Samuel Ting

Quarks, Gluons and New Particles in Nature

Category: Lectures

Date: 1 July 1982

Duration: 35 min

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Subtitles: EN

Samuel Ting (1982) - Quarks, Gluons and New Particles in Nature

This is Samuel Ting’s second lecture held at the Lindau Meetings, three years after the first one. The two lectures are connected and tell more or less the same story, the story of a travelling high-energy physicist, who moves from one accelerator to another in search of higher and higher energies

Good morning. Even though I have worked in Germany close to 20 years, my knowledge of German language is exceedingly limited. So since there are more people understand English than Chinese, I will give his lecture in English. What I would like to do today is to go over some experiments on photons, leptons, quarks and gluons. The study of proton has progressed a lot since 1911 when Rutherford first measured the size of atom and found it of a distance approximately 10^-8 centimetres. In 1930's through the work of Yukawa, study of nuclear force one extend the range to 10^-13 centimetres. In '53 through the fundamentally important experiment by Hofstadter, which measured the size of the proton. In '63 Gell-Mann and Zweig postulated that inside proton there are 3 kind of elementary particles known as quarks. And from this one can explain the spectroscopy of the known particles at that time. Then in 1968 came a very important experiment by Taylor and collaborators SLAC, which measured electron scattering from a proton at a given momentum transfer Q^2. And the cross section for this inelastic process is proportional to the point like cross section known as Mott cross section times the energy difference between initial and final energy times a form factor W2. The measurement of this cross section shows W2 as function of loss energy mu goes down as 1 over mu. Independent of Q^2. That means W2 goes as 1 over mu, therefore Sigma is equal to Sigma_Mott. That means, you begin to see scattering, a large momentum transfer and some relatively small angles, scattering from points inside. And shows therefore indeed the point like structures inside the proton. If you plot the same curve as function of Q^2, but compare with normalised to the Mott cross section. You see the elastic cross section of course goes down because of the structure of the proton. The inelastic cross section, at different energies, is essentially almost constant and shows you are now probing the points inside the proton. This of course is really a fundamentally important experiment. From this then, one can explain a lot of phenomena, once you know the protons are made out of point like particles and you assume there are three kind of point like particles known as quarks. On explanation of experiment is: With energetic photon on a nuclear target, produce 2 kind of elementary particles, one we call the Rho which is resonance from 2 pions. Another is called Omega, is a resonance of 3 pions. But since Rho and Omega has the quantum numbers like a photon and therefore when the energy is high enough you can ignore the mass and therefore these are really like elastic scattering of photons. And therefore Rho and Omega sometimes, a small probability, one part in 10^8 of the time, goes to the electron positron pair. And this of course will interfere in the final state. And this is the plotted mass spectrum as function of the electron positron mass. You see a Rho, which is a broad peak and interfere with, a coherent interference with Omega. The shape and exact size indeed can be explained by assuming 3 quarks. And the 3 quark relative interaction does explain all the phenomenon here. The next question is of course how many quarks exist in nature besides the 3 known quarks? Then in 1974 experiments were carried out, one at SLAC by Richard's group. Another at Brook Haven by my collaborators and myself on proton on a beryllium target. Produce electron positron pairs. This plotted on this axis is the mass of the electron positron pair. Compared to the relative yield. The yield is essentially flat except the mass of 3.1 billion electron volt, you have a very sharp peak, which exits for a very short time and decay to electron positron pair and you measure this electron positron pair, which we call the J particle. This particle has a rather long lifetime, about 10,000 times longer than all the other known particles. It is very heavy, 3 times the mass of the proton. And after the discovery of this particle, a family of particles were discovered, which by emission and absorption of gamma rays are related to this particle. Because the long lifetime and heavy mass and because the close family associated with this particle it can not be explained by the known 3 quarks. And therefore a fourth quark is necessary. Once you have 4 quarks you can naturally ask why there should be only 4, where is the fifth one, sixth one, seventh one and so forth. To look for more quarks, another experiment was carried out at the highest energy accelerator, the 3,000 billion electron volt in the laboratory system into a section storage ring at CERN. Where you have a 30 billion electron volt proton and a 30 billion electron vote proton collide with each other. The process therefore would be proton-proton goes to heavy electron known as a muon, mu+, mu- plus X. In this quark model therefore, this process can be visualised very simply in the following way: Proton consists of many quarks. Another proton consists of many quarks and also antiquarks. So antiquark and a quark can annihilate, goes to a photon, goes to a mu-, mu+. If this is the case, the cross section can be visualised as a point like cross section of a quark, antiquark go to mu+, mu-. Which is this term. And a distribution function of quarks and antiquarks. Which is this term. In other words if you express in terms of variable tau, which is m^2 divide by S, the total centre mass energy. The forward