the photon, when the energy is high enough, has a very small probability to change itself to a particle, which I call the Rho, in the field of a nucleus, and the Rho again decays to a photon, decays to e+ e-. If this of course is the case, this being a strong interaction, it changes very slowly with the angle, goes at 1 over Zeta to the 3rd. This being an electro dynamic process goes at 1 over Zeta to the 7th or to the 8th. And therefore, if you go to a large opening angle, this will predominate. Or indeed, when you go to 15 degrees of opening angle, the deviation from the electrodynamics increases. And there’s another one at a mass of larger than 1 GeV, again you have another particle. Next slide please. Let me now summarise what I call photons and heavy photons. Photons, which we call gamma rays, lifetime is stable, its spin is 1, parity -1, charge-conjugation minus 1, its mass is zero. Then there are 3 particles which we’ll call the Rho, the Omega, the Phi. One of them has a mass of 760, charge-conjugation -1, parity -1, spin 1, same as the photon. It has a lifetime of 10 to the minus 24 seconds and then goes to a pair of pions. And there is Omega, which has a lifetime of 10 to the minus 24 seconds, has a mass of 785 MeV, has the same quantum numbers as the photon, it goes to Pi+, Pi-, Pi0. And then you have the Phi, which has a mass of 1020 MeV, has a charge-conjugation again -1, parity -1, spin 1, goes to 3 Pis or 2 ks. Because this particle has exactly the same quantum number as a photon, so you may imagine from a high energy photon interact with a particle, you can visualise a photon change itself for a very short time to a Rho, to an Omega, to a Phi and the Rho, Omega and Phi interact with the photon. Let me give you a few examples. In classical optics, when you have a lamp which generates light of electronvolt, use a focusing lens with a screen, the hole of the screen of a millimetre matched with the wavelengths of the lamp, with a black cloth with a hole of a millimetre, and behind the focusing lens you have another screen, of course you see diffraction patterns. The larger the radius, the smaller, the light is more intensely focused together. The smaller the radius, the light is more diffuse. To change to a high energy sense, you change the lamp to a 6 billion electronvolt accelerator which generates light to billion electronvolt. To match with the wavelengths, then you need a scattering centre with a dimension of 10 to the minus 13 centimetres, that means nuclear target. And then you change the screen to 1,000 pound detector. Next slide please. This is what I call the modern lamp, which generates light of 6 billion electronvolts and this is the Deutsches Elektronen-Synchrotron located in DESY. And this is the electron accelerator, which accelerates light, the electron to 6 billion electronvolt and therefore generates light to that energy. Next slide. This is a reaction of gamma ray on nuclear target produce Rho meson, which are the same as the photons on nuclear target. This is what is plotted in this axis is a mass of Pi+, Pi-. You can see, near the mass of the Rho you see enhancement correspond to Rho, decay to Pi+, Pi-. The yield as function of angle decreases as the diffraction pattern, changes from beryllium to carbon to aluminium to copper, all the way to uranium. You can see, the larger the nucleus the light is more focused together, and the smaller the nucleus, the light is more diffuse. In the same way as the classical diffraction scattering. Next slide please. I can give you one more example between classical optics and heavy photon production. In classical optics, if you have a light source with a focusing lens, you have a black cloth with 2 slits behind it, of course you see an interference pattern. If in front of one of the slits you put a very thin piece of glass with known thickness, you see a shift of the diffraction pattern. And the amount of shift is the measure of index of reflection of this piece of glass. To change to high energies then, you can visualise that transmission of light goes through a slit, producing photon production of the electron positron pair in the field of nucleus. Adding this piece of glass can be visualised as the photon changes itself to a Rho, Rho decays to a photon, to an e+ e-. In this analysis in D2 near the mass of the Rho, you shall see a shift of interference pattern. What is plotted in here is the measurement of interference pattern, where the positron goes to the left, the electron goes to the right, minus the reversed one as function with the mass from 610 to 640 and 640 to 670, and as function of a difference of momentum transfer between the 2 particles. What is important to know is, away from the mass of the Rho, you see no interference. When you go to the mass of the Rho, which is 760 to 790 or 730 to 760 because of its width, you see a sharp interference pattern. In the same analogy as the classical case. The next slide please. I can give you one more example. Once you put a piece of glass in front of one slit, it doesn’t prevent you to add another piece of glass with known thickness, and again you see a shift of the intensity pattern and this will be a measurement of the index reflection of the second piece of glass. In the high energy sense then instead, transmitting light without a piece of glass is the pair production in the Bethe Heitler sense, and adding the first piece of glass, which corresponds to photon, goes to Rho, Rho decays to a photon to e+ e-, that’s the first case. Adding the second piece of glass, then would correspond to a photon goes to Omega, Omega goes to a photon, goes to e+ e-. Since the Rho and the Omega have a mass very close and the width is very wide, you will expect interference between these two. In the same way you expect interference between these two. This I took many years to observe, mainly because of experimental difficulties. The first is the Rho to ee, compare Omega to Pi Pi-ratio is 1 part in 10 to the minus 5. And therefore you need a rejection against Pi Pi larger than 10 to the 8th. And in the second the width of Omega is very narrow, it’s about 10 MeV. That means the resolution with the detector has to be 5 million electronvolt. Once you have done that, you measure the mass of the e+ e-, and then you see the experimental point agrees with the prediction of Rho plus Omega decay to e+ e- with interference. And this will be the dotted line, will