Mesons

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Elementary Particals

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Mesons

  1. 1. By Mrs. Samia Rehman Dogar Associate Prof Federal College Of Education H-9,Islamabad samia42001@yahoo.com Elementary Particles
  2. 2. Elementary Particles  Atoms  From the Greek for “indivisible”  Were once thought to be the elementary particles  Atom constituents  Proton, neutron, and electron  After 1932 these were viewed as elementary  All matter was made up of these particles
  3. 3. Discovery of New Particles  New particles  Beginning in 1945, many new particles were discovered in experiments involving high-energy collisions  Characteristically unstable with short lifetimes  Over 300 have been cataloged  A pattern was needed to understand all these new particles
  4. 4. Elementary Particles – Quarks Physicists recognize that most particles are made up of quarks Exceptions include photons, electrons and a few others The quark model has reduced the array of particles to a manageable few Protons and neutrons are not truly elementary, but are systems of tightly bound quarks
  5. 5. Fundamental Forces  All particles in nature are subject to four fundamental forces  Strong force  Electromagnetic force  Weak force  Gravitational force  This list is in order of decreasing strength
  6. 6. Nuclear Force  Holds nucleons together  Strongest of all the fundamental forces  Very short-ranged  Less than 10-15 m  Negligible for separations greater than this
  7. 7. Electromagnetic Force  Is responsible for the binding of atoms and molecules  About 10-2 times the strength of the nuclear force  A long-range force that decreases in strength as the inverse square of the separation between interacting particles
  8. 8. Weak Force Is responsible for instability in certain nuclei Is responsible for decay processes Its strength is about 10-5 times that of the strong force Scientists now believe the weak and electromagnetic forces are two manifestions of a single interaction, the electroweak force
  9. 9. Gravitational Force  A familiar force that holds the planets, stars and galaxies together  Its effect on elementary particles is negligible  A long-range force  It is about 10-41 times the strength of the nuclear force  Weakest of the four fundamental forces
  10. 10. Explanation of Forces  Forces between particles are often described in terms of the actions of field particles or exchange particles  The force is mediated, or carried, by the field particles
  11. 11. Forces and Mediating Particles
  12. 12. Paul Adrian Maurice Dirac  1902 – 1984  Understanding of antimatter  Unification of quantum mechanics and relativity  Contributions of quantum physics and cosmology  Nobel Prize in 1933
  13. 13. Antiparticles  Every particle has a corresponding antiparticle  From Dirac’s version of quantum mechanics that incorporated special relativity  An antiparticle has the same mass as the particle, but the opposite charge  The positron (electron’s antiparticle) was discovered by Anderson in 1932  Since then, it has been observed in numerous experiments  Practically every known elementary particle has a distinct antiparticle  Among the exceptions are the photon and the neutral pi particles
  14. 14. Dirac’s Explanation The solutions to the relativistic quantum mechanic equations required negative energy states Dirac postulated that all negative energy states were filled These electrons are collectively called the Dirac sea Electrons in the Dirac sea are not directly observable because the exclusion principle does not let them react to external forces
  15. 15. Dirac’s Explanation, cont An interaction may cause the electron to be excited to a positive energy state This would leave behind a hole in the Dirac sea The hole can react to external forces and is observable
  16. 16. Dirac’s Explanation, final  The hole reacts in a way similar to the electron, except that it has a positive charge  The hole is the antiparticle of the electron  The electron’s antiparticle is now called a positron
  17. 17. Pair Production  A common source of positrons is pair production  A gamma-ray photon with sufficient energy interacts with a nucleus and an electron-positron pair is created from the photon  The photon must have a minimum energy equal to 2mec2 to create the pair
  18. 18. Pair Production, cont  A photograph of pair production produced by 300 MeV gamma rays striking a lead sheet  The minimum energy to create the pair is 1.022 MeV  The excess energy appears as kinetic energy of the two particles
  19. 19. Annihilation  The reverse of pair production can also occur  Under the proper conditions, an electron and a positron can annihilate each other to produce two gamma ray photons e- + e+ 
