Chapter 5 Particles –Energy states After the discovery of electrons in 1897, protons, neutrons, pions, kaons, sigmas, rhos, and other particles have been discovered. Some of them in cosmic rays and some generated in particle collision experiments. "Everything should be made as simple as possible, but not simpler." Albert Einstein Enrico Fermi, the father of nuclear reactors, once said “Young man, if I could remember the names of all these particles, I would have been a botanist”. Botanists classify and organize plants into families according to their structures and functions. They also seek relationships among living organisms. These methods applied to study of particles are led to the Standard Model for the material worlds. Studies of radioactivity developed machines to detect particles. These machines also detected many particles in the cosmic rays during the 1930s. After the World War II, physicists have created these particles using accelerators. The study of these particles opened a field called particle physics or high-energy physics. New breed of particle physicists work to gain simplified understanding of the materials. They are the modern-day philosophers pursuing the ultimate understanding of the material world. Ordinary matter consists of molecules and atoms. Radioactivity revealed the components of atoms and nuclei. These components are electrons, protons, and neutrons. Theoretical and experimental physicists poke closer to these particles. They saw more some structure features of protons and neutrons and suggested that nucleons were composed of quarks. Furthermore, quarks are the components of other particles. A standard model summarizes the relations of quarks, electrons, particles and their forces of interaction. Particles cannot be visualized as tiny analogues of ordinary material objects such as billiard balls or dust particles. Their properties are different from those of ordinary (or classical) particles. They are better called energy states. Atoms and molecules also have various energy states, and their transitions lead to atomic phenomena. The change of states in particles is usually considered a decay process. Some particle states such as the protons are stable for an indefinite time, but others exist for a short time. We will take a look at these particles in this Chapter. 149 Particles and Antiparticles When we discussed the beta decay process, properties of positrons were introduced, but the concept of antiparticles was left out. Every particle (state) has a corresponding antiparticle (another state). Particles and antiparticles have the same mass, but they have opposite electric charge, and magnetic moment. We generally believe the theory because of its simplicity and symmetry. The Concept and Discovery of Antiparticles Antimatter is a common term used by experts, fictionists, and ordinary writers. Antimatter refers to antiparticles such as positrons, antiprotons, antineutrons etc. Theoretically, antiparticles can also combine to give antiatoms and antimolecules, forming a world of antimatter. How did the concept of antiparticles originate? When and who initiated this concept? What is the theory about? Why do we believe in it? Do neutral particles have antiparticle partners? What experimental evidences show the existence of antiparticles? In 1930, Paul A.M. Dirac studied the electrons. He combined the theory of relativity with quantum mechanics and derived a wave function for free electrons. His theory predicted four energy states, two of which due to the spin. His theory suggested the quantum number +1/2 and –1/2 for the spin, which has already been detected. Two other states are for electrons with positive charges. Some basic considerations are given below concerning the background leading to his suggestion of antiparticles. The linear momentum p is the product of the mass m and its velocity v , p = m v. For a particle with rest mass m0 moving at a speed v close to that of light, c, its mass m increases according to Einstein theory of relativity, m0 m= . 2 v 1 ( c) This m is the relativistic mass. The total energy E of the particle is Dirac, Paul Adrien Maurice (1902-1984), shared the Nobel Prize for Physics with the Austrian physicist Erwin Schrödinger in 1933. He is known for his work in quantum mechanics and for his theory of the spinning electron. 150 2 E=mc = m0 c 2 1 (v c)2 . Dirac studied these relationships, and from the above two equations he derived the following equation, Derivation of Dirac's equation E2 = p 2 c 2 + (m0 c 2 ) 2. Square both side of Einstein's equation In which, the quantity m0 c 2 is the energy corresponding to the rest mass. Taking the square E 2 = (m c2)2 = m02 c 4/(1-(v/c)2) root from the above equation gives two expressions for energy: From this, we have E+ = + {(p2 c2 + (m0 c2 )2}1/2 > m0 c2 (m c2)2 - m 2 v 2c 2 = m02 c 4 E– = – {(p2 c2 + (m0 c2 )2}1/2 < m0 c2 Thus, E 2 = p 2 c 2 + (m0 c 2)2 and The total energy E+ increases as the particle gathers speed, but the total energy E– decrease as the particle gathers speed. The E– of a moving particle is less than that of the rest mass. This notion of negative energy is contrary to the case of ordinary particles, represented by E+. Dirac further worked out the wave function for particles fitting the E– description. Such a particle should have opposite spin, charge, and magnetic moment with respect to the normal particle. Such a particle is called an antiparticle. The Concept of Antiparticle Positive Energy States Zero Energy States In the case of electrons, Dirac further suggested that in a vacuum, all negativeenergy states are occupied, and positive Negative Energy States energy states are completely empty. Uniformly occupied or unoccupied (a) (b) ( c) (d) energy states are unobservable, see (a). When an electron occupies a positive energy state, it is observable, see (b), as is when one of the filled energy states become unoccupied, (c), it’s an antielectron. However, from the unobservable condition in (a), an electron can be promoted to the positive energy states, and at the same time create a vacancy in the negative energy states. This electron-antielectron pair (d) is observable but energy is required for this promotion. However, an electron-antielectron pair (d) is unstable, and the electron will return to the negative-energy state in a process called annihilation. The particle and antiparticle vanish, releasing energy in the form of photon. 