There are two types of elementary particles: fermions and bosons. Fermions obey the Pauli exclusion principle and have half-integer spin, while bosons do not obey PEP and have integer or zero spin. Fermions are further divided into leptons, which do not feel the strong force, and quarks, which do feel the strong force. Quarks combine to form composite particles called hadrons, which are divided into baryons containing three quarks and mesons containing two quarks. The four fundamental forces are electromagnetic, strong, weak, and gravity, and are mediated by gauge bosons.

1. Particle Physics

2. Elementary Particle
A particle with no internal structure.

3. Three types of elementary particles
Quarks
Leptons
Exchange Particles (Gauge Bosons)

4. elementary Gauge Bosons
particles
that feel
strong force
FERMIONS – follow Pauli
exclusion principle
elementary
particles DO NOT follow
that do not Pauli exclusion
feel strong principle
force

5. FERMIONS
Two types of fundamental particles are classified
as FERMIONS (they follow Pauli’s exclusion
principle and have ½ spin numbers)
Present theory states that these particles cannot
be broken down into even “smaller” particles.
These two classes of fundamental particles are.
Leptons – do not feel the strong force
Quarks – feel the strong force

6. Leptons
There are six types of lepton and each has an
antiparticle (opposite charge).
Family -1 charge zero charge
1 electron (e) electron-neutrino ( e)
2 muon ( ) muon-neutrino ( )
3 tau ( ) tau-neutrino ( )
Each lepton has a designated lepton number of +1. The
antiparticles of each lepton are -1. For any interaction, the
sum of all the lepton numbers must remain constant. This
is the lepton number conservation law.

7. Quarks (isolated quarks have never been detected)
There are six types of quarks and consequently six
types of anti-quarks (with opposite charge).
Family +2/3 charge -1/3 charge
1 up (u) down (d)
2 charm (c) strange (s)
3 top (t) bottom (b)
Quarks and anti-quarks combine to form composite
particles called HADRONS: two families of hadrons
3 quarks = baryon (ex. protons and neutrons)
2 quarks = meson (ex. pions)

8. Fermions Bosons
elementary particles elementary particles
gauge bosons
HADRONS
composite particles composite particles
baryons mesons
(made of 3 quarks) (one quark + one anti quark)

9. Elementary Particles

10. Exchange Particles – Mediate Fundamental Forces
gauge bosons
graviton
gluon (gravity)
(strong)
photon w+, w -, z0
(electromagnetic) (weak)
electroweak
Range: gravity, electromagnetic >> strong > weak
Strength: strong > electromagnetic >> weak >> gravity
Mass: weak >>>> strong, gravity, electromagnetic

11. The Higgs Boson
Not discovered yet, only theorized
An exchange particle that gains mass
when it interacts with other particles.
The existence of Higgs is important
because it is fundamental to theories
about how particles have mass. If it
doesn’t exist, much of the current theory
will need to be revised.

12. Classifying Particles
There are many different properties used to
classify a particle. These intrinsic properties are
expressed as quantum numbers.
Quantum numbers tell us about
-electric charge
- spin
- strangeness
-.charm
- color (not actual color)
- lepton number
- baryon number

13. Pauli’s Exclusion Principle
No two particles in a closed system (such as an
atom) can have the same set of quantum numbers.
All fermions follow the PEP
Bosons do not follow the PEP

15. Quantum Number – electrical charge
Fundamental particles can have positive, negative
or no charge.
An ANTIPARTICLE has the identical mass to a
particle but opposite charge (if charged) and
opposite spin (if there is spin).

16. Classifying Particles
There are many different properties used to
classify a particle. These intrinsic properties are
expressed as quantum numbers.
Quantum numbers tell us about
-electric charge
- spin
- strangeness
-.charm
- color (not actual color)
- lepton number
- baryon number

17. Quantum Number - SPIN
All fermions have non-integer spin
example electrons +½ (or – ½ )
All bosons have integer (or zero) spin

18. Classifying Particles
There are many different properties used to
classify a particle. These intrinsic properties are
expressed as quantum numbers.
Quantum numbers tell us about
-electric charge
- spin
- strangeness
-.charm
- color (not actual color)
- lepton number
- baryon number

19. Particles - Summary
All observed particles
fermions bosons
½ integral spin zero or integral spin
obey Pauli exclusion do not obey Pauli exclusion
mesons gauge
leptons quarks (2 quarks) bosons
Hadrons
baryons
(3 quarks)

20. Fundamental Interactions
The four fundamental interactions of nature are:
electromagnetic, strong, weak, and gravity
The electromagnetic and the weak interactions are two
aspects of the same interaction, the electroweak interaction

21. Mediation of Fundamental Forces
The fundamental forces are mediated by the
exchange of particles. These particles are called
exchange bosons.
A Feynman diagram can be used to show how
interactions between particles are mediated by
bosons.
The electromagnetic force is
mediated by photons. These
photons are unobservable
and are termed virtual
photons to distinguish them
from real ones.

22. Exchange Particles : the nature of force
All four of the fundamental forces involve the
continuous exchange of “virtual” particles
The creation of “virtual” particles is a breach of
conservation laws (as they are created from nothing) so
they can only exist for a short period of time.
The maximum range of an exchange force is dictated
by the Heisenberg uncertainty principle.

23. The Heisenberg Uncertainty Principle (HUP)
It is impossible to make precise measurements of both the
position and momentum (velocity) of electrons or any other
particles.
The very act of measuring changes these quantities. The
more precise one measurement is, the less precise the other
one becomes.
.

24. Implications of the Uncertainty Principle
HUP can be applied to the h
relationship between energy E t
and time.
4
Here, the uncertainty principle implies that the life time
of a virtual particle is inversely proportional to its
mass (energy)
The more massive the exchange particle, the shorter its life.
Why is the range of the strong and weak nuclear force very
small compared to the infinite range of the electromagnetic
and gravitational force?