  20. 20. Antimatter, final  In 1955 a team produced antiprotons and antineutrons  This established the certainty of the existence of antiparticles  Every particle has a corresponding antiparticle with  equal mass and spin  equal magnitude and opposite sign of charge, magnetic moment and strangeness  The neutral photon, pion and eta are their own antiparticles
  21. 21. Hideki Yukawa 1907 – 1981 Nobel Prize in 1949 for predicting the existence of mesons Developed the first theory to explain the nature of the nuclear force
  22. 22. Mesons Developed from a theory to explain the nuclear force Yukawa used the idea of forces being mediated by particles to explain the nuclear force A new particle was introduced whose exchange between nucleons causes the nuclear force It was called a meson
  23. 23. Mesons The proposed particle would have a mass about 200 times that of the electron Efforts to establish the existence of the particle were made by studying cosmic rays in the late 1930’s Actually discovered multiple particles Pi meson (pion) Muon Not a meson
  24. 24. Pion  There are three varieties of pions  + and -  Mass of 139.6 MeV/c2  0  Mass of 135.0 MeV/c2  Pions are very unstable  For example, the - decays into a muon and an antineutrino with a lifetime of about 2.6 x10-8 s
  25. 25. Muons  Two muons exist  µ- and its antiparticle µ+  The muon is unstable  It has a mean lifetime of 2.2 µs  It decays into an electron, a neutrino, and an antineutrino
  26. 26. Richard Feynman  1918 – 1988  Developed quantum electrodynamics  Shared the Noble Prize in 1965  Worked on Challenger investigation and demonstrated the effects of cold temperatures on the rubber O-rings used
  27. 27. Feynman Diagrams A graphical representation of the interaction between two particles Feynman diagrams are named for Richard Feynman who developed them A Feynman diagram is a qualitative graph of time on the vertical axis and space on the horizontal axis Actual values of time and space are not important The actual paths of the particles are not shown
  28. 28. Feynman Diagram – Two Electrons  The photon is the field particle that mediates the interaction  The photon transfers energy and momentum from one electron to the other  The photon is called a virtual photon  It can never be detected directly because it is absorbed by the second electron very shortly after being emitted by the first electron
  29. 29. The Virtual Photon  The existence of the virtual photon seems to violate the law of conservation of energy  But, due to the uncertainty principle and its very short lifetime, the photon’s excess energy is less than the uncertainty in its energy  The virtual photon can exist for short time intervals, such that ΔE   / 2Δt
  30. 30. Feynman Diagram – Proton and Neutron (Yukawa’s Model)  The exchange is via the nuclear force  The existence of the pion is allowed in spite of conservation of energy if this energy is surrendered in a short enough time  Analysis predicts the rest energy of the pion to be 100 MeV / c2  This is in close agreement with experimental results
  31. 31. Nucleon Interaction – (Yukawa’s Model)  The time interval required for the pion to transfer from one nucleon to the other is  The distance the pion could travel is cDt  Using these pieces of information, the rest energy of the pion is about 100 MeV 2 2 2  D   D R t E m c
  32. 32. This concept says that a system of two nucleons can change into two nucleons plus a pion as long as it returns to its original state in a very short time interval It is often said that the nucleon undergoes fluctuations as it emits and absorbs field particles These fluctuations are a consequence of quantum mechanics and special relativity
  33. 33. Nuclear Force  The interactions previously described used the pion as the particles that mediate the nuclear force  Current understanding indicate that the nuclear force is more fundamentally described as an average or residual effect of the force between quarks
  34. 34. Feynman Diagram – Weak Interaction An electron and a neutrino are interacting via the weak force The Z0 is the mediating particle  The weak force can also be mediated by the W  The W and Z0 were discovered in 1983 at CERN
  35. 35. Classification of Particles  Two broad categories  Classified by interactions  Hadrons – interact through strong force  Leptons – interact through weak force  Note on terminology  The strong force is reserved for the force between quarks  The nuclear force is reserved for the force between nucleons  The nuclear force is a secondary result of the strong force
  36. 36. Hadrons  Interact through the strong force  Two subclasses distinguished by masses and spins  Mesons  Decay finally into electrons, positrons, neutrinos and photons  Integer spins (0 or 1)  Baryons  Masses equal to or greater than a proton  Half integer spin values (1/2 or 3/2)  Decay into end products that include a proton (except for the proton)  Not elementary, but composed of quarks
  37. 37. Leptons  Do not interact through strong force  Do participate in electromagnetic (if charged) and weak interactions  All have spin of ½  Leptons appear truly elementary  No substructure  Point-like particles