151 The positron (e+ or +) is the first antiparticle discovered by Carl D. Anderson in 1932, when he was a graduate student in R. Millikan’s (he determined the charge on an electron) research group. Anderson made the discovery while studying cosmic and gamma () rays. He built a cloud chamber to study cosmic ray tracks under the influence of a magnetic field, and he observed some tracks similar to those of electrons but bent in the opposite direction. Millikan thought these tracks were due to protons, but Anderson considered them too thin to be tracks of protons. He first considered them tracks of electrons entering the cloud chamber from the opposite direction, and he Illustration of Electron and Positron inserted a lead plate in the chamber to see Tracks Observed by C. Anderson the effect in order to solve the puzzle. Lead When the particle passed the lead, its track plate bent more, due to loss of energy. Then, he reviewed Dirac’s proposal of unoccupied e+ negative energy states and called the antiparticle positron. e– Anderson observed more positrons, and he often saw equal numbers of positron and electron tracks in his cloud chamber. At the end of positron tracks, two tracks due to gamma photons were observed, and they are due to the annihilation of positrons and electrons. A year later, the creation of an electron-positron pair using high-energy photon was confirmed, and this process is called pair production. Today, positrons and electrons are created routinely using high-energy photons. These particles are created for particle accelerator. Electrons and positrons move in opposite directions in circular accelerators accelerated using electric and magnetic fields. When high-energy electrons and positrons collide, particles with rest mass heavier than electrons are produced. For example, the Brookhaven National Laboratory on Long Island, New York and CERN (Conseil Européen pour la Recherche Nucleaire or European Organization for Nuclear Research) on the outskirts of Geneva are some of the laboratories dedicated to this type of research. The term positron rather than antielectron is used more often, and its symbol is e+ or +. However, the symbol for an antimatter particle is usually a bar over the corresponding matter particle symbol. For example, an antiproton is denoted by p , pronounced p-bar. Annihilation and Pair Production + + – 2 e– + + – Skill Building Questions: 1. What are the four states of electron according to Paul Dirac's results? e+ e– Carl D. Anderson (1905-1991) with Victor Francis Hess of Austria, won the Nobel Prize for Physics in 1936 for his discovery of the positron, or positive electron, the first known particle of antimatter. 152 2. Derive the equation E2 = p2 c2 + (m0 c2 )2 using the statements given in this section. (Only for those interested in derivation of formulas. It is not difficult.) 3. What properties are the same for particle-antiparticle pairs and what properties are different? Compare the properties of an electron and a positron? 4. Describe the experiment leading to the discovery of positron and explain annihilation and pair production. Antiprotons and Antineutrons Dirac’s proposal regarding particle-antiparticle pairs is valid for any particle or matter. Existence of antiparticles for protons and neutrons is very important. There was a race to detect antiprotons and antineutrons. Do protons and neutrons have antiparticles? What are the properties (behavior) of antiprotons and antineutrons? Where and how can they be detected? Do all particles have antiparticles? After the discovery of positrons, everyone is convinced that for every particle, there is an antiparticle. A race was on to be first to find antiprotons and antineutrons. Researchers in Berkley built an accelerator called Bevatron to produce particles (protons) with a kinetic energy of 6.2 GeV, and they hope to find antiprotons when the high-energy protons collide with a target of fixed protons in liquid hydrogen. In the mean time, there is a belief that antiprotons are present in cosmic rays. Simultaneously, many groups looked for antiprotons. One group led by Emilio Segrè looked for negatively charged particles with mass similar to that of a proton among particles generated from the bombardment of high-energy protons. They used a variety of methods for the detection of fast moving particles. They realized that an antiproton would annihilate a proton when it stroke one, converting both masses to pure energy. When they observed a set of star-like tracks called starburst, they thought the energy of annihilation re-materialized quickly, converting to many charged particles including positive and negative Star-like Tracks Produced by an Antiproton Annihilating a Proton in a Bubble Chamber antiproton Segrè, Emilio (Gino) (1905 - 1989) Owen Chamberlain (1920-) received the Nobel Prize for Physics in 1959 for the discovery of the antiproton, an antiparticle having the same mass as a proton but opposite in electrical charge. 153 pions (+ and –). In a magnetic field, these pions bend in different directions due to their positive and negative charges. The starburst is a typical annihilation signature of antiprotons. A typical one produces 8 pions, 4 – and 4 +. Energies of these particles were estimated from the tracks, and their total energy is equal to the kinetic energy of the antiproton plus the rest masses of two protons. More than 100 antiprotons have been detected before their observations were published in 1955. Segrè and Chamberlain group members included Clyde Wiegand and Tom Ypsilantis. The star-like tracks were captured in a 72-inch liquid hydrogen bubble chamber, and how such a chamber works will be discussed in a Chapter discussing interactions of ionizing particles with material. After the discovery of antiprotons, attention was directed to antineutrons. A year later, Bruce Cork, also working in Berkley, thought that when an antiproton neutralized its charge with a proton, instead of annihilating it, the proton and antiproton pair will convert to a neutron and antineutron pair, p + p n + n. Cork observed the annihilation of an antineutron and a neutron also produces a star-burst of charged pions and photons. Particles and antiparticles have opposite electrical charge in the case of electrons and protons, but antineutrons, have opposite magnetic moment of neutrons. The detection of antiprotons and antineutrons not only further confirms and generalizes the Dirac's theory of antiparticles. Skill Building Questions: 1. What are the properties of protons and antiprotons? 2. The production of neutron and antineutron from proton and antiproton is endothermic. What is the minimum energy input to the system for this reaction? 3. There so much more matter in the universe than antimatter, why? Why did matter "win" instead of antimatter? (The answers to these questions are still being sought for.) Antihydrogen Antielectron and antiproton are components of an antihydrogen atom, but … Is a system consisting of an antielectron and an antiproton stable? Has an antihydrogen ever been created or detected? Can antiatoms form antimolecules? A system consisting of an antielectron and an antiproton is an anti-hydrogen atom. As long as these atoms do not contact normal hydrogen atoms and they are stable theoretically. 154 Predictions of something to be discovered or made are frontiers of science. A search in the Encyclopædia Britannica Online on June 8, 1999 <http://www.eb.com:180/bol/topic?eu=123833&sctn=1> resulted in the following description. In early 1996 a team of physicists reported achieving that goal, although fleetingly, in the Low Energy Antiproton Ring (LEAR) storage facility at CERN (European Laboratory for Particle Physics) near Geneva. The team, led by Walter Oelert of the Institute for Nuclear Physics Research, Jülich, Ger., began with a stored beam of antiprotons circulating in the ring and squirted a jet of xenon atoms across its path. Interaction between the xenon and the antiprotons sometimes produced pairs of electrons and positrons, and very occasionally one of the positrons teamed up with an antiproton, forming an atom of antihydrogen. Once made, the neutral antiatoms left the magnetic confinement of the storage ring and were detected. Over the course of three weeks, the experimenters reported the detection of nine such antiatoms, which existed for an average of 40 billionths of a second before they annihilated with normal matter. The antideuterium, a composite antiparticle consisting of an antiproton and an antineutron, was first produced with a 30-GeV (giga-electron volt) synchrotron at Brookhaven National Laboratory, Long Island, N.Y., in 1965. Antideluterium is an antiparticle of deuterium nucleus. Skill Building Questions: 1. What is the difference between an antiparticle and an antiatom? 