cross section then, is nothing but a function of tau only. Now this quantity has been measured. Again the quark-antiquark distribution function measured in 1975, very precisely, W2 as function X at SLAC. With this then, you in principle, if the theory is right, you can calculate what would be the behaviour of proton-proton goes to mu+, mu-. And whether you would see scaling or not. So this is a measured cross section as function of tau. So if the theory is correct, independent of energy, if you express in this quantity square root of tau all the cross section should be on top of each other. Indeed whether there is 62 GeV or 44 GeV the cross sections when you express in this form, are exactly the same. To measure the cross section itself of course is quite different, it's only the same when you express it in units of tau. So this shows a direct test of this fact that protons are made out of point like particles without any further assumptions of nuclear physics. The next question of course, how many quarks- antiquark particles exist? This is a direct measured cross section, this sigma DM as function of the mass of mu+, mu-. This enhancement comes from the fourth quark, which I called the J, which is the cc-bar, the fourth quark. And this additional peak is the confirmation of the existence of the fifth quark, which is called Upsilon, which come from the bb-bar quark. So now we know there are 5. The question of course is where is the sixth one, seventh one and so forth. To look for the sixth one, means you have to travel once more and this time a serious experiment was carried out at the highest energy electron positron accelerator in the world, the 38 billion electron volt colliding beam accelerator located in Hamburg, Germany. And there positron and electron were accelerated with existing ... (inaudible 11.44) and then they did collide in 4 intersection regions. And at this intersection region you have e+, e- colliding, total energy 38 GeV and you can look whether there are new quarks or new particles exist. The detector for this type of experiment has become rather large. This is one of the detectors, this is the size of a person, e+, e- collision occurring here and surrounded with 1,000's of channels of detectors in the field of a super conducting magnet. And also with liquid argon calorimeters to measure the energy. So practically all the modern technology is used, fast electronics, fast computers, low temperature cryogenics, everything necessary is being used. And also it's not cheap. So the first question I would like to discuss, is on the existence of free quarks, free quarks as being reported at Stanford and this is a search of free quarks, a measurement of energy loss as function of a parity momentum. For a detector where Kaon would have this curve, proton have this curve, neutron, triton. And if you have charge 1/3, charge 2/3 quark with a mass of 5 GeV, you should be here. And therefore this shows there's no free quark has been found at PETRA. The next question is what is the size of the quark. Now to determine the size of the quark, one can plot the measured electron-positron to hadron cross section compared to electron positron to mu+, mu- cross section. This result is essentially the sum of the quark card square as function of energy. So the first three quark u, d, s, give R about two, u, d, s, c give R about 3.5, u, d, s, c and b give R about 3.9. And now we assume that between quarks, the forces transmitted by another particle, which we call gluons of which I will speak a little more. With this gluon, if it's like really the correction, the R changes a little bit. From the flatness you know that the quark are point like and indeed a simple comparison with the theory will say its dimension is 10^-18 m, which means it's one part in 10,000 of a typical atomic nucleus. It also shows there's no sharp structures here. So now we have mentioned the search for the free quark, the size of the quark, let us now concentrate on the search for the sixth quark. The sixth quark, which we call the top quark, people give different names, ok of course you can use your own name. The first 3 are u, d, s, the fourth one is called charm, the fifth one is called beauty and this one is called top. This is the measurement, the latest measurement on search for the sixth quark. What is plotted here is the same unit R by controlling the electron positron energy in 20 MeV intervals. And if there is a sharp new particle of course, there should be a sharp resonance. And this you have seen in here. Every time when there exists a c there's a sharp spike here, when you see a b there's a sharp spike here. The question is, whether there are sharp spikes in between, in here. And the latest experimental result does not seem to indicate the existence of a sharp peak. And this by itself is somewhat puzzling because the first 3 quark give particles of mass 1 GeV, c give a mass of 3 GeV, b give a mass of 9 GeV. So you have 1, 3, 9 you imagine the next one should be 27 and we have gone to 37. The search was also extended in here and nothing was there. You can search for new quarks in many ways and this is another way of searching. What is plotted here is a variable, which I call thrust, which is nothing but in the centre mass system the sum of the parallel momentum compared with the total momentum. And so this of course you have a certain distribution. And this distribution, this measured points agrees with the simple 5 quark with gluon theory, which is known as QCD and agree with the data. If there exists a sixth quark and because sixth quark is produced more or less at rest so when it decays it decays somewhat isotropically. And therefore there will be more yield at a low thrust. Ideally if you have 1/3 quark you have this axis, you have 2/3 quark you have this axis. And this clearly is nothing. There is another way to exclude the 6 quarks. Now let me discuss a little bit about the physics of gluons. The gluon is a particle, which