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6. The Standard Model page 22
Summary of The Standard Model
The Standard Model is the basis of our picture of particle physics. It describes the fundamental particles, the forces between them and the ways that they combine to make other particles. In this resource, we have looked at the discoveries and reasoning that led to this picture.

It says that:

  • there are two families of fundamental particles – leptons and quarks (