be the Rho to e+ e- alone, which does not agree with the data. Next slide please. So we now know that photons and heavy photons are almost the same and they do transform back and forth into each other. We also learned how to use high intensity flux of 10 to the 11 gamma rays per second. And to obtain a Pi Pi rejection of larger than 10 to the 8th. And to have a mass resolution of 5 MeV. And then you can ask yourself: Why should there only be 3 heavy photons, all of them at a mass of 1 BeV? And go to higher mass, in order to search for higher mass, you go to a high energy accelerator, and then you go to Brookhaven National Laboratory in Long Island. Next slide please. This is the aerial view of the Brookhaven accelerator and this is the injection system and this is the 30 billion electronvolt proton synchrotron and this is the experimental area. Next slide please. So accelerating is carried out in the first stage by Cockcroft-Walton electrostatic accelerator, it accelerates protons to kilo electronvolt range. Next one. You guide the photon through a linear accelerator, accelerated to a few hundred million electronvolts. Next slide please. After that you bring the proton beam into an alternating gradient electron synchrotron. After 2 seconds you reach the speed of light and energy of 30 billion electronvolts. And then you make a slight change of magnet current, you bring all the protons out. Next slide please. Now to go to higher mass, to look for new particles, the first question is: How do you set up the detector? The first answer is: You do not know, because you do not know the property of the new detector. But you do know the following: For ordinary heavy photons like the Rho, we know that when a photon hits a nuclear target, the Rho decays to a electron positron pair, the maximum yield is at 15 degrees in the laboratory. Since we know nothing else, we assume the new particle is produced like the Rho meson. Next slide please. The main difficulty for this type of experiment is the following. When a proton interacts with a target, lets say beryllium, you produce many, many particles like Pis, ks, protons, Rho0s, K-stars and many others. The number of the electron positron pairs compared to Pi pair is even smaller, it´s 1 part in 10 to the 8th. And that means, to obtain enough e+ e- events, you need a 10 to the 12th protons per second. And to obtain a Pi Pi-rejection to 1%, you need a rejection of 1 part in 10 to the 10. A typical city like Stuttgart or Munich, during the rainy season you have about 10 to the 10th drops per second, now if one of them has a particular colour, you have to find that one. And that is why an experiment of this type is normally somewhat difficult. Next slide please. The detectors for this and subsequent detectors are very similar. This is the top view and this is the side view. The beam on a target, M0, M1, M2, are basically bending magnets. A0, A, B and C are position measuring chambers, from the top view you measure the production angle. Bending is on the vertical view, so A, B, C, A0 measure the momentum and C0, C – at Cherenkov counters - measure the velocity therefore identifies the electron. At the end again you have shower counters. Very similar to all the detectors used by us before. The dimension of this is somewhat larger now, this is 70 metres by 10 metres. Next slide please. Now I give you a few examples of how this type of experiment are done, just to give you a feeling what the kind of problems one encounter in high energy physics. When you have a 10 to the 12 protons on a target, the first thing you ought to do is to divide the target into many small pieces. When you divide this into many small pieces, you have one advantage, is because a real electron positron from one interaction you can trace back to one point. By accidental having electron in this way, in this point, positron in another point, you can reconstruct them and reject them. The next slide please. This is a picture of the target, one target in here, another in here, another in here and they are made alternatively with beryllium and sulphur. Industrial sulphur has an advantage, when you have a proton beam go through and it shines light and becomes illuminated and so if you have a close circuit television, you can monitor whether the beam is on target or not. Next slide please. One of the main difficulties for all this type of experiment, to search for real phenomena, is the rejection against hadrons, and for this experiment we need a rejection of 1 part in 10 to the 10. You do this with Cherenkov counters with gas. Now with Cherenkov counters, you have the following problem: For example you have a pion entering the Cherenkov counter, in the Cherenkov counter there is gas with atomic electrons, and so there’s a finite chance a pion will interact with atomic electron and kick the atomic electron out. And therefore a Pi has become an electron and you begin to make a mistake. To reduce this problem, the first thing you do is use a gas with the smallest number of atomic electron, that is hydrogen. The second thing you have to do is – to kick out the electron is always low momentum and therefore you can use two Cherenkov counters with a strong magnetic field between them, and you sweep the electrons away from the first counter so it doesn’t get into the second one. These things are somewhat difficult to make, they are rather large, and with gas, hydrogen, but very thin, the front is 125 micron and the back is again 125 micron thick. And you have a focusing mirror which is 1 metre in diameter, 3 mm thick and focus the light into a photo tube. The next slide please. The detectors are basically measuring the ionisation which, known as proportional chambers, there are 8,000 wires, each has its own amplifier and the spacing is about a millimetre and so when a particle goes through them, you lost energy in the gas by ionisation, the ionisation is picked up by the wire and then it´s amplified. So there are 8,000 amplifiers, each one amplified a signal, therefore you measure the position to about a millimetre. The next slide please. Now, to protect yourself for this type of experiment, when you have a 10 to the 12 protons per second and a 10% interaction target, you have 10 to the 11 interactions, each interaction produces 10 particle, so again you have 10 to the 12 particles are produced. Most are protons, kaons, pions, gamma rays, electrons and neutrons. You use