2. Is the system consisting of an electron and an antiproton stable, why or why not? 3. What is common between an antiatom and an antideuterium? (Both are composite antiparticles.) 155 The Standard Model and Particles The standard model (SM) describes material structures in terms of fundamental components (or building blocks). Chapter 3 mentioned the SM. At that time, ordinary material was described in terms of electrons, neutrinos, down and up quarks. In the standard model, two types of fundamental particles are leptons and quarks. Electrons, positrons, antineutrinos, and neutrinos in beta decays are considered first generation leptons. Their heavier ancestors are muons, – and +, and muon neutrinos, and v . Their properties are the same as their first generation counterparts, except that the muon mass is 207 times that of an electron. The still heavier ancestors called taus are 3500 times the rest mass of the electrons. They are even heavier than protons. The neutrinos have almost zero mass. All leptons have spin ½. Antiparticles have opposite spins, so do their associated neutrinos. Charges on corresponding leptons for all generations are the same. Particles of the Standard model Leptons Quarks Charge Mass (GeV) First Generation Second Generation Third Generation – – Electron, e Muon, , Tau, –, + Positron, e Antimuon, + Antitau, +, Up, u; +2/3; 0.3 Muon-neutrino, Tauneutrino, Neutrino, v e, Antineutrino, e. Anti-muon-neutrino, v , Antitauneutrino, v Down, d Strange s; Charm, c Bottom b; Top, t –1/3 –1/3; +2/3 –1/3; +2/3 0.3 0.5 1.5 4.8 176 Charged particles interact with each other via gauge bosons, whereas the quarks interact with one another via strong-force carriers called gluons. Quarks as Fundamental Particles Up, down, strange, charm, bottom, and top are the six fundamental quarks of three generations in SM. Ordinary matter is made up of first-generation quarks and leptons: up (u) and down (d) quarks, and electrons (e–). Particles created by high-energy particle collision contain heavier quarks s, c, b, and t and leptons and . How did the concept of quarks come about? Are the quarks real particles? What compelling evidences show their existence? Why are quarks considered elementary? 156 We have mentioned in an earlier Chapter that the concept of quarks came from the study of relationship among particles such as baryons and mesons. In 1961, Murray Gell-Mann and Yuval Ne'eman proposed a particle classification scheme called the Eightfold Way, based on the mathematical symmetry group SU(3). In 1964 Gell-Mann introduced the name quark from a passage in James Joyce's novel Finnegans Wake. George Zweig developed a similar theory independently that same year and called his fundamental particles aces. Mesons (for example pions and kaons) consist of a quark and an antiquark, whereas baryons (protons and neutrons) consist of three quarks. Gell-Mann and Ne'eman postulated three quark types: u, d, and s. Each carries a fractional electric charge, has a mass, and a spin 1/2. They obey the Pauli exclusion principle. No theory or experiment seems to suggest that they can be resolved into something smaller. Quarks always seem to occur in combination with other quarks or antiquarks. No single free quark has been detected yet. Since no free quark has been detected, investigations continue. Quantum chromodynamics (QCD) of strong interactions, developed in 1977, introduced the concept of colour, which has nothing to do with the colours of the everyday world, but as a property of quarks, similar to the charge as a property of electron. The colours red, green, and blue are ascribed to quarks, and their opposites, minus-red, minus-green, and minus-blue, to antiquarks. According to QCD, all combinations of quarks must contain equal mixtures of these imaginary colours so that they will cancel out one another, with the resulting particle having no net colour. Similar to exchange of photons between charged particles, massless gluons exchange between particles having color. Just as photons carry electromagnetic force, gluons transmit the strong forces that bind quarks together. Quarks change their colour as they emit and absorb gluons, and the exchange of gluons maintains proper quark colour distribution. Further support for the quark model comes from the study of interaction between pions and protons. Pions with certain kinetic energies seem to form a temporary resonance particle with protons. These unstable resonance particles decay within 10–23 seconds. By studying the direction of the decay products, the proton was considered to have three centers or quarks. Studies of electron scattering gave the same conclusions. Quarks charm ( c) and bottom (b) found in the late 1970s as components of excited kaons and extended the number of quarks to 5. Quarks are believed to occur in pairs, and a top (t) quark will complete the pairs. A theory suggests it carries a +2/3 electric charge; its partner, the bottom quark, has a charge of -1/3. In 1995 two independent groups at Fermi National Accelerator Laboratory reported that they had found the top quark. A weighted average of their results gives the top quark a mass of 176 +/- 12 GeV (giga electron volts). (The next heaviest quark, the bottom, has a mass of 4.8 GeV.) Masses of u and d are believed to be equal (~ 0.3 GeV or 1/3 of the masses of proton or neutron ~310 MeV). No direct measurement is available. 157 High energy particle-antiparticle collision Eight JP = ½+ Baryons experiments produce baryons sigmas (–, 0, & +, mass ~1.192 GeV) lambda (, mass 1.115 udd uud GeV), and cascade (– & 0, mass 1.317 GeV). n p Using the quark model, these baryons are related to the stable proton p and neutron n. dds uds uus – 0 Such a relationship in terms of their quark + 0 composition is shown here. Baryons and are composed of the same quarks, uds, but the quarks in them have different spin – 0 dss uss combinations, corresponding to uds 0 and to uds, where the arrows indicate the spin of the quarks in the composite particles. Possible spin combinations and energy states in these composite particles are of great interest in the field of particle physics. Compared to atoms or atomic nuclei, particles are relatively simple systems. Well, if you think that combination of u, d, and s quarks should produce more particles than 6, you are right. For J P = 3/2+, there are 10 more combinations, and all corresponding particles have been detected. These particles have very short half-lives and they are often called resonances. As such, their masses have not been measured accurately. Seven of the ten J P = 3/2+ resonances are excited states of the J P = ½+ baryons. Ten J P = 3/2+ Resonances ddd ddu duu uuu – 0 + 2+ dds dus uus – 0 + dss uss – 0 sss – Three new resonances or baryons containing the same 3 quarks are interesting. From the charges of these resonances, we get a sense of why quarks are considered to have their respective fractional charges, -1/3 for d and s, +2/3 for u. By combining the u, d, and s quarks, we have shown some interesting relationships for mesons and baryons. When these are combined with the charm quark c, the bottom b and top t quarks, many more particles are possible. In fact, discoveries of particles containing c, b, and t quarks confirm the existence of c, b, and t quarks. A free quark has not been detected, despite great effort for their search. Review Questions: 1. What are the quark compositions for mesons? What forces hold the quarks together in mesons? 