  38. 38. Leptons, cont  Scientists currently believe only six leptons exist, along with their antiparticles  Electron and electron neutrino  Muon and its neutrino  Tau and its neutrino  Neutrinos may have a small, but nonzero, mass
  39. 39. Conservation Laws  A number of conservation laws are important in the study of elementary particles  Already have seen conservation of  Energy  Linear momentum  Angular momentum  Electric charge  Two additional laws are  Conservation of Baryon Number  Conservation of Lepton Number
  40. 40. Conservation of Baryon Number  Whenever a baryon is created in a reaction or a decay, an antibaryon is also created  B is the Baryon Number  B = +1 for baryons  B = -1 for antibaryons  B = 0 for all other particles  Conservation of Baryon Number states: the sum of the baryon numbers before a reaction or a decay must equal the sum of baryon numbers after the process
  41. 41. Conservation of Baryon Number and Proton Stability There is a debate over whether the proton decays or not If baryon number is absolutely conserved, the proton cannot decay Some recent theories predict the proton is unstable and so baryon number would not be absolutely conserved For now, we can say that the proton has a half- life of at least 1033 years
  42. 42. Conservation of Baryon Number, Example  Is baryon number conserved in the following reaction?   Baryon numbers:  Before: 1 + 1 = 2  After: 1 + 1 + 1 + (-1) = 2  Baryon number is conserved  The reaction can occur as long as energy is conserved pnppnp 
  43. 43. Conservation of Lepton Number  There are three conservation laws, one for each variety of lepton  Law of Conservation of Electron-Lepton Number states that the sum of electron-lepton numbers before the process must equal the sum of the electron-lepton number after the process  The process can be a reaction or a decay
  44. 44. Conservation of Lepton Number, cont  Assigning electron-lepton numbers  Le = 1 for the electron and the electron neutrino  Le = -1 for the positron and the electron antineutrino  Le = 0 for all other particles  Similarly, when a process involves muons, muon-lepton number must be conserved and when a process involves tau particles, tau- lepton numbers must be conserved  Muon- and tau-lepton numbers are assigned similarly to electron-lepton numbers
  45. 45. Conservation of Lepton Number, Example  Is lepton number conserved in the following reaction?   Check electron lepton numbers:  Before: Le = 0 After: Le = 1 + (-1) + 0 = 0  Electron lepton number is conserved  Check muon lepton numbers:  Before: Lµ = 1 After: Lµ = 0 + 0 + 1 = 1  Muon lepton number is conserved    e e
  46. 46. Strange Particles Some particles discovered in the 1950’s were found to exhibit unusual properties in their production and decay and were given the name strange particles Peculiar features include Always produced in pairs Although produced by the strong interaction, they do not decay into particles that interact via the strong interaction, but instead into particles that interact via weak interactions They decay much more slowly than particles decaying via strong interactions
  47. 47. Strangeness  To explain these unusual properties, a new quantum number, S, called strangeness, was introduced  A new law, the conservation of strangeness, was also needed It states that whenever a reaction or decay occurs via the strong force, the sum of strangeness numbers before the process must equal the sum of the strangeness numbers after the process  Strong and electromagnetic interactions obey the law of conservation of strangeness, but the weak interaction does not
  48. 48. Bubble Chamber Example of Strange Particles The dashed lines represent neutral particles At the bottom, - + p  Λ0 + K0 Then Λ0  - + p and 0 0 - K + µ +    
  49. 49. Creating Particles  Most elementary particles are unstable and are created in nature only rarely, in cosmic ray showers  In the laboratory, great numbers of particles can be created in controlled collisions between high-energy particles and a suitable target
  50. 50. Measuring Properties of Particles A magnetic field causes the charged particles to curve This allows measurement of their charge and linear momentum If the mass and momentum of the incident particle are known, the product particles’ mass, kinetic energy, and speed can usually be calculated The particle’s lifetime can be calculated from the length of its track and its speed
  51. 51. Resonance Particles  Short-lived particles are known as resonance particles  They exist for times around 10-20 s  In the lab, times for around 10-16 s can be detected  They cannot be detected directly  Their properties can be inferred from data on their decay products
  52. 52. Murray Gell-Mann 1929 – Studies dealing with subatomic particles Named quarks Developed pattern known as eightfold way Nobel Prize in 1969