transmit forces between quarks. The first I think important contribution PETRA has made, is the discovery of 3 jet events, which I will explain to you in a minute, compared with quantum chromodynamics, the physics of quarks and gluons. Do not let this slide scare you. This slide says the following. When you have e+, e- annihilate to a photon, which is the quark and antiquark. Quark and antiquark pick out other quarks and antiquarks from the c and combine themselves into hadrons. And therefore produce P's and K's. Because e+, e- is a high momentum and therefore this quark and antiquark are produced very energetically, therefore you have a stream of particles, very much collimated and therefore known as jets. So you have, normally you have two jets. If there are gluons and then gluons can be emitted from quarks and the gluon by itself can decay again into quark, antiquark and then can fragment into another jet. And therefore if there exists gluon when the energy is high enough you will see three jet events. And when the energy is even higher you can see 4 jet events and multi jet events. This is the observation of three-jet event. e+, e- collision and this is the energy distribution of the event, you're looking perpendicular to the event plane, you will see two lobes. That's because initially you have e+, e- on a line so whatever is produced along this line has to be conserved. Therefore this is nothing but a conservation momentum. You're looking from top of the event, you see 1 lobe, 3 lobe, a second lobe and 3 lobe. A quark, an antiquark and a gluon jet. The size and the distribution with the gluon jet agrees with our knowledge, with our theory known as quantum chromodynamics. That is then the first indication of the physics of gluon, in e+, e-. Much more study has been carried out and this is example of the first three jet events. Here is the plot of the transverse momentum distribution for the hadrons. If you have two quarks, the event of course is more collimated. You have three quarks the event of course is more, you have large perpendicular momentum, indeed there is the gluon distribution compared with the data. And this is the low energy data when you have 2, only 2 quarks at 12 GeV, at 30 GeV when you have 3 quarks, 2 quarks plus gluon, this event distribution. You can also study the spin of the gluon. To do that you transfer into a system, where the antiquark and gluon are back to back and you measure this angle of distribution, angle of distribution is a measured event as function of this and cosine of this angle. If gluon has spin zero you have this distribution, if gluon has spin 1 you have this distribution. This clearly shows gluon has spin 1. Then you can also measure the strength of coupling between gluons and quarks. Now when you have e+, e- go to photon, go to qq-bar, produce a gluon, produce 3 jet. This is some of the time. Most of the time go to 2 jet. The ratio between 2 jet and 3 jet event clearly is a measurement of this coupling concept. The only 2 free parameter in the theory is one is the coupling constant another is the momentum distribution of the quarks. And so this curve is the rate of the 2 jet versus 3 jet. And this curve is the rate of the momentum distribution out of the production plane of the 3 jet event. They intersecting here shows the quark has 300 MeV, like the hadron and the coupling constant is about .2. And you can make even more detailed study of physics of gluons and quarks. And that is to study events, which are final stage as a muon. When you have e+, e- produce quarks, the heaviest quark so far found is the b quark bb-bar produce mass of nine GeV. b quark of course can decay to a c quark plus a muon. And therefore if you take a muon, give you some handling of, which quark you select. And so you have 1 jet, another jet and take it on a muon. Just give you a more sensitive way to isolate the processes. The first thing you can do, is to search, give you a more sensitive way to search for new quarks. This again is a thrust distribution of all the events that have one muon producing in them. The points are the measurement and the green line is the 5 quark model with gluon, clearly in agreement with the data. You have a charged 1/3 quark with 8 GeV, charged 2/3 quark with 8 GeV, you have a distribution like this. And that again shows there is no additional and new quarks. You can also refine your measurement to study the ray of how the b quark decays into c quarks. If you measure the transverse momentum distribution of the muons, where you have the ordinary quark u, d, s, because they are very light, so the transverse momentum distribution is peaked forward. c quark is a little bit heavier so the transverse momentum distribution is a little bit broader. b quark is the heaviest so you have a most broad transverse momentum distribution. So the measurement of the transverse momentum distribution enable you to identify this process. And indeed, this is the measured transverse momentum distribution and from this distribution you get a b quark branching ratio about 8%. Which is a rather small number. Another thing you can do in studying the physics of quarks and gluons associated with the muon, is a precision study of quantum chromodynamics. And you do this in the following way: You have e+, e- produce a qq-bar, which fragment into hadrons. And if this q decays to another q' of course emit a muon. And by tagging on this muon you have a more sensitive way to isolate various processes. Indeed the green points are the measurement of the thrust distribution without muon. If the model is exactly correct, muon and hadron should not be different. And the red points are the measurements with muon. And this clearly shows they are the same. And shows the theory again is correct in this sensitive test. So much on quarks and gluons. The next topic I would like to touch on is the measurement of the size of muon, electron and tau, leptons, in other words test of quantum electrodynamics. Let