2. What are the quark compositions for neutron and protons? Sigma zero and lambda have the same quark compositions, but the two particles are different, why? 158 Hadrons Mesons and baryons are collectively called hadrons. They are composed of quarks. Since there are six quarks, their combinations and permutations give rise to a large number of hadrons. What particles have been detected? How are they related to each other? How are they related to the material world? In these relationships, what particles are fundamental or basic? How are materials related to these fundamental particles? Via what forces do particles interact with one another? What forces hold particles together in composite particles? What are responsible for the exerting the forces between particles? We have mentioned leptons as fundamental particles. They combine with baryons to form atom-like systems. Hadrons (mesons and baryons) are composed of fundamental particles called quarks. Ordinary materials are composed of 1st generation particles. Particles containing quarks of 2nd and 3rd generations are formed when large quantities of energy materialize (forming a bound state). Their lifetimes are short, decaying into particles (states) made up of first generation leptons and quarks. Mesons were originally defined as particles with masses heavier than electrons, but lighter than protons. For example, pions (pi-plus +, pi-minus –, and pi-zero 0) and kaons (K+, K–, and K0) are well known. However, the J/psi (J/), D, and Upsilon (Y) discovered in the 1970s with properties similar to those of pions and kaons are much heavier, 3.1 GeV for J/ and 9.5 Gev for Y. Their existence called for a review of the fundamental particle theory. We now consider particles consisting of a quark and an antiquark mesons. Since quarks have half spin, mesons have integral spin, 0 or 1. Quark contents and properties of some mesons are listed below: 159 == Properties of Some Mesons == Name Symbol Mass Lifetime Charge Spin Quarks Pi-zero 0 Pi-plus + Pi-minus – 135 MeV 140 MeV “ 0.8x10–16 s 2.6x10–8 s “ 0 +1 –1 0 0 0 u u or d d ud du K-zero K0 K-plus K+ K-minus K– 498 MeV 494 MeV “ 10–8 to 10–10 s 0 1.2x10–8 s +1 “ –1 0 0 0 ds us su J/psi D-zero D-plus Upsilon 3.1 GeV 1.87 GeV “ 9.46 GeV 10–20 s 10–12 s 4x10–13 s 10–20 s 1 0 0 1 cc cu cd bb J/ D0 D+ Y 0 0 +1 0 Pi-plus and pi-minus form a particle-antiparticle pair, so do K-plus and K-minus. Pi-zero and its antiparticle are the same, but K-zero has an antiparticle. These are inferred from their quark compositions. Some mesons contain quarks from different generations. The meson J/psi (J/) was independently discovered in 1974 by two teams working at two facilities: B. Richter at SLAC and S. Ting at BNL. This discovery called for and confirmed the existence of a charm quark (c ) for the first time. The discovery of D was made in 1976 by a team led by Goldhaber at SLAC and BNL. A year later, the Upsilon was discovered by Lederman’s team at Fermilab. Baryons consist of three quarks and they have some common properties. For example, they have half-integral spins of ½, 3/2, 5/2 .... The most familiar baryons are the proton and the neutron. All but protons are short-lived. Some baryons consist of quarks from different generations, similar to some mesons. Properties of some, but not all, baryons are listed below: The discovery of this particle, thought to be composed of a charmed quark and its antiquark, led to a significant expansion and refinement of the quark model. For this discovery Samuel Ting and Burton Richter were awarded the 1976 Nobel Prize for Physics. Leon Max Lederman, Melvin Schwartz and Jack Steinberger, received the Nobel Prize in 1988 for physics for having established the identity of muon neutrinos. 160 == Properties of Some Baryons == Name Symbol Mass Lifetime Charge Spin Quarks Proton Antiproton p 938.3 MeV p “ stable +1 -1 ½ -½ uud uud Neutron Antineutron n 939.6 MeV n “ 600 s 0 0 ½ -½ udd udd Lambda Antilambda 1.115 GeV “ 2.6x10–10 s “ 0 0 ½ -½ uds uds Sigma 0 1.192 GeV 6x10–20 s Sigma-plus + 1.189 GeV 0.8x10–10 s Sigma-minus – (anti particle of +) 0 +1 -1 ½ ½ -½ uds uus uus Charmed lambda +1 ½ udc c 2.28 GeV 2x10–13 s Note that the lambda consists of a strange quark. This neutral particle is similar to a neutron, and a nucleus containing this particle is a hypernucleus. The charmed lambda c contains a charm quark, it’s charge same as a proton. This table lists some baryons involving only four quarks. Some baryons contain b and t quarks are not listed here. The baryons containing the b and t quarks are not listed here yet, due to a lack of definitive information about them, leaving an opportunity for you to carry out some self-study. Review Questions: 1. What are the first generation leptons and quarks? What are second generation leptons and quarks? 2. What are mesons? According to the definition you have given, how many mesons can be derived from six quarks? 3. What are baryons? Illustrate with some examples. Force Carriers Particles interact with one another. Attraction, repulsion, or the recognition of each other's existence is an interaction, and such a phenomenon is due to a force between the particles involved. Without such a force, particles will be indifferent of each other. 161 Is it possible to have an interaction without an exchange of something? How many kinds of interactions or forces are there among various particles? What are exchanged for these forces and how do we recognize them? An interaction is an exchange of something. An exchange usually results in an attraction or repulsion, which we call a force. Without an exchange, particles will ignore each other, and they do not experience the existence of each other. The Newton's law of gravity and Coulomb's law of electromagnetic attraction represent two types of commonly known forces. As long as particles or objects have masses, they experience gravitation attraction of each other. As long as the objects or particles are electrically charged, they experience Coulomb forces or electromagnetic forces. Generally, we assume that gravitons and photons exchange to give rise to these forces. Many properties for photons have been introduced. They interact with charged particles. Thus, you may find the Feynman diagram (to be discussed later) a fine presentation of the electromagnetic force. Exchange of photon, zigzag line, makes electrons repulse each other, and photons are called force carriers. Feynman Diagram for Electron Scattering space e e’ time The existence of the gravitons as force carriers is very reasonable and physicists have postulated their existence, but their properties are unknown. In fact, we do not know how to detect them. Their existence is important from theoretical point of view, but their search progresses slowly. Gravitational and electromagnetic forces cannot confine nucleons in a nucleus due to electromagnetic repulsion of the protons. Thus, there must be other types of force acting between nucleons. For this type of interactions, the quantum field theory suggests strong force and weak force being responsible. Exchange of the force carrier gluons, equivalent to photons of the electromagnetic force, gives rise to the strong force. These