Elementary Particals

1. By
Mrs. Samia Rehman Dogar
Associate Prof
Federal College Of Education
H-9,Islamabad
samia42001@yahoo.com
Elementary Particles

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. 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. 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. 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. 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. 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. 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. 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. 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. Forces and
Mediating Particles

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Murray Gell-Mann
1929 –
Studies dealing
with subatomic
particles
Named quarks
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 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. 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
 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. 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. Quark Composition
of Particles – Examples
 Mesons are quark-
antiquark pairs
 Baryons are quark
triplets

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. 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. 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. Particle Properties

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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. The Standard Model – Chart

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. Particle Paths After a Collision

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. A Brief History of the Universe

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 = H R
H is called Hubble’s
constant
H 17 x 10-3 m / s ly

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. 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. 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