1. Re-arrangement By Thang Hai Book Publishing Co.,
Chapter 31
Particle Physics
Re-arrangement By Thang Hai Book Publishing Co.,

2. In this image from the NA49 experiment at CERN, hundreds of subatomic particles are created in the collision of high-energy nuclei with a lead target. The aim of the experiment is to create a quark-gluon plasma, in which the force that normally locks quarks within protons and neutrons is broken.
Fig. 31-CO, p. 1048

3. 31.1 Atoms as Elementary Particles
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

4. Discovery of 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

5. 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

6. 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

7. Nuclear Force Holds nucleons together
Strongest of all the fundamental forces
Very short-ranged
Less than m
Negligible for separations greater than this

8. 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

9. 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

10. 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 times the strength of the nuclear force
Weakest of the four fundamental forces

11. 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

12. Forces and Mediating Particles

13. 31.2 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

14. 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

15. 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

16. 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
Fig 31.1

17. 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

18. 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

19. Pair Production, cont Fig 31.2
A photograph of pair production produced by 300 MeV gamma rays striking a lead sheet
The minimum energy to create the pair is MeV
The excess energy appears as kinetic energy of the two particles

20. 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+ ® 2g

21. 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

22. Figure 31.3: PET scans of the brain of a healthy older person (left) and that of a patient suffering from Alzheimer’s disease (right). Lighter regions contain higher concentrations of radioactive glucose, indicating higher metabolism rates and therefore increased brain activity.
Fig. 31-3, p. 1052

23. 31.3 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

24. 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

25. Mesons, cont
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

26. Pion There are three varieties of pions Pions are very unstable
+ and -
Mass of MeV/c2 0
Mass of 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

27. Muons Two muons exist The muon is unstable µ- and its antiparticle µ+
It has a mean lifetime of 2.2 µs
It decays into an electron, a neutrino, and an antineutrino

28. 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

29. 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

30. 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
Fig 31.4

31. 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

32. 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
Fig 31.5

33. Nucleon Interaction – More About 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

34. Nucleon Interaction, final
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

35. 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

36. 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
Fig 31.5

37. 31.4 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

38. 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

39. Leptons Do not interact through strong force All have spin of ½
Do participate in electromagnetic (if charged) and weak interactions
All have spin of ½
Leptons appear truly elementary
No substructure
Point-like particles

40. 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

41. Table 31-2, p. 1056

42. 31.5 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

43. 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

44. 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

45. Conservation of Baryon Number, Example
Is baryon number conserved in the following reaction?
Baryon numbers:
Before: = 2
After: (-1) = 2
Baryon number is conserved
The reaction can occur as long as energy is conserved

54. Figure 31. 6: (Interactive Example 31
Figure 31.6: (Interactive Example 31.2) This detector at the Super Kamiokande neutrino facility in Japan is used to study photons and neutrinos. It holds metric tons of highly purified water and photomultipliers. The photograph was taken while the detector was being filled. Technicians use a raft to clean the photodetectors before they are submerged.
Fig. 31-6, p. 1059

55. 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

56. 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

57. 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 = = 1
Muon lepton number is conserved

60. 31.6 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

61. 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

64. 31.7 Bubble Chamber Example of Strange Particles
The dashed lines represent neutral particles
At the bottom,
- + p  Λ0 + K0
Then Λ0  - + p and
Fig 31.7

65. 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

66. 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

67. Resonance Particles
Short-lived particles are known as resonance particles
They exist for times around s
In the lab, times for around s can be detected
They cannot be detected directly
Their properties can be inferred from data on their decay products

68. 31.8 Murray Gell-Mann 1929 – Studies dealing with subatomic particles
Named quarks
Developed pattern known as eightfold way
Nobel Prize in 1969

69. 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

70. 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
Fig 31.8

71. 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
Fig 31.8

72. 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
Fig 31.9

73. Figure 31. 10: Discovery of the Ω− particle
Figure 31.10: Discovery of the Ω− particle. The photograph on the left (a) shows the original bubble-chamber tracks. The drawing on the right (b) isolates the tracks of the important events. The K− particle at the bottom collides with a proton to produce the first detected Ω− particle plus a K0 and a K+.
Fig a, p. 1064

74. Figure 31. 10: Discovery of the Ω− particle
Figure 31.10: Discovery of the Ω− particle. The photograph on the left (a) shows the original bubble-chamber tracks. The drawing on the right (b) isolates the tracks of the important events. The K− particle at the bottom collides with a proton to produce the first detected Ω− particle plus a K0 and a K+.
Fig b, p. 1064

75. 31.9 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

76. 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

77. 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

78. Quark Composition of Particles – Examples
Mesons are quark-antiquark pairs
Baryons are quark triplets
Fig 31.11

79. Active Figure
31.11
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80. Table 31-3, p. 1066

81. 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

82. Table 31-4, p. 1067

83. 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

84. Table 31-5, p. 1067

85. 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

86. Table 31-6, p. 1068

87. 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

88. 31.10 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

89. 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

90. 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

91. 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

92. 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
Fig 31.12

93. 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
Fig 31.12

94. 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
Fig 31.13

95. Figure 31.13: (a) A nuclear interaction between a proton and a neutron explained in terms of Yukawa’s pion-exchange model. Because the pion carries charge, the proton and neutron switch identities. (b) The same interaction, explained in terms of quarks and gluons. Note that the exchanged _ud quark pair makes up a π− meson.
Fig , p. 1069

96. 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

97. 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

98. 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

99. 31.11 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

100. The Standard Model – Chart
Fig 31.14

101. 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

102. Figure 31.15: A view from inside the Large Electron–Positron (LEP) Collider tunnel, which is 27 km in circumference.
Fig , p. 1071

103. Particle Paths After a Collision
Fig 31.16

104. 31.12 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

105. A Brief History of the Universe
Fig 31.17

106. 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

107. 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
Fig 31.18

110. 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

111. Figure 31.19: Red shift, or speed of recession, versus magnitude (which is related to brightness) of 18 faint galaxy clusters. Significant scatter of the data occurs, so the extrapolation of the curve to the upper right is uncertain. Curve A is the trend suggested by the six faintest clusters. Curve C corresponds to a Universe having a constant rate of expansion. If more data are taken and the complete set of data indicates a curve that falls between B and C, the expansion will slow but never stop. If the data fall to the left of B, expansion will eventually stop and the Universe will begin to contract.
Fig , p. 1075

114. Figure 31.20: (Example 31.6) The galaxy labeled with mass m is escaping from a large cluster of galaxies contained within a spherical volume of radius R. Only the mass within the sphere slows the escaping galaxy.
Fig , p. 1075

116. 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

117. 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