about 5 tons of uranium, 100 tons of lead to stop electromagnetic particles like gamma rays, electrons and muons. What's left is strong interaction particles, protons, kaons and pions, and you use about 10,000 tons of concrete to stop them. What's left then are soft neutrons which are walking in all directions, which are very difficult to stop. And for that, the best way of course is to use hydrogen, but hydrogen is quite explosive, so the next thing you can do is use water. But we already buried our detector under 10,000 tons of concrete, so you cannot be sure water is still there. So the next best thing you can do is salt, because salt contains a large amount of hydrogen. And so you need about 5 tons of salt to stop all the soft neutrons. Even with that the radiation in the target area one hour after the beam stop is still 5 R per hour. And this of course makes the experiment somewhat difficult since we only had one graduate student at that time. Next slide please. This is a picture of the detector and these are the Cherenkov counters. And these are various shielding of lead and uranium, and there are chambers behind the measured position. The next slide please. So this is a measurement probably you have seen before. It’s a mass of the electron position pair, between 2.5 and 3.5 billion electronvolt, and number of event per 25 MeV. Basically there’s nothing except a mass of 3.5 billion electronvolt. You see a very, very sharp pronounced peak. The next slide please. This type of experiment has been tried over the last 25 years by many people and nothing has been found. So the first time you see a sharp peak, the first thing you suspect is probably something is wrong with your detector, because the detector, as you see, is quite complicated. Now to check this, the first thing you can do is to change your magnet current. Once you change your magnet current, let´s say by 10%, you move all your trajectory to a different position. And if the peak is still there, of course this shows that it´s not due to instrumentation. Your peak moved away, you know, this is not a real phenomenon. And many other checks can be performed, but the most important one is to check whether you have problems of scattering, a particle scattering from the edge of the magnet into a detector. And to do that you perform the experiment twice: once with a large counter, another time with a smaller counter. And this way you can know how much is from the scattering from the edge of the magnet. Next slide please. This is a check on the measurement of the particle by lower the momentum by 10%. The blue one is all the current in the normal setting and the red one is all the magnet current lowered by 10%. In this way the peak is the same place, shows what you observed is a real phenomenon. Next slide please. Since this work and since the work of Richter´s group at SLAC, many, many other work was carried out, and particularly the work at Frascati in Italy also measured electron positron to hadrons, which is a reverse process as what we did. And compared this with the electron positron to the electron positron, you can measure the width, the natural width of this peak and it´s found to the order of kilo electronvolt, 70 kilo electronvolt. That means it lives about 1,000 times longer than the other particles. Now what is the significance of this particle? The only thing we know now is that it´s lifetime is 1,000 to 10,000 times longer than ordinary particles. Next slide please. Before this new particle, before 1974, there are about 200 or 300 subatomic particles which I call the old particles. Most of the particles live only a very short time. Normally, the heavier the mass, the shorter the lifetime. Next slide please. In fact our understanding of the atomic nucleus in the last 50 years has gone through many, many changes. In the ‘20’s we viewed the proton as a very small object in the heart of a hydrogen atom. In the ‘50’s we view it as a small object with mesons in its vicinity. In the ‘60’s, through elastic scattering, we view the proton as a fairly large object compared with the electron. It is denser at the centre than at the edges. In the ‘70’s we view it as a large object containing much smaller objects known as quarks. A theoretical picture before ’74 is there are basically 3 quarks with exchanging gluons between them. Next slide please. So in this picture the proton is a combination of three quarks with two lying up, one lying down. A neutron is again quarks with different alignment. Next slide please. The new particle, which we call a J particle, Richter´s group called it a Psi particle, we know it has something to do with light quanta because it goes through the electron positron pair. It´s heavy and stable, lives 100 to 1,000 times longer than the other particles and therefore it must have some hidden reason behind it. Next slide please. Some of the important reasons were discovered both at SLAC and at DESY. In the DESY setup you have a linear accelerator. It accelerates electrons and positrons and you accelerate them in an electron-synchrotron, and then you guide them into a colliding beam machine with the electron in one way, positron another way. At a certain point you let them collide. In this way then you can systematically study the new particles. Next slide please. So the work at SLAC and DESY and at Frascati has shown that, besides the J particle at a mass of 3.1. You have many, many other states. There’s one at 3.7, 3.4 and many, many states. And these states transform into each other by emitting and absorbing of gamma rays. Very similar to the simplest item of the electron positron known as positronium. In fact it’s the triplet state of positronium, and compared this and this, you see they are very similar together. Means that, whatever this particle is, it probably is a bound state of another new quark and anti quark together. Next slide please. The discovery of the J particle indicates a nucleon besides three ordinary quarks, you have one more quark and people call it the charm quark and charm in my opinion is an unfortunate name. You can either call it ugly or anything else. Once you have four quarks, nothing prevents you to say there’s a 5th one, there’s a 6th one, there’s a 7th one and there are more and more. In fact the work at Fermi lab by Lederman’s group already give indication that there may be a 7th one. Next slide please. We know that photons, with the mass zero

