1. Particle Physics and Cosmology
Chapter 46
Particle Physics
and Cosmology

2. 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

3. Discovery of New Particles
Beginning in the 1940s, many “new” particles were discovered in experiments involving high-energy collisions
Characteristically unstable with short lifetimes
Over 300 have been catalogued
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
The quark model has successfully predicted new quark combinations

6. Original Quark Model Three types or flavors
u – up
d – down
s – strange
Quarks have fractional electrical charges
-1e/3 and +2e/3
Associated with each quark is an antiquark
The antiquark has opposite properties (charge, baryon number and strangeness)

8. Fundamental Forces
All particles in nature are subject to four fundamental forces

9. Strong Force Attractive force between nucleons
Strongest of all the fundamental forces
Very short-ranged
Less than m (1 fm)

10. Electromagnetic Force
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

11. Weak Force Is responsible for instability in certain nuclei
Decay processes
Its strength is about 10-5 times that of the strong force

12. Gravitational Force Its effect on elementary particles is negligible
A long-range force
It is about times the strength of the strong force
Weakest of the four fundamental forces

13. Explanation of Forces
Forces between particles are often described in terms of the actions of field particles or exchange particles
Field particles are also called gauge bosons
The interacting particles continually emit and absorb field particles
The emission of a field particle by one particle and its absorption by another manifests itself as a force between the two interacting particles
The force is mediated, or carried, by the field particles

14. Forces and Mediating Particles

15. Paul Adrien Maurice Dirac
1902 – 1984
British physicist
Understanding of antimatter
Unification of quantum mechanics and relativity
Contributions of quantum physics and cosmology
Nobel Prize in 1933

16. Dirac’s Description of the Electron
Dirac developed a relativistic quantum mechanical description of the electron
It successfully explained the origin of the electron’s spin and its magnetic moment
The solutions to the wave equation 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

17. Dirac’s Explanation
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

18. Dirac’s Explanation, cont.
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

19. Antiparticles For every particle, there is an antiparticle
From Dirac’s version of quantum mechanics that incorporated special relativity
This has been verified for all particles known today
Some particles are their own antiparticles
Photon and po
An antiparticle of a charged particle 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

20. 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

21. 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 1.02 MeV
The excess energy appears as kinetic energy of the two particles

22. Quick Quiz
Given the identification of the particles in the figure on the right, what is the direction of the external magnetic field in the bubble chamber shown in the figure below?
(a) into the page (b) out of the page (c) impossible to determine

23. 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

24. Hideki Yukawa 1907 – 1981 Japanese physicist
Nobel Prize in 1949 for predicting the existence of mesons
Developed the first theory to explain the nature of the nuclear force

25. 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
Exchange of mesons between nucleons causes the nuclear force

26. 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 done by studying cosmic rays in the 1930s
Actually discovered multiple particles
pi meson (pion)
muon
Found first, but determined to not be a meson

28. Richard Feynman 1918 – 1988 American physicist
Developed quantum electrodynamics
Shared the Nobel Prize in 1965
Worked on Challenger investigation and demonstrated the effects of cold temperatures on the rubber O-rings used

29. Feynman Diagram – Two Electrons
The photon is the field particle that mediates the electromagnetic force between the electrons
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

30. Feynman Diagram – Proton and Neutron (Yukawa’s Model)
The exchange is via the nuclear force
Analysis predicts the rest energy of the pion to be ~130 MeV / c2

31. 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 c∆t
Using these pieces of information, the rest energy of the pion is about 100 MeV

32. 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

33. 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

34. Nuclear Force and Strong Force
Historically, the nuclear force was called the strong force
Now the strong force is reserved for the force between quarks
Or between particles made from quarks
The nuclear force is the force between nucleons
It is a secondary result of the strong force
Sometimes called residual strong force

35. Classification of Particles
Two broad categories
Classified by interactions
Hadrons – interact through strong force
Leptons – interact through weak force

37. Hadrons Interact through the strong force
Two subclasses distinguished by masses and spins
Mesons
Integer spins (0 or 1)
Decay finally into electrons, positrons, neutrinos and photons
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

38. Quark Composition of Particles – Examples
Mesons are quark-antiquark pairs
Baryons are quark triplets

41. Leptons Do not interact through strong force All have spin of 1/2
Leptons appear truly elementary
No substructure
Point-like particles

42. 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
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

46. Conservation of Lepton Number
There are three conservation laws, one for each variety of lepton
The 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

47. 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

48. 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

49. Strange Particles
Some particles discovered in the 1950s 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

50. Strangeness
To explain these unusual properties, a new quantum number S, called strangeness, was introduced
A new law, the law of conservation of strangeness was also needed
It states that the sum of strangeness numbers before a reaction or a decay 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

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

52. Murray Gell-Mann 1929 – American physicist
Studies dealing with subatomic particles
Named quarks after “Three Quarks for Muster Mark” from Finnigan’s Wake by James Joyce
Developed pattern known as eightfold way
Nobel Prize in 1969

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. An Eightfold Way for Baryons
A hexagonal pattern for the eight spin 1/2 baryons
Strangeness 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. 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. 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. 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. 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

59. 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/, was discovered that was shown to be a charm quark and charm antiquark pair

60. 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

62. 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. 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

65. 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 color charge
The color has nothing to do with the visual sensation from light, it is simply a name

66. 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

67. 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

68. 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

69. 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

70. 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

71. 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

72. 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

73. 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 actually yet include the gravitational force

74. The Standard Model – Chart

75. 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

77. Particle Paths After a Collision

78. 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

79. A Brief History of the Universe

80. Cosmic Background Radiation (CBR)
CBR represents the cosmic “glow” left over from the Big Bang
The radiation had equal strengths in all directions
The curve fits a black body at 2.7K
There are small irregularities that allowed for the formation of galaxies and other objects

81. CBR, cont.
The COBE satellite found that the background radiation had irregularities that corresponded to temperature variations of K

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. Hubble’s Law, cont. Hubble’s law can be written as v = HR
H is called the Hubble constant
H » 17 x 10-3 m/s•ly

84. Remaining Questions About the Universe
Will the universe expand forever?
Today, astronomers and physicists are trying to determine the rate of expansion
It depends on the average mass density of the universe compared to a critical density
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

85. Another Remaining Question About the Universe
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
The dark energy results in an effective repulsive force that causes the expansion rate to increase

86. Some Questions in Particle Physics
Why so little antimatter in the Universe?
Is it possible to unify electroweak and strong forces?
Why do quarks and leptons form similar but distinct families?
Are muons the same as electrons apart from their difference in mass?
Why are some particles charged and others not?
Why do quarks carry fractional charge?
What determines the masses of fundamental particles?
Can isolated quarks exist?

87. A New Perspective – String Theory
String theory is one current effort at answering some of the previous questions
It is an effort to unify the four fundamental forces by modeling all particles as various vibrational modes of an incredibly small string

88. String Theory, cont. The typical length of a string is 10-35 m
This is called the Planck length
According to the string theory, each quantized mode of vibration of the string corresponds to a different elementary particle in the Standard Model

89. Complications of the String Theory
It requires space-time to have ten dimensions
Four of the ten dimensions are visible to us, the other six are compactified (curled)
Another complication is that it is difficult for theorists to guide experimentalists as to what to look for in an experiment
Direct experimentation on strings is impossible

90. String Theory Prediction – SUSY
One prediction of string theory is supersymmetry (SUSY)
It suggests that every elementary particle has a superpartner that has not yet been observed
Supersymmetry is a broken symmetry and the masses of the superpartners are above our current capabilities to detect