  53. 53. The Eightfold Way  Many classification schemes have been proposed to group particles into families  These schemes are based on spin, baryon number, strangeness, etc.  The eightfold way is a symmetric pattern proposed by Gell-Mann and Ne’eman  There are many symmetrical patterns that can be developed  The patterns of the eightfold way have much in common with the periodic table  Including predicting missing particles
  54. 54. An Eightfold Way for Baryons  A hexagonal pattern for the eight spin ½ baryons  Stangeness vs. charge is plotted on a sloping coordinate system  Six of the baryons form a hexagon with the other two particles at its center
  55. 55. An Eightfold Way for Mesons  The mesons with spins of 0 can be plotted  Strangeness vs. charge on a sloping coordinate system is plotted  A hexagonal pattern emerges  The particles and their antiparticles are on opposite sides on the perimeter of the hexagon  The remaining three mesons are at the center
  56. 56. Eightfold Way for Spin 3/2 Baryons  The nine particles known at the time were arranged as shown  An empty spot occurred  Gell-Mann predicted the missing particle and its properties  About three years later, the particle was found and all its predicted properties were confirmed
  57. 57. Quarks  Hadrons are complex particles with size and structure  Hadrons decay into other hadrons  There are many different hadrons  Quarks are proposed as the elementary particles that constitute the hadrons  Originally proposed independently by Gell-Mann and Zweig
  58. 58. Original Quark Model  Three types or flavors  u – up  d – down  s – strange  Associated with each quark is an antiquark  The antiquark has opposite charge, baryon number and strangeness  Quarks have fractional electrical charges  +1/3 e and –2/3 e  Quarks are fermions  Half-integral spins
  59. 59. Original Quark Model – Rules  All the hadrons at the time of the original proposal were explained by three rules  Mesons consist of one quark and one antiquark  This gives them a baryon number of 0  Baryons consist of three quarks  Antibaryons consist of three antiquarks
  60. 60. Quark Composition of Particles – Examples  Mesons are quark- antiquark pairs  Baryons are quark triplets
  61. 61. Additions to the Original Quark Model – Charm  Another quark was needed to account for some discrepancies between predictions of the model and experimental results  A new quantum number, C, was assigned to the property of charm  Charm would be conserved in strong and electromagnetic interactions, but not in weak interactions  In 1974, a new meson, the J/Ψ was discovered that was shown to be a charm quark and charm antiquark pair
  62. 62. More Additions – Top and Bottom  Discovery led to the need for a more elaborate quark model  This need led to the proposal of two new quarks  t – top (or truth)  b – bottom (or beauty)  Added quantum numbers of topness and bottomness  Verification  b quark was found in a Y- meson in 1977  t quark was found in 1995 at Fermilab
  63. 63. Numbers of Particles  At the present, physicists believe the “building blocks” of matter are complete  Six quarks with their antiparticles  Six leptons with their antiparticles
  64. 64. Particle Properties
  65. 65. More About Quarks  No isolated quark has ever been observed  It is believed that at ordinary temperatures, quarks are permanently confined inside ordinary particles due to the strong force  Current efforts are underway to form a quark-gluon plasma where quarks would be freed from neutrons and protons
  66. 66. Color  It was noted that certain particles had quark compositions that violated the exclusion principle  Quarks are fermions, with half-integer spins and so should obey the exclusion principle  The explanation is an additional property called the color charge  The color has nothing to do with the visual sensation from light, it is simply a name
  67. 67. Colored Quarks Color “charge” occurs in red, blue, or green Antiquarks have colors of antired, antiblue, or antigreen These are the quantum “numbers” of color charge Color obeys the Exclusion Principle A combination of quarks of each color produces white (or colorless) Baryons and mesons are always colorless
  68. 68. Quantum Chromodynamics (QCD) QCD gave a new theory of how quarks interact with each other by means of color charge The strong force between quarks is often called the color force The strong force between quarks is mediated by gluons Gluons are massless particles When a quark emits or absorbs a gluon, its color may change
  69. 69. More About Color Charge Particles with like colors repel and those with opposite colors attract Different colors attract, but not as strongly as a color and its anticolor The color force between color-neutral hadrons is negligible at large separations The strong color force between the constituent quarks does not exactly cancel at small separations This residual strong force is the nuclear force that binds the protons and neutrons to form nuclei