Samuel Ting (1979)

Light Rays, Massive Light Rays, and new Particles in Nature

Samuel Ting (1979)

Light Rays, Massive Light Rays, and new Particles in Nature

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The Lindau Meetings in physics, chemistry and medicine are held during a week at the beginning of summer. Around the same time as Samuel Ting in 1979 lectured for the first time in Lindau, the German electron-positron collider PETRA in Hamburg was taking data with several large detectors and Ting was member of one of the teams. The title of his talk included the phrase “New Particles in Nature”. When the data analysis from the three detectors at PETRA was finally complete, a new particle had been discovered: the gluon, the carrier of the strong nuclear force, the force that acts between the quarks of the atomic nucleus and keeps it together. At least some of the students and young researchers in the audience at Lindau must have made the connection to Ting’s talk when they, later on, heard about the discovery. Even though Ting’s name was not on top of the list of discoverers, he mentions PETRA at the end of his very interesting talk. As a true experimental physicist, Ting shows N slides, where N is a large number. But at the same time he tells his story in such a clear way that there is no real difficulty in following his arguments. He starts by describing his early work looking for new particles at DESY in Hamburg, then moves over the Atlantic to Brookhaven. There he made the discovery of the fourth quark (charm), for which he only two years later received the 1976 Nobel Prize in Physics shared with Burton Richter, the latter having made the same discovery independently at Stanford. At the time of his Lindau lecture, also the fifth quark (bottom) had been discovered and Ting mentions that some theories predicted many more quarks. We now know that, according to the Standard Model, there was only one more quark to be discovered, the sixth (top), which because of its large mass was not clearly detected until 1995. On his way from one large accelerator to the next, Ting’s story is that of a travelling physicist!

Anders Bárány

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