carriers exchange between particles such as quarks that have a property called color. This term is absolutely unrelated to the color we see, but a name for properties similar to the electric charge for the electron. Only particles with color experience strong force by exchanging gluons. Radioactive beta decay inter-converts neutrons and protons. The half-lives of beta decays are long. Interactions leading to these conversions have been called weak interactions, because they are weak compared to the force binding quarks together. Furthermore, the force carriers of weak interactions, W and Z have been detected. Although their lifetime are very short. These bosons are heavy, with masses about 100 times the mass of a proton, and their heavy masses cause the force appear weak. In radioactive decays, the strength of the weak force is about 100,000 times less than the strength of the electromagnetic force. However, it is now known that the weak force has intrinsically the same strength as the electromagnetic force, and these two apparently distinct forces are believed to be different manifestations of a single 162 "electroweak" force ("weak nuclear force" Encyclopædia Britannica Online <http://www.eb.com:180/bol/topic?eu=78363&sctn=1> [Accessed June 9, 1999]). The discussion above introduced four types of forces: gravitational, electromagnetic, strong, and weak. Force carriers are called gauge bosons. As a summary, the four forces and their carriers are tabulated below: Forces and Force Carriers (Gauge Bosons) Gravitational Electromagnetic Carrier Mass (GeV) Decay mode Lifetime Charge Spin graviton 0 photon (hv) 0 - - stable 0 unkown stable 0 1 Weak Strong W+, W–, Z0 8 gluons (g) W : 83.3 ~0 Z: 91.2 W e, + Z e ,e ; 10–25 s stable +1, -1, 0 0 1 1 Some theories predict the existence of one or more Higgs particles, which are massive force carriers of other fields not familiar to us. Higgs particles are responsible for the masses of all the other particles. Higgs particle or particles have yet to be found, yet more complicated theories predict additional particles. For example, supersymmetry predicts existence of gauginos, sleptons and squarks; additional weak bosons W' and Z' has been predicted; GUT theories suggest X and Y bosons; the technicolor models require Majorons, familons, axions, paraleptons, ortholeptons, technipions; Some theories suggests existence of B', a hadron with 4th generation quarks, magnetic monopoles, excited leptons e*, etc. These "exotica" have yet to be confirmed. The search is on! Review Questions: 1. How many kinds of force exist in nature? 2. What are the Coulomb's law and the law of gravity? What are the similarities and differences? What are the gauge bosons for the gravitational and the electromagnetic forces? 3. What are strong and weak interactions? What are the force carriers? 163 High-Energy Particles The previous section summarized the standard model that has given a fundamental description regarding the structure and interactions (forces) of the material world, from atoms to rare encountered high-energy particles. A lot of theoretical and experimental work has gone into the study to give such a summary, and the development from an experimental point of view is also very interesting. In fact, radioactivity and high energy particles are not printed symbols in publications or created in sophisticated laboratories, many of them are around us. Cosmic Rays After its discovery, radioactivity was detected in all kinds of places: under water, under ground tunnels, caves, sea level, mountain tops, and high above in the atmosphere. For example, Wulf observed high radioactivity at the top of Eiffel Tower. Soon after, Victor Hess went high up with balloons and detected very high radioactivity in the atmosphere. So did Millikan’s and Rutherford’s groups. These discoveries astonished them. What particles are actually responsible for this apparent high radioactivity? What is the source of the high radioactivity high up in the atmosphere? Is the origin of high radioactivity terrestrial or extraterrestrial? Millikan thought high-altitude radioactivity was gamma rays from outer space and coined the phrase cosmic rays for them, and others found penetrating positively charge particles, at sea level and below. Cosmic rays are also present underground. The planet Earth is constantly bombarded by cosmic rays. Measurements by space ship indicate that primary galactic cosmic rays are high-energy protons (83%), helium nuclei (16%) and other nuclei. Protons with energy 2 GeV are the most abundant, but some have energy as high as 20 TeV (1012 eV). High-energy particles penetrate deep into the ocean and solid Earth. They interact with terrestrial atoms producing secondary cosmic rays of subatomic or high-energy particles. Intensities of cosmic rays vary with altitude, geological locations, and year. Generally speaking, intensity at the equator is lower than that at a polar region. Cosmic rays cause interesting phenomena such as aurora. Skill Building Questions: 1. What are the components of primary galactic cosmic rays? What are secondary cosmic rays? 2. What is the energy distribution among particles in the primary cosmic rays? (Further reading and research required) 164 Leptons First generation leptons are electrons, neutrinos and their antiparticles. All these are particles emitted in the beta decay process. What are higher generation leptons? What are their masses and spins? How were they discovered? How do these particles relate to the fundamental particles of the material world? In 1936, Anderson and his colleagues observed some denser tracks of positively and negatively charged particles in the cloud chamber than those made by electrons and positrons. They thought there were two types of electrons, and they calculated their masses to be 1/9th of that of protons, or almost 207 times heavier than that of electron. In additional to antielectrons, they have discovered the electron-like muons. Muons are charged, and they lose energy by knocking electrons out of atoms (ionization). Muons were at first thought to be pions, the particle predicted by Yukawa in 1935 as the strong force carriers. However, muons never interact with nuclei, but decay by the weak interaction with a lifetime of 10–6 seconds into electrons, electron neutrinos (1st generation leptons) and muon neutrinos, + e+ + e + – e– + v e + v These properties made muons the second-generation leptons, not strong force carriers. The third generation lepton tau was discovered in the Leptons collision between high-energy (3.6 GeV) electronGenerations positron beams. Properties of taus are similar to 1st 2nd 3rd those of electron-positron, muon antimuon pairs e–, e+ –, + –, + except that their masses are 3500 times those of v e, e v , v , electrons, 20 times those of muons, and 2 times those of protons. The pair discovered by Martin Perl and his colleague working at SPEAR are called taus, s, being the first letter for the Greek word “third”. Their existence has been predicted, and we believe that they should have their own accompanying neutrinos. Their discovery in 1974 expanded the number of leptons to six (12 if antiparticles are considered separately) in three generations. The rest mass for tau is 1.784 GeV. Taus have many decay modes, and their decays produce electrons, muons, or hadrons. The pure following leptonic decays of taus are not very common, but they have been observed, 165 – e– + v e + v + e+ + e + With the discovery of the taus, three generations of leptons are known. The electron, muon, and tau masses are 0.511, 107 and 1784 MeV respectively. These compared to a rest mass of 938 MeV for the proton. Unlike the electron, which appears to be stable, the muon decays after an average lifetime of 2.2 microseconds into an electron, a neutrino, and an antineutrino. This process, like the beta decay of a neutron into a proton, an electron, and an antineutrino, occurs via the weak nuclear force. Neutrinos at every generation are produced in the decays, and whether the neutrinos have a mass is an important question. Skill Developing Questions 1. What are muons? What their charges and masses are and how do they decay? What are the second generation leptons? 