  70. 70. Quark Structure of a Meson A green quark is attracted to an antigreen quark The quark – antiquark pair forms a meson The resulting meson is colorless
  71. 71. Quark Structure of a Baryon  Quarks of different colors attract each other  The quark triplet forms a baryon  Each baryon contains three quarks with three different colors  The baryon is colorless
  72. 72. QCD Explanation of a Neutron- Proton Interaction  Each quark within the proton and neutron is continually emitting and absorbing gluons  The energy of the gluon can result in the creation of quark- antiquark pairs  When close enough, these gluons and quarks can be exchanged, producing the strong force
  73. 73. Elementary Particles – A Current View Scientists now believe there are three classifications of truly elementary particles Leptons Quarks Field particles These three particles are further classified as fermions or bosons Quarks and leptons are fermions Field particles are bosons
  74. 74. Weak Force  The weak force is believed to be mediated by the W+, W-, and Z0 bosons  These particles are said to have weak charge  Therefore, each elementary particle can have  Mass  Electric charge  Color charge  Weak charge  One or more of these charges may be zero
  75. 75. Electroweak Theory  The electroweak theory unifies electromagnetic and weak interactions  The theory postulates that the weak and electromagnetic interactions have the same strength when the particles involved have very high energies  Viewed as two different manifestations of a single unifying electroweak interaction
  76. 76. The Standard Model A combination of the electroweak theory and QCD for the strong interaction form the standard model Essential ingredients of the standard model  The strong force, mediated by gluons, holds the quarks together to form composite particles  Leptons participate only in electromagnetic and weak interactions  The electromagnetic force is mediated by photons  The weak force is mediated by W and Z bosons The standard model does not yet include the gravitational force
  77. 77. The Standard Model – Chart
  78. 78. Mediator Masses Why does the photon have no mass while the W and Z bosons do have mass? Not answered by the Standard Model The difference in behavior between low and high energies is called symmetry breaking The Higgs boson has been proposed to account for the masses Large colliders are necessary to achieve the energy needed to find the Higgs boson  In a collider, particles with equal masses and equal kinetic energies, traveling in opposite directions, collide head-on to produce the required reaction
  79. 79. Particle Paths After a Collision
  80. 80. The Big Bang This theory states that the universe had a beginning, and that it was so cataclysmic that it is impossible to look back beyond it Also, during the first few minutes after the creation of the universe all four interactions were unified All matter was contained in a quark-gluon plasma As time increased and temperature decreased, the forces broke apart
  81. 81. A Brief History of the Universe
  82. 82. Hubble’s Law The Big Bang theory predicts that the universe is expanding Hubble claimed the whole universe is expanding Furthermore, the speeds at which galaxies are receding from the earth is directly proportional to their distance from us This is called Hubble’s Law
  83. 83. Hubble’s Law, cont Hubble’s Law can be written as v = H R H is called Hubble’s constant H 17 x 10-3 m / s ly
  84. 84. Remaining Questions About The Universe  Will the universe expand forever?  Today, astronomers are trying to determine the rate of expansion  The universe seems to be expanding more slowly than 1 billion years ago  It depends on the average mass density of the universe compared to a critical density  The critical density is about 3 atoms / m3  If the actual density is less than the critical density, the expansion will slow, but still continue  If the actual density is more than the critical density, expansion will stop and contraction will begin
  85. 85. More Questions Missing mass in the universe The amount of non-luminous (dark) matter seems to be much greater than what we can see Various particles have been proposed to make up this dark matter Exotic particles such as axions, photinos and superstring particles have been suggested Neutrinos have also been suggested  It is important to determine the mass of the neutrino since it will affect predictions about the future of the universe
  86. 86. Another Question Is there mysterious energy in the universe? Observations have led to the idea that the expansion of the universe is accelerating To explain this acceleration, dark energy has been proposed It is energy possessed by the vacuum of space The dark energy results in an effective repulsive force that causes the expansion rate to increase

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