me summarise by saying the measurement at DESY on muon electron tao, shows quantum electrodynamics is correct. And if you want to express a radius or a size, muon electron tau are smaller than 10^-16 centimetres. And this shows the measurement as function of energy, the cross section of e+, e- goes to tau+, tau-. Compared with the prediction of electrodynamics. This agreement enables you to parameterise the size of the tau. Tau let me remind you is twice the mass of the proton, its measured size but now it's 1,000 times smaller than the size of the proton. Next question you can ask, is how many leptons exist. We now know there's electron, there's the mu, there's tau, tau is a mass of 1,8 billion electron volt. The question is how many more this type of family exist. This is the measurement a number of events as function of heavy lepton mass. The solid curve is the prediction of the number of events if heavy lepton exists. The red curve is a 95% upper limit confidence level. From this, one can see between two GeV and 16 GeV there's no more heavy leptons. Next question I will deal with, is the theory of Weinberg Salam on the fact of Z0. The first experiment, the first important experiment was carried out again in 1978 by the group of Taylor on polarised electron scattering from nucleus. When you have an electron with a polarisation about 40% scattered from a nucleus, 2 time contribute, one is the photon, another is the Z0, which carries the weight force. And so because of Z0 you have a parity violation in fact. And so you measure the asymmetry, which is the difference between right handed cross section minus left handed cross section compared to the sum. And this is the measurement of precession of spin vector in the analysing magnet. This experimental asymmetry normalised after you remove the polarisation in Q^2 dependents, as a function of incident energy, which is measured in the analyser magnet, in the magnet. And you see a spin precession. And this in units of 10^-5 and shows parity indeed is violated in this process. A direct comparison with the Weinberg theory is shown here. This is the measured asymmetry normalised to Q^2, versus a quantity, which is called rapidity and this is the Weinberg angle and this is the data, the data says the angle is about .2. And these are other possible models, which clearly are ruled out by this theory. There is of course another way to study the weak neutral current and that is in the time-like region. And that is by comparing the mu+, mu- production from e+, e-, which has a photon term and Z0 boson term. And then if you measure the forward, minus backward distribution, mu+, mu- and you will see an asymmetry. The theory of Weinberg and Salam is minus 9.2%, measurement by the TASSO group, so minus 16 plus or minus 3.2%. To this is the data compared with the theory and this is the pure electromagnetic interaction, clearly is ruled out. A more precise measurement has been carried out recently by my group and that is shown here. Measurement of forward, backward asymmetry is a very difficult process, because the effect is very small and therefore the first thing you want to do is calibrate your detector. Make sure detector has no asymmetry. And this is the measurement of a cosmic ray asymmetry and shows the detector is symmetrical to 1%. Measurement of mu+, mu- are low energy, 14 GeV, 22 GeV shows no asymmetry. And again in agreement with the theory. Measurements at high energies clearly show asymmetry. Indeed the asymmetry measurement, so now you know this systematic error is less than 1%, you have some confidence to quote these recorded errors, so minus 8.4, plus minus 2.1%. The Weinberg Salam is minus 7.6%. And this also sets a limit of Z0 larger than 51 GeV. In the next months time PETRA will go to 45 GeV with a luminosity of 10^31. With this one should be able to measure charge asymmetry within one year to an accuracy of 13 plus or minus 1.8%. And therefore you will be able to find the mass of Z0 to 25% accuracy. And that should be therefore the program for DESY for the next few years' time. Of course, you can always continue search for the top quark, up to 45 GeV, that's the highest energy at PETRA. Professor ... (inaudible 32.00) current prediction is 38 plus or minus two GeV. Theory is always a little bit ahead of experiment. It of course is interesting if it's correct. After that at this moment there's a very important plan at DESY to use the existing electron positron colliding beam as the pre accelerator to do a large energy electron proton accelerator. And that means you will have a proton in one direction, electron in another direction, colliding here and you can perform experiments. If this accelerator is visualised, you can then pretty much see what partical physics would be until the end of the century. Until the end of this century the United States you have a proton, antiproton collider, 2,000 GeV, the basic reaction of course quark, antiquark. Since each quark is 1/3 of the energy so the total quark, antiquark energy is 660 GeV. The distance you can probe is about 5, 10^-18. You study strong interaction. In Geneva at CERN there's an e-, e+, which has 130 GeV, 130 GeV, in this case you have pure lepton, lepton interaction. The total centre mass is 260 GeV, 10^-17 centimetre is essentially a study you like to win. In Hamburg, if this project goes ahead you have electron proton, which is just a cross between these two, that's why it makes it so interesting. You have a 30 GeV electron, 820 GeV proton and therefore you have a lepton quark interaction. This is the lepton, lepton, quark, quark, this is the lepton, quark. And this will probe 100 GeV centre mass, the distance is 3*10^-17, you will study electroweak and neutral current combined. That means for particle physics at least if all these projects go ahead, one can certainly do many interesting physics until the turn of this century, at, which time I will be retired anyway. Thank you. Applause.