2. Why is the third generation electron equivalent called tau? Is it possible that tau particles are present in the cosmic rays? Masons Using photographic emulsions as detectors for the study of cosmic rays was C.F. Powell, working in the Cavendish Laboratory of C. Wilson, the inventor of the cloud chamber. They recorded precise tracks of many particles in his photographic emulsions. In some of the plates they exposed to cosmic rays carried high up by balloons, they observed some tracks similar to muon but slightly heavier, 1/7th (as opposed to 1/9th) of the proton mass. What did Powell discover? What are the properties of these particles, and into what do they decay? These particles are unstable, and after traveling a fraction of a millimeter, decay into muons. Powell' group thought they discovered the strong force carrier pion that had been previously predicted by Yukawa as strong force carriers. As we shall see later that pions decay into leptons, and they are not strong force carriers. What Powell discovered turned out to be pion plus ( +) and pion minus ( –), which are unstable with average lifetimes of 10–8 seconds. They travel a very short distance in the emulsion before decaying into muons, accompanied by muon neutrino or antineutrino, different from electron neutrinos.: + + + – – + v . Powell, Cecil Frank (1903-1969) winner of the Nobel Prize for Physics in 1950 for his development of the photographic method of studying nuclear processes and for the discovery of the pion (pi-meson). 166 These are most common modes of decay. Pions + – and muons + – are particleantiparticle pairs. Muons are often referred to as heavy electrons and heavy positrons, 207 times heavier. Muons and muon neutrinos (4 particles) are the 2nd generation leptons, similar to electrons and electron neutrinos. Pions decay by another mode, converting to electrons and positrons accompanied by the emission of electron neutrinos and antineutrinos + e+ + e Bubble Tracks from Decay of K– Interpreted for the Presence of 0 e+ e– – e– + v Lead plate A neutral pion (pi-zero), 0, has a short lifetime of 10–-16 seconds – (compared to of 10–8 seconds for 0 the charge pions). Thus, its detection was much more difficult K– than the charged pions. Two years later R. Bjorkland and colleagues of e+ + e – the Lawrence Berkeley Laboratory 0 (LBL) discovered 0. They bombarded metal targets by K– 0 + – accelerated protons, and the experiments produced many pions with their tracts recorded by a hydrogen bubble chamber. They observed tracks corresponding to the diagram shown here together with many other (not shown) tracks on the same photograph. The 0 left no tracks, neither did its decay products, two photons. The two pairs of positrons and electrons produced by the rays when they struck the lead plate implied the existence of the photons from 0 decay as predicted. They further calculated all the energy of the decay products, and the results matched the decay scheme as shown. The diagram on the left shows only the tracks, but particles of the tracks are indicated on the diagram on the right. The rest masses of the three pions 0, +, and – are 134.97, 139.57, and 139.95 MeV respectively. The similarity of their rest masses suggests that they contain similar quarks: (0: u u or d d , often represented as (u u - d d )/2; –: d u ; +: u d ). As a summary, their decay modes are given below: o + + e+ + e – e– + v e 167 G. Rochester and C. Butler discovered Dark Lines Imitating Kaon Tracks Kaons in 1947, from tracks recorded in Observed in 1947 their cloud chamber at Manchester K+ University. Gamma rays leave tracks in cloud chambers, but not in bubble K0 chambers. They saw two -ray tracks Lead Plate Lead Plate originating from the same point forming a V, and they thought a particle of zero charge named kaon, K0 was responsible for + the gamma rays. A track with a sharp kink appeared on another photograph. After their analysis of these tracks, they concluded that a kaon-plus has decayed into a muon and a muon-neutrino. The latter leaves no tracks in the cloud chamber. Diagrams are sketched to convey images of their observed tracks, but photographs are much more complicated. Kaons, like pions, are mesons. Their masses are intermediate between those of electron and that of nucleon. Kaons are heavier than pions. Their masses are: K0 498 MeV, K, 494 MeV. The discovery of kaon confirms the presence of a heavier quark strang. Kaons have many decay modes K0 2 or + + – K+ + + v K– – + v or + + 0 or + + ++ – or 0 + e+ + ve or – +0 or – + ++ – or 0 + e– + v e The transition, K0 + + – is forbidden, because the strangeness S is 1 before the decay, but S is 0 after the transition. The strangeness is attributed to the strange quark (s) for kaons. The process can only decay by weak interaction. Thus, the lifetimes for kaons are in the order of 10–10 seconds, long compared to the lifetime (10–16 seconds) of 0. This behavior is due to the strange quark and anti quark in the kaons. Note that K+ and K– form a particle-antiparticle pair, whereas the antiparticle of K0 is K 0. Light Mesons with JP = 0– S ds us Binding by gluons, a quark and an antiquark forms K0 K+ a meson. Ideally, three quarks (d, u, and s) uu or dd combine to give a total of 8 (23) mesons, but two du – 0 + ud I3 of the combination give rise to the same meson 0. – 0 K K Their relationship led to the Eightfold Way, su sd proposed by Gell-Mann and Ne'eman in 1961, given in the diagram here. Quark-compositions of mesons have been given in the summary earlier and the diagram. The diagram is more interesting if you notice the directions in which the quantum numbers change. The strange quantum number S (1 for K0 and K+, 0 for ’s and -1 for K–, and K 0) and spin quantum 168 number I3 (-1 for –, - ½ for K0 and K–, 0 for p0, ½ for K+ and K 0, and 1 for p+) change systematically. Since the angular momentum quantum numbers J for these particles are 0, and parity P = -1, the state JP is represented by 0–. Furthermore, high-energy collision experiments between particles and antiparticles have produced excited states of kaons: *K–, *K0, *K+ with mass ~890 MeV, and excited states of pions called rho (–, 0, & +, mass ~ 765 MeV). These excited states correspond to the same set of 8 mesons described earlier, but their angular momentum quantum numbers J = 1 with parity P = -1, thus, J P = 1– compared to J P = 0– for ground-state pions and kaons. Formations of groups of mesons and excited mesons suggest the existence of a quark charm. Other mesons J/psi (J/), D0, D+, and Upsilon (Y) were discovered in 1974, 1976, and 1977 respectively. The existence of J/ signifies the existence of the charm quark. Review Questions: 1. Make a table listing all properties of pions and kaons? 