1. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 1 Departamento de Física Fac. Ciencias Exactas - UNLP 14.1Early Discoveries 14.2The Fundamental Interactions 14.8Accelerators 14.3Classification of Elementary Particles 14.4Conservation Laws and Symmetries 14.5Quarks 14.6The Families of Matter 14.7Beyond the Standard Model CHAPTER 14 Elementary Particles “If I could remember the names of all these particles, I’d be a botanist.” Enrico Fermi Steven Weinberg (1933 - ) “I have done a terrible thing: I have postulated a particle that cannot be detected.” Wolfgang Pauli (after postulating the existence of the neutrino)

2. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 2 Departamento de Física Fac. Ciencias Exactas - UNLP Elementary Particles Finding answers to some of the basic questions about nature is a foremost goal of science: What are the basic building blocks of matter? What is inside the nucleus? What are the forces that hold matter together? How did the universe begin? Will the universe end, and if so, how and when?

3. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 3 Departamento de Física Fac. Ciencias Exactas - UNLP 14.1: Early Discoveries In 1930 the known elementary particles were the proton, the electron, and the photon. Thomson identified the electron in 1897, and Einstein defined the photon in 1905. The proton is the nucleus of the hydrogen atom. Despite the rapid progress of physics in the first couple of decades of the twentieth century, no more elementary particles were discovered until 1932, when Chadwick proved the existence of the neutron. That would have seemed sufficient… James Chadwick (1891-1974)

4. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 4 Departamento de Física Fac. Ciencias Exactas - UNLP But particle physics measurements were happening… Energetic particles collide with stationary particles in a “bubble chamber,” vaporizing the nearby matter and leaving a visible track. A magnetic field (pointing into the screen) causes charged particles to take curved paths.

5. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 5 Departamento de Física Fac. Ciencias Exactas - UNLP The Positron In 1928 Dirac introduced the relativistic theory of the electron when he combined quantum mechanics with special relativity. He found that his wave equation had negative, as well as positive, energy solutions. His theory can be interpreted as a vacuum being filled with an infinite sea of electrons with negative energies. If enough energy is transferred to the “sea,” an electron can be ejected with positive energy leaving behind a hole that is the positron, denoted by e +. Paul Dirac (1902-1984) E 0 Vacuum Electron & positron Positron!

6. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 6 Departamento de Física Fac. Ciencias Exactas - UNLP Anti-particles Dirac’s theory yields anti-particles, which: Have the same mass and lifetime as their associated particles. Have the same magnitude but are opposite in sign for such physical quantities as electric charge and various quantum numbers. All particles, even neutral ones (with some exceptions like the neutral pion), have antiparticles. Magnetic field into screen

7. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 7 Departamento de Física Fac. Ciencias Exactas - UNLP The Positron Carl Anderson identified the positron in cosmic rays. It was easy: it had positive charge and was light. Carl Anderson (1905-1991) Anderson’s cloud chamber photo of positron track Cosmic rays are highly energetic particles, mostly protons, that cross interstellar space and enter the Earth’s atmosphere, where their interaction with particles creates cosmic “showers” of many distinct particles.

8. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 8 Departamento de Física Fac. Ciencias Exactas - UNLP Positron-Electron Interaction The ultimate fate of positrons (anti-electrons) is annihilation with electrons. After a positron slows down by passing through matter, it is attracted by the Coulomb force to an electron, where it annihilates through the reaction: All anti-matter eventually meets the same fate. A lot of energy is released in this process: all of the matter is converted to energy. Star Trek’s “dilithium crystals” supposedly contain anti-matter, which powers the Enterprise.

9. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 9 Departamento de Física Fac. Ciencias Exactas - UNLP Feynman Diagrams Feynman presented a particularly simple graphical technique to describe interactions. It predicts that, when two electrons approach each other, according to the quantum theory of fields, they exchange a series of photons called virtual photons, because they cannot be directly observed. The action of the electromagnetic field (for example, the Coulomb force) can be interpreted as the exchange of photons. In this case we say that the photons are the carriers or mediators of the electromagnetic force. Example of a Feynman space- time diagram. Electrons interact through mediation of a photon. The axes are normally omitted.

10. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 10 Departamento de Física Fac. Ciencias Exactas - UNLP Yukawa’s Meson The Japanese physicist Hideki Yukawa had the idea of developing a quantum field theory that would describe the force between nucleons—analogously to the electromagnetic force. To do this, he had to determine the carrier or mediator of the nuclear strong force analogous to the photon in the electromagnetic force which he called a meson (derived from the Greek word meso meaning “middle” due to its mass being between the electron and proton masses). Hideki Yukawa (1907-1981)

11. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 11 Departamento de Física Fac. Ciencias Exactas - UNLP Yukawa’s meson, called a pion (or pi-meson or  -meson), was identified in 1947 by C. F. Powell (1903–1969) and G. P. Occhialini (1907–1993) Charged pions have masses of 140 MeV/c 2, and a neutral pion  0 was later discovered that has a mass of 135 MeV/c 2, a neutron and a proton. Yukawa’s Meson Feynman diagram indicating the exchange of a pion (Yukawa’s meson) between a neutron and a proton.

12. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 12 Departamento de Física Fac. Ciencias Exactas - UNLP Other Mesons, Quarks, and Gluons Yukawa’s pion is responsible for the nuclear force. Later we’ll see that the nucleons and mesons are part of a general group of particles formed from even more fundamental particles: quarks. The particle that mediates the strong interaction between quarks is called a gluon (for the “glue” that holds the quarks together); it’s massless and has spin 1, just like the photon. Computed image of quarks and gluons in a nucleon

13. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 13 Departamento de Física Fac. Ciencias Exactas - UNLP The Weak Interaction In the 1960s Sheldon Glashow, Steven Weinberg, and Abdus Salam predicted that particles that they called W (for weak) and Z should exist that are responsible for the weak interaction. They have been observed. Sheldon Glashow (1932- ) Abdus Salam (1926-1996)

14. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 14 Departamento de Física Fac. Ciencias Exactas - UNLP The Graviton It has been suggested that the particle responsible for the gravitational interaction be called a graviton. The graviton is the mediator of gravity in quantum field theory and has been postulated because of the success of the photon in quantum electrodynamics theory. It must be massless, travel at the speed of light, have spin 2, and interact with all particles that have mass-energy. The graviton has never been observed because of its extremely weak interaction with objects.

15. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 15 Departamento de Física Fac. Ciencias Exactas - UNLP The Fundamental Interactions One of the main goals of particle physics is to unify these forces (to show that they’re all just different aspects of the same force), just as Maxwell did for the electric and magnetic forces many years earlier.

16. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 16 Departamento de Física Fac. Ciencias Exactas - UNLP The Fundamental Interactions A finite range effectively confines the particle, which, by the uncertainty principle, determines energy and mass. Photons and gravitons are massless. W and Z bosons are heavy.

17. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 17 Departamento de Física Fac. Ciencias Exactas - UNLP 14.8 Accelerators Particle accelerators generate high enough energies to create particles 1 GeV/c 2 or greater.

18. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 18 Departamento de Física Fac. Ciencias Exactas - UNLP Accelerators There are three main types of accelerators used presently in particle physics experiments: synchrotrons, linear accelerators, and colliders.

19. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 19 Departamento de Física Fac. Ciencias Exactas - UNLP Synchrotron Radiation One difficulty with cyclic accelerators is that when charged particles are accelerated, they radiate electromagnetic energy called synchrotron radiation. This problem is particularly severe when electrons, moving very close to the speed of light, move in curved paths. If the radius of curvature is small, electrons can radiate as much energy as they gain. Physicists have learned to take advantage of these synchrotron radiation losses and now build special electron accelerators (called light sources) that produce copious amounts of photon radiation used for both basic and applied research.

20. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 20 Departamento de Física Fac. Ciencias Exactas - UNLP Linear Accelerators Linear accelerators or linacs typically have straight electric-field-free regions between gaps of RF voltage boosts. The particles gain speed with each boost, and the voltage boost is on for a fixed period of time, and thus the distance between gaps becomes increasingly larger as the particles accelerate. Linacs are sometimes used as pre-acceleration device for large circular accelerators.

21. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 21 Departamento de Física Fac. Ciencias Exactas - UNLP Colliders Because of the limited energy available for reactions like that found for the Tevatron, physicists decided they had to resort to colliding beam experiments, in which the particles meet head-on. If the colliding particles have equal masses and kinetic energies, the total momentum is zero and all the energy is available for the reaction and the creation of new particles.

22. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 22 Departamento de Física Fac. Ciencias Exactas - UNLP Large Hadron Collider Counter- propagating protons will each have an energy of 7 TeV, giving a total collision energy of 14 TeV. The LHC can also be used to collide heavy ions such as lead (Pb) with a collision energy of 1,150 TeV.

23. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 23 Departamento de Física Fac. Ciencias Exactas - UNLP 14.3: Classification of Elementary Particles Particles with half-integral spin are called fermions and those with integral spin are called bosons. This is a particularly useful way to classify elementary particles because all stable matter in the universe appears to be composed, at some level, of constituent fermions. Fermions obey the Pauli Exclusion Principle. Bosons don’t. Photons, gluons, W ±, and the Z are called gauge bosons and are responsible for the strong and electroweak interactions. Gravitons are also bosons, having spin 2. Fermions exert attractive or repulsive forces on each other by exchanging gauge bosons, which are the force carriers.

24. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 24 Departamento de Física Fac. Ciencias Exactas - UNLP One other boson that has been predicted, but not yet detected, is necessary in quantum field theory to explain why the W ± and Z have such large masses, yet the photon has no mass. This missing boson is called the Higgs particle (or Higgs boson) after Peter Higgs, who first proposed it. The Standard Model proposes that there is a field called the Higgs field that permeates all of space. By interacting with this field, particles acquire mass. Particles that interact strongly with the Higgs field have heavy mass; particles that interact weakly have small mass. The Higgs boson is very heavy, and it hasn’t been observed yet. The search for the Higgs boson is of the highest priority in elementary particle physics. Simulated event featuring the appearance of the Higgs boson. The Higgs Boson

25. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 25 Departamento de Física Fac. Ciencias Exactas - UNLP Boson Properties

26. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 26 Departamento de Física Fac. Ciencias Exactas - UNLP Leptons: electrons, muons, taus & neutrinos The leptons are perhaps the simplest of the elementary particles. They appear to be point-like, that is, with no apparent internal structure, and seem to be truly elementary. Thus far there has been no plausible suggestion they are formed from some more fundamental particles. Each of the leptons has an associated neutrino, named after its charged partner (for example, muon neutrino). There are only six leptons plus their six antiparticles.

27. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 27 Departamento de Física Fac. Ciencias Exactas - UNLP Muon and tau decay The muon decays into an electron, and the tau can decay into an electron, a muon, or even hadrons. The muon decay (by the weak interaction) is:   e e

28. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 28 Departamento de Física Fac. Ciencias Exactas - UNLP Hadrons Hadrons are particles that act through the strong force. Two classes of hadrons: mesons and baryons. Mesons are particles with integral spin having masses greater than that of the muon (106 MeV/c 2 ). (Mesons are made up of pairs of quarks—a quark and an anti-quark.) They’re unstable and rare. Baryons have masses at least as large as the proton and have half- integral spins. Baryons include the proton and neutron, which make up the atomic nucleus, but many other unstable baryons exist as well. The term "baryon" is derived from the Greek βαρύς (barys), meaning "heavy," because at the time of their naming it was believed that baryons were characterized by having greater mass than other particles. (They’re made up of three quarks.) All baryons decay into protons.

29. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 29 Departamento de Física Fac. Ciencias Exactas - UNLP The Hadrons

30. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 30 Departamento de Física Fac. Ciencias Exactas - UNLP Particles and Lifetimes The lifetimes of particles are also indications of their force interactions. Particles that decay through the strong interaction are usually the shortest-lived, normally decaying in less than 10 −20 s. The decays caused by the electromagnetic interaction generally have lifetimes on the order of 10 −16 s, and The weak interaction decays are even slower, longer than 10 −10 s.

31. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 31 Departamento de Física Fac. Ciencias Exactas - UNLP Fundamental and Composite Particles We call certain particles fundamental; this means that they are not composed of other, smaller particles. We believe leptons, quarks, and gauge bosons are fundamental particles. Although the Z and W bosons have very short lifetimes, they are regarded as particles, so a definition of particles dependent only on lifetimes is too restrictive. Other particles are composites, made from the fundamental particles.

32. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 32 Departamento de Física Fac. Ciencias Exactas - UNLP 14.4: Conservation Laws Physicists like to have clear rules or laws that determine whether a certain process can occur or not. It seems that everything occurs in nature that is not forbidden. Certain conservation laws are already familiar from our study of classical physics. These include mass-energy, charge, linear momentum, and angular momentum. These are absolute conservation laws: they are always obeyed. Additional conservation laws will be helpful in understanding the many possibilities of elementary particle interactions. Some of these laws are absolute, but others may be valid for only one or two of the fundamental interactions.

33. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 33 Departamento de Física Fac. Ciencias Exactas - UNLP Baryon Conservation In low-energy nuclear reactions, the number of nucleons is always conserved. Empirically this is part of a more general conservation law for what is assigned a new quantum number called baryon number that has the value B = +1 for baryons and −1 for anti-baryons, and 0 for all other particles. The conservation of baryon number requires the same total baryon number before and after the reaction. Although there are no known violations of baryon conservation, there are theoretical indications that it was violated sometime in the beginning of the universe when temperatures were quite high. This is thought to account for the preponderance of matter over anti-matter in the universe today.

34. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 34 Departamento de Física Fac. Ciencias Exactas - UNLP Lepton Conservation The leptons are all fundamental particles, and there is a conservation of leptons for each of the three kinds (families) of leptons. The number of leptons from each family is the same both before and after a reaction. We let L e = +1 for the electron and the electron neutrino; L e = −1 for their antiparticles; and L e = 0 for all other particles. We assign the quantum numbers L μ for the muon and its neutrino and L τ for the tau and its neutrino similarly. Thus three additional conservation laws.

35. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 35 Departamento de Física Fac. Ciencias Exactas - UNLP Strangeness The behavior of the K mesons seemed very odd. There is no conservation law for the production of mesons, but it appeared that K mesons, as well as the Λ and Σ baryons, were always produced in pairs in the p + p reaction. One would expect the K 0 meson to also decay into two photons very quickly, but it does not. A new quantum number was defined: Strangeness, S, which is conserved in the strong and electromagnetic interactions, but not in the weak interaction. The kaons have S = +1, lambda and sigmas have S = −1, the xi has S = −2, and the omega has S = −3. When the strange particles are produced by the p + p strong interaction, they must be produced in pairs to conserve strangeness.

36. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 36 Departamento de Física Fac. Ciencias Exactas - UNLP Strangeness is strange…  0 can decay into two photons by the strong interaction, it is not possible for K 0 to decay at all by the strong interaction. The K 0 is the lightest S = 0 particle, and there is no other strange particle to which it can decay. It can decay only by the weak interaction, which violates strangeness conservation. Because the typical decay times of the weak interaction are on the order of 10 −10 s, this explains the longer decay time for K 0. Only ΔS = ±1 violations are allowed by the weak interaction.

37. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 37 Departamento de Física Fac. Ciencias Exactas - UNLP Hypercharge One more quantity, called hypercharge, has also become widely used as a quantum number. The hypercharge quantum number Y is defined by Y = S + B. Hypercharge, the sum of the strangeness and baryon quantum numbers, is conserved in strong interactions. The hypercharge and strangeness conservation laws hold for the strong and electromagnetic interactions, but are violated for the weak interaction.

38. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 38 Departamento de Física Fac. Ciencias Exactas - UNLP Symmetries Symmetries lead directly to conservation laws. Three symmetry operators called parity, charge conjugation, and time reversal are considered.

39. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 39 Departamento de Física Fac. Ciencias Exactas - UNLP The conservation of parity P describes the inversion symmetry of space, Inversion, if valid, does not change the laws of physics. The conservation of parity is valid for the strong and electromagnetic interactions, but not for the weak interaction (experimentally). The Conservation of Parity P

40. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 40 Departamento de Física Fac. Ciencias Exactas - UNLP Charge conjugation C Charge conjugation C reverses the sign of the particle’s charge and magnetic moment. It has the effect of interchanging every particle with its antiparticle. Charge conjugation is valid for the strong and electromagnetic interactions, but it is also not conserved in the weak interactions. Even though both C and P are violated for the weak interaction it was believed that when both charge conjugation and parity operations are performed (called CP), conservation was still valid.

41. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 41 Departamento de Física Fac. Ciencias Exactas - UNLP Time Reversal T Here time t is replaced with –t. When all three operations are performed (CPT), where T is the time reversal symmetry, conservation holds. We speak of the invariance of the symmetry operators, such as T, CP, and CPT.

42. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 42 Departamento de Física Fac. Ciencias Exactas - UNLP Unifying all these interactions proved difficult. In the 1950s, it was rumored that Heisenberg had done it, and just the details remained to be sketched in. But nothing ever emerged from Heisenberg. So Wolfgang Pauli responded with the following: “Below is the proof that I am as great an artist as Rembrandt; the details remain to be sketched in.”

43. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 43 Departamento de Física Fac. Ciencias Exactas - UNLP The Weak Interaction: The Electroweak Theory In the 1960s Sheldon Glashow, Steven Weinberg, and Abdus Salam unified the electro- magnetic and weak interactions into what they called the electroweak theory, much as Maxwell had unified electricity and magnetism into the electromagnetic theory a hundred years earlier. Sheldon Glashow (1932- ) Abdus Salam (1926-1996)

44. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 44 Departamento de Física Fac. Ciencias Exactas - UNLP Unification of the Strong and Electroweak Interactions: The Standard Model Over the latter half of the 20 th century, numerous physicists combined efforts to generate The Standard Model. It is a widely accepted theory of elementary particle physics at present. It is a relatively simple, comprehensive theory that explains hundreds of particles and complex interactions with six quarks, six leptons, and three force-mediating particles. It is a combination of the electroweak theory and quantum chromodynamics (QCD), but does not include gravity.

45. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 45 Departamento de Física Fac. Ciencias Exactas - UNLP Quarks Three quarks were proposed, named the up (u), down (d), and strange (s), with the charges +2e/3, −e/3, and −e/3, respectively. The strange quark has the strangeness value of −1, whereas the other two quarks have S = 0. Quarks are believed to be essentially point-like, just like leptons. With these three quarks, all the known hadrons (at the time) could be specified by some combination of quarks and anti-quarks. Murray Gell-Mann (1929- ) In 1963 Murray Gell-Mann and, independently, George Zweig proposed that hadrons were formed from fractionally charged particles called quarks. The quark theory successfully described the properties of the particles and reactions and decay.

46. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 46 Departamento de Física Fac. Ciencias Exactas - UNLP Charm, Truth, and Beauty A fourth quark called the charmed quark (c) was proposed to explain some additional discrepancies in the lifetimes of some of the known particles. A new quantum number called charm C was introduced so that the new quark would have C = +1 while its anti-quark would have C = −1 and particles without the charmed quark have C = 0. Charm is similar to strangeness in that it is conserved in the strong and electromagnetic interactions, but not in the weak interactions. This behavior was sufficient to explain the particle lifetime difficulties. Two additional quarks, top and bottom (or truth and beauty), were also required to construct some exotic particles (the Upsilon-meson).

47. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 47 Departamento de Física Fac. Ciencias Exactas - UNLP Quark Properties The spin of all quarks (and anti-quarks) is 1/2.

48. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 48 Departamento de Física Fac. Ciencias Exactas - UNLP Quark Description of Particles Baryons normally consist of three quarks or anti-quarks. A meson consists of a quark-anti-quark pair, yielding the required baryon number of 0. 1/3e -2/3e 2/3e -1/3e 2/3e 1/3e 2/3e -2/3e -1/3e 2/3e -1/3e 1/3e 2/3e -1/3e -2/3e 2/3e -2/3e

49. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 49 Departamento de Física Fac. Ciencias Exactas - UNLP Quark Description of Particles A meson consists of a quark-anti-quark pair, which gives the required baryon number of 0. Baryons normally consist of three quarks. The structure is quite simple. For example, a  − consists of ud, which gives a charge of (−2e/3) + (−e/3) = −e, and the two spins couple to give 0 (−1/2 + 1/2 = 0). A proton is uud, which gives a charge of (2e/3) + (2e/3) + (−e/3) = +e; its baryon number is 1/3 + 1/3 + 1/3 = 1; and two of the quarks’ spins couple to zero, leaving a spin 1/2 for the proton (1/2 + 1/2 − 1/2 = 1/2).

50. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 50 Departamento de Física Fac. Ciencias Exactas - UNLP Other Particles What about the quark composition of the Ω −, which has a strangeness of S = −3? Its quark composition is sss. And its charge is 3(−e/3) = −e, and its spin is due to three quark spins aligned, 3(1/2) = 3/2. There is no other possibility for a stable omega (lifetime ~10 −10 s) in agreement with the table.

51. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 51 Departamento de Física Fac. Ciencias Exactas - UNLP Quantum Chromodynamics (QCD) Because quarks have spin 1/2, they are all fermions. According to the Pauli exclusion principle, no two fermions can exist in the same state. Yet we have three identical strange quarks in the Ω − ! This is not possible unless some other quantum number distinguishes each of these quarks in one particle. A new quantum number called color circumvents this problem and its properties establish quantum chromodynamics (QCD). Discovery of the  -

52. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 52 Departamento de Física Fac. Ciencias Exactas - UNLP There are three colors for quarks we call red (R), green (G), and blue (B) with anti-quark color antired ( ); antigreen ( ) and antiblue ( ). (A “bar” above the symbol is usually used to describe the “anti-color”). Color is the “charge” of the strong nuclear force, analogous to electric charge for electromagnetism. ColorColor

53. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 53 Departamento de Física Fac. Ciencias Exactas - UNLP ColorColor Quantum electrodynamics and quantum chromodynamics, are similar in structure; color is often called color charge and the force between quarks is sometimes referred to as color force. Earlier we saw that gluons are the particles that hold the quarks together. Below is a Feynman diagram of two quarks interacting. A red quark comes in from the left and interacts with a blue quark coming in from the right. They exchange a gluon, changing the blue quark into a red one and the red quark into a blue one.

54. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 54 Departamento de Física Fac. Ciencias Exactas - UNLP ColorColor A color and its anti-color cancel out. We call this colorless (or white). All hadrons are colorless. In the figure, the gluon itself must have the color in order for the diagram to work. Quarks change color when they emit or absorb a gluon, and quarks of the same color repel, whereas quarks of different color attract.

55. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 55 Departamento de Física Fac. Ciencias Exactas - UNLP Color To finish the story we should mention that the six different kinds of quarks are referred to as flavors. There are six flavors of quarks (u, d, s, c, b, t). Each flavor has three colors. Finally, how many different gluons are possible? Using the three colors red, blue, and green, there are nine possible combinations for a gluon. They are Note in the diagram that the gluon is and not. The combination does not have any net color change and cannot be independent. Therefore, there are only eight independent gluons, and that is what quantum chromodynamics predicts. Gluons can interact with each other because each gluon carries a color charge. Note that in this case gluons, as the mediator of the strong force, are much different from photons, the mediator of the electromagnetic force.

56. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 56 Departamento de Física Fac. Ciencias Exactas - UNLP Quark- anti-quark creation Physicists now believe that free quarks cannot be observed; they can only exist within hadrons. This is called confinement. This occurs because the force between the quarks increases rapidly with distance, and the energy supplied to separate them creates new quarks.

57. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 57 Departamento de Física Fac. Ciencias Exactas - UNLP Confinement When a high-energy gamma ray is scattered from a neutron, a free quark cannot escape because of confinement. For high enough energies, an anti-quark-quark pair is created (for example, ), and a pion and proton are the final particles.

58. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 58 Departamento de Física Fac. Ciencias Exactas - UNLP The Families of Matter The three generations (or families) of matter. Note that both quarks and leptons exist in three distinct sets. One of each charge type of quark and lepton make up a generation. All visible matter in the universe is made from the first generation; second- and third-generation particles are unstable and decay into first-generation particles.

59. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 59 Departamento de Física Fac. Ciencias Exactas - UNLP The Families of Matter Most of the mass in the universe is made from the components in the first generation (electrons and u and d quarks). The second generation consists of the muon, its neutrino, and the charmed and strange quarks. The members of this generation are found in certain astrophysical objects of high energy and in cosmic rays, and are produced in high-energy accelerators. The third generation consists of the tau and its neutrino and two more quarks, the bottom (or beauty) and top (or truth). The members of this third generation existed in the early moments of the creation of the universe and can be created with very high energy accelerators.

60. El núcleo y sus radiaciones Clase 30 Curso 2009 Página 60 Departamento de Física Fac. Ciencias Exactas - UNLP Three families: 1897-2000 Particle masses in MeV; 1 MeV  1.8  10  27 gram