25. The uncertainty in the energy of a virtual photon
is 7.1 × 10-19 J. Determine the uncertainty in the time for
the electromagnetic interaction between two electrons
exchanging the virtual photon.
.
34
h 6.6 10 17
t 19
7.4 10 s
4 E 4 (7.1 10 )

26. Range of Interactions of Exchange Particles.
The range of a virtual particle (and hence the force it mediates) is
governed by the equation below (from HUP)
h h is Planck’s constant
R c is the speed of light
m is the REST MASS of the virtual particle
4 mc
We see here again that
range is inversely proportional to the rest mass

27. The strong force has a range of about 10-15 m. Calculate the rest
mass of the related exchange particle. What type of particle is
this?
34
h 6.6 10 28
R 15 8
2 10 kg
4 mc 4 (10 )(3.0 10 )
this is a gluon

28. FEYNMAN DIAGRAMS
Exchange forces are often pictured with Feynman diagrams.
At each vertex in a Feynman diagram, conservation laws
such as charge, lepton number and baryon number must be
obeyed

29. Different lines are drawn for different particles. There are
some variations in the conventions that are applied.
or W and Z bosons
sometimes gluons

30. Interactions
Interactions are illustrated using Feynman
diagrams. Here are two examples:
Gluon exchange holds A meson interaction
quarks together. (which at the quark
level involves gluons)
holds nucleus together

33. Practice : Draw Feynman diagrams to illustrate the following
a) an electron absorbing a photon of energy
b) a positron (anti-electron) emitting a photon of energy
c) an electron-positron pair annihilation to form a photon
d) Formation of an electron and positron from a photon

34. Review Problem

35. Review Problem

36. Review Problem

37. Review Problem

38. Review Problem

39. Review Problem

40. Quarks (isolated quarks have never been detected)
There are six types of quarks and consequently six
types of anti-quarks (with opposite charge).
Generation +2/3 charge -1/3 charge
1 up (u) down (d)
2 charm (c) strange (s)
3 top (t) bottom (b)
Quarks and anti-quarks combine to form hadrons.
There are two classes of hadrons
3 quarks = baryon (ex. protons and neutrons)
2 quarks = meson (ex. pions)

41. Here are some examples of baryons and mesons.

42. Baryons (three quarks)
Baryon numbers are examples of quantum
numbers.
Baryon numbers are +1 and -1 (anti-particles)
respectively. The baryon number is conserved
in any interaction.
All other particles have a baryon number of zero.
(only a Baryon can be +1 or -1)

43. Individual quarks have baryon
numbers of 1/3 (or -1/3)
Protons consist of two up
quarks and one down. This is
written as uud and referred to
as up, up, down.
Note that the overall baryon number is
1/3 + 1/3 + 1/3 = 1
And the overall electrical charge would be equal to
+ 2/3 + 2/3 + (-1/3) = +1

44. Charges in quarks
EZ to remember
Proton UUD
Neutron UDD
make sense?

45. Quarks and Spin
Recall
All fermions have non-integer spin
ex. electrons have spin number ½
ex. protons have spin number ½
ex. quarks have spin number ½
All bosons have integer (or zero) spin

46. There are two spin states referred to as UP and
DOWN
So
spin number +½ UP
spin number - ½ DOWN
In a proton, the two up quarks cannot have the
same spin number.

47. Quarks and QCD
Quarks also have different “colors”.
The color force between quarks is mediated by gluons.
quarks come in three colors: red, blue, green
anti-quarks are : anti-red (cyan), anti-blue (yellow) and
anti-green (magenta)

48. The “colorless” property of bound quarks is called
confinement.
Only combinations of color-neutral (add to white) quarks
have been found.
Baryons R + G + B = white
Mesons color + anti-color = white
The combination though must always be color neutral
(white or colorless). This is why particles consisting of 4
quarks have never been found.

49. Strangeness – yet another quantum number
Depends on number of
strange (-1) and anti-strange
(+1) quarks in a composite
particle.
Only conserved in interactions
involving gluons and photons.
(not the WEAK force)

50. Interactions
You do not need to worry about the composition of
baryons (other than protons and neutrons) or
mesons. You should however be able to apply
conservation laws to interactions. They are:
Conservation of mass-energy.
Conservation of baryon and lepton numbers.
Conservation of electrical charge
Conservation of angular momentum. Each particle
has a spin number. The total spin before and after
the interaction remains the same.

51. Practice Problem
A common process examined is beta decay.
neutron  proton + electron + anti-neutrino
The anti-neutrino is required to conserve the
lepton number : zero = zero + 1 – 1
uud
To convert a neutron to a
?
proton a down quark must
change its flavor.
udd

52. Beta decay continued:
For udd  uud conversion
All quarks have baryon number of 1/3 so baryon
number is conserved. Charge however is not
conserved. A negative charge must be removed.
uud
Beta decay is mediated by
the weak force. The weak
w-
force boson w – changes the
flavor of the up quark in the
neutron.
udd

53. Interactions and Other Processes
e-
uud
w- Arrows pointing down
in a Feynman diagram
indicate anti-particles,
udd NOT direction.
The electron and anti-neutrino lepton
numbers are + 1 and -1 so lepton number is
conserved, as is electrical charge.

64. Elementary Particles Composite
Particles
Do not feel strong force
Color combinations
= white
Lepton # = 1
Obey PEP
(anti leptons = -1)
Baryons
Baryon # = 1
Feel strong force
Baryon # = 1/3
(anti quarks = -1/3)
Hadrons
Gauge Bosons
Mesons
Obey PEP
graviton
&
Do Not
Higgs
(undetected) Strong
EM
Weak