2. What are the quark compositions (qq), charges (Q), and spins (I) of mesons? Make a table for them. 3. How can the masses of pions and kaons be explained in terms of quarks? Gauge Bosons as Force Carriers Photons have been known as electromagnetic force carriers since the 1920s. While mediating electromagnetic interactions, they manifest themselves as radio, IR, visible, X- and rays. What are force carriers? Does the gravitational force have a force carrier? Are there equivalent force carriers for the strong and weak interactions? What will be produced when high-energy particle-antiparticle pairs annihilate? In 1972-3, high-energy particle experiments indicated that neutrinos interact with nuclei causing some of them to move slightly. A neutral force is responsible for this interaction. The weak interaction may be mediated by a neutral particle, and Z0 has been suggested as the weak force carrier. S. Glashow, A. Salam, and S. Weinberg united electromagnetic force with the weak force in their electroweak theory. The observation and theory gave the scientific community the confidence to award them the Nobel prize for physics in 1979, although the three force carrier particles, W+, W– and Z had not been observed at the time. Their masses, however, have been estimated to be about 90 GeV (actually 83.3 and 91.1 GeV for W and Z respectively given earlier in this Chapter). The discovery of positrons, antiprotons, and antineutrons excited many researchers in the high-energy research community, because they expected the collision and annihilation to produce new particles. Charged particles and antiparticles are ideal for particle accelerators and collision experiments. These particles can be accelerated in opposite directions using the same electric and magnetic 169 fields, stored in ring-shape paths, and focused to collide head-on with each other. Energy from the annihilation of high-energy particles produces new particles. Accelerators making use of e– and e+ were Adding a Ring to a Linear Accelerator built in many research centers. Attached to the famous 3-km Stanford Linear Accelerator SPEAR Center (SLAC), the Stanford Positron Electron Asymmetric Rings (SPEAR) was SLAC added to store high-energy electrons and positrons from the SLAC for collision experiments. This was a joint project between Princeton and Stanford. In other projects, the Brookhaven National Laboratories (BNL) on Long Island, New York, the Deutsches Elektronen Synchrotron Laboratory (DESY), the European Center for Nuclear Research (CERN) are some of the many laboratories making use of the electron positron beams. Later, p and p were accelerated to 30 GeV in Fermilab in Chicago. These experiments led to the discovery of many particles, including W+, W– and Z, which were observed in 1983 and 1984 by teams working in DESY. The tau boson mentioned earlier is also produced by accelerated particles. The formation of W and Z can be written as p + p W+ + H– p + p W– + H+ p + p Z0 + H0 where H represents one of the hadrons such as proton, antiproton, or neutron, or others. Theory indicates that the W and Z particles have very short lifetimes, ~ 10–24 seconds. The W and Z so produced decay by emitting electrons, positrons and muons, and their corresponding neutrino and antineutrinos. These heavy bosons have many modes of decay and they also emit protons and neutrons as accompanying particles. For example, W+ l+ + l ; ( l = e, or ) W– n + – + v e (n = neutron) or Z0 + + – + v + Z0 hadrons Discoveries of W, and Z convince us that there are four types of forces interacting among various particles: Gravitational force: the long-range force that stabilizes the solar system and governs the motion of stars and planets in the universe. This force is acting on all matter including subatomic particles, but due to their tiny mass, the force is insignificant compared to other types. Physicists suggest the graviton as the force carrier, but the method for its detection has not been invented. If gravitons exist, they are everywhere. 170 Electromagnetic force: the long-range force that stabilizes the atomic system of nuclei and electrons. This is also known as the Coulomb interaction. The strength of the force is proportional to the charge, not to the mass. Electric charges experience forces exerted by each other via photon exchanges. Photons can be absorbed, or scattered elastically or inelastically. Photons are the force carriers of electromagnetic force. Strong interaction: the force that stabilizes the atomic nuclei and among quark containing particles. In the 1930s, based on the distance (10–15 m) between nucleons. H. Yukawa estimated the force carriers of strong interaction to be pions () with a mass of 1/7th of a proton. Later, 8 gluons not pions were suggested and confirmed to be the strong force carriers, exchange of which gives rise to the strong force. These carriers exchange between particles such as quarks that have a property called color. There are 8 colors and 8 types of gluons*. Weak interaction: the force creates electron before its emission from a nucleus as a beta particle. Compared to the strong interaction, the force exerted by weak interaction is only some percent of strong interaction, and thus so named. The force carriers are W+, W–, Z. A summary is given in the table below: Type Gravity Electromagnetic Weak Strong particles Act on carrier All particles Charged particles All particles Colored The Basic Forces Force Range Strength System Graviton, g Photon, h v 1/r2 1/r2 10-39 1/137 Solar system Atom W and Z Gluon <10-17 m <10-15 m 10-5 1 Atomic nucleus Skill Building Questions: 1. Which one of the four forces is always present for any particle? Which one is only acting on nucleons? 2. What is the force carrier of electromagnetic force? What are force carriers of weak interactions? 3. Why are particle and antiparticle pairs ideal to have for particle accelerators? 4. How are W+, W–, Z particles produced? * A news release read: The 1995 EPS High Energy and Particle Physics (HEPP) Prize has been awarded to Paul Söding, DESY-Institute of High-Energy Physics, Zeuthen, Björn Wiik and Günther Wolf, Deutsches Elektronen-Synchrotron (DESY), Hamburg, and Sau Lan Wu, University of Wisconsin, Madison, WI, USA, for the first evidence for three-jet events in e+e- collisions at DESY's PETRA collider while working with the TASSO Collaboration. 171 Feynman Diagrams Feynman represented transformations and interactions of particles by diagrams based on mathematical and theoretical considerations. Feynman diagrams are particularly useful to convey particle physics ideas, because they let us visualize the particle process. What do Feynman diagrams show? How do particles interact with one another? Feynman diagrams show the interactions of particles by way of force carriers. Solid lines represent particles (usually Fermions), and zigzag lines represent force carriers (bosons). Normal progressing in time goes from the left to right. An arrow pointing to the right represents location change of a particle as time progresses, and a left-pointing arrow represents a particle moving in reverse time. When the arrow or line changes direction, the particle changes state. For example, the Feynman diagram Feynman Diagram for Electron Scattering representing the scattering of two electrons is space shown here. The electron e emits a photon, which is absorbed by e’, because the zigzag line moves to the right. The two electrons e experience the presence of each other by exchange the photon. This diagram represents the interaction of any charged particle via a e’ photon. time For the strong interaction between quarks via the force carrier gluon, a similar diagram may be used. The solid lines represent quarks, and the zigzag line represents a gluon. For strong interactions, types of gluons must match the types of quarks. The types are often labeled by colors: red, green, yellow, etc. reflecting the colour (a property) of quarks. Feynman Diagram for the Stable Hydrogen Atom electron proton The interactions between the proton and the electron in a hydrogen atom are the constant exchange of photons between the two particles. The Feynman diagram is shown here. Since no photon is absorbed or emitted, this state is stable. Of course, when a photon is absorbed or emitted, the process can also be represented by zigzag lines entering or leaving the stable hydrogen atom. Feynman diagram let us visualize the mystery of force, force carriers, and change of state. 