This is Samuel Ting’s second lecture held at the Lindau Meetings, three years after the first one. The two lectures are connected and tell more or less the same story, the story of a travelling high-energy physicist, who moves from one accelerator to another in search of higher and higher energies. Ting seems immediately to have understood the idea behind the Lindau Meetings and, as a number of other Nobel Laureates, fallen in love with it. Understanding the idea partly means that he knows that most of the students and young researchers in the audience are different from meeting to meeting. So he could in principle tell more or less the same story every time. But Ting, even as a Nobel Laureate, is an extremely active physicist and he cannot resist telling the latest news from his work. So after an historic introduction, involving a long list of Nobel Laureates such as Rutherford, Yukawa, Hofstadter, Gell-Mann and Taylor, he concentrates on his own work. This involves the discovery of the 4th quark, the charm quark, for which he received the 1976 Nobel Prize in Physics together with Burton Richter. He then moves on to the work he has done looking for more quarks, since at the time of the lecture the number of quarks was still an open question. The 5th quark had been discovered, but Ting could not find any signs of the 6th quark. So he instead brings up the question of how large the quarks are and concludes that according to his experiments they are pointlike. At the PETRA accelerator in Hamburg, where very high energy beams of electrons and positrons were made to collide, he worked in a team which discovered the so-called three jet events. These are signatures of the existence of the carrier of the strong force, the gluon. The discovery of the gluon has not been recognised with a Nobel Prize in Physics, maybe because there were so many collaborators in the experiments. So far (2012) the physics prize has not been given to whole groups.Anders Bárány