172 Feyman diagrams for the decay of kaons into pions with emission of electrons and positrons, Feynmam Diagrams for Kaon Decays K– 0 + e– + K+ 0 + e+ + , e are given here. The two diagrams are essentially the same, if the positron is equivalent to an electron moving backward in time. We simply represent the neutrino by in both cases. On the other hand, we may also reverse the arrow for antineutrino, and make the diagrams look different. There are many ways to draw Feynman diagrams. A process may take place in various time sequences, and each may be represented by a Feynman diagram. In his book (see Further reading) QED, Feynman drew the time axis vertically, and the space axis horizontally. The choice is a personal preference as long as his guide lines are followed. The beta decay is a process involving weak interactions. In this process, a neutron is converted into a proton, with the emission of an electron. If we look at the quark level, a neutron (udd) and a proton (uud) differ by one quark. Converting a d quark in neutron into a u quark changes the neutron into a proton. The force carrier W– mediates the conversion of a quark d into u. The charge on a d quark is -1/3, and the charge on a u quark is 2/3. Thus, the net charge change is –1, which is emitted as the particle. – K– e+ K+ Feynman Diagram for Beta Decay v e e– W– udd uud+ Skill Building Questions: 1. Draw a Feynman diagram for the electron-positron annihilation process producing two photons. 2. Draw a Feynman diagram for the pair production process of gamma rays. 3. Draw a Feynman diagram corresponding to the reaction: e– + e– + . What is the force carrier in this case? Explain your choice. Particle Weak Interactions and Decay Feynman diagrams shows the similarity between strong and weak particle interactions. However, the two interactions have their unique features. 173 How can the strong and weak interactions between particles be distinguished? Usually, formations of particles and their decays are represented by a equation. For example, n + e p+ + e– The force of interaction between particles can be a strong or a weak force. A reaction involving only strong forces rearranges quarks or creates quark-antiquark pairs from other quark-antiquark pairs. Rearrangement does not change one type of quark into another type. The following two processes rearrange quarks by strong forces. 0 + n p+ + – u u + udd = uud + d u K– + p+ 0 + 0 s u + uud = uds + u u An example of creating a quark-antiquark pair from other quark-antiquark pair is given below: – + p+ K0 + 0 d u + uud = d s + uds In this case, a gluon converts u u in to s s . In reactions or decays involving weak interactions, a weak force carrier is involved. The force carrier can convert one type of quark into The Feynman Diagram of another type of quark. For example, the decay a Weak Interaction of L0 is such a case. u L0 – + p+ uds = d u + uud In this process, s is converted into d mediated by a weak force carrier W–, which decays into a u u pair. The five quarks split forming a +, –, and a proton p+. A Feynman diagram is shown. s u d Skill Building Questions: 1. What causes particles to decay? 2. Exemplify reactions mediated by a strong force by weak force carriers? 174 W– u d u d Exercises 1. What is an antiparticle? How did the concept of antiparticle come about? How is a particle different from its antiparticle? 2. Give the mass, charge, spin, magnetic moment, and parity for electron and a positron:. 3. Make a table listing mass, charge, spin, magnetic moment, and parity of protons, antiprotons, neutrons, and antineutrons. 4. Is the process of converting a pair of proton and antiproton into neutron and antineutron exothermic or endothermic? How much energy is required or released in this conversion? How can the energy be supplied or removed in this process? 5. Give six particles for each of these categories: gauge bosons, leptons, mesons, baryons. 6. What study leads to the requirement quarks? What are quarks? 7. What are leptons, hadrons, mesons, and baryons? 8. How many mesons can be made up using u, d, and s quarks? What are these mesons? 9. How many baryons can be made up using u, d, and s quarks? (Further reading and research is required.) 10. How many u and d quarks do the nuclei of 2D, 4He, 56Fe, and 235U have? 11. What are the products for the annihilation of a proton and an antiproton? Write an equation to represent the reaction. 12. What are the forces acting between particles? What are the force carriers in each case? What force binds the quarks and antiquarks in mesons? 13. What baryon have +1, 0 and -1 charges? 14. What is the most abundant particle in galactic cosmic rays? Protons at what energy are most common in cosmic rays? What are secondary cosmic rays? 15. Give the decay scheme of pions and kaons. 16. How are an electron and a positron represented in a Feynman diagram. 17. Draw Feynman diagrams for the scattering of an electron by a proton, annihilation of an electron-positron pair, and beta-decay process. 175 Further Reading and Literature Cited The Particle Explosion, by F. Close, M. Marten, & C. Sutton, Oxford University Press (1987). This book has fantastic photographs of particle tracks in cloud chambers, emulsions, and bubble chambers. QED, The Strange Theory of Light and Matter, by Richard P. Feynman, Princeton University Press (1985). The Particle Data Book, published every two years; the most recent edition is Physical Review D Vol.54, page 1-720 (1996). Many topics are given in this biannual review, and it lists details of the properties of particles. Some of the reviews in it are very interesting. These articles and tables are also available on the inter-net. For example: Particle Properties Data: http://pdg.lbl.gov/ Particle Physics Booklet: http://pdg.lbl.gov/rpp/booklet/contents.html Particle Properties: http://durpdg.ac.uk/hepdata/part Computer Readable Files: http://pdg.lbl.gov/computer_read.html Standard Model Matter Particles: http://www.pdg.lbl.gov.80/www/cpep/sm_matter_particles.html A Tour of the Subatomic Zoo, by C. Schwarz, American Institute of Physics (AIP, 1992). l Introduction to High Energy Physics, by Perkins . Experimental Foundations of Particle Physics, by Cahn and Goldhaber. Internet sites About hypernucleus.: http://www.phy.bnl.gov/~bnlhyp/me.html Nobel prize about quarks: http://www.nobel.se/laureates/physics-1990-press.html The particle adventure home page containing animations http://wwwpdg.cern.ch/pdg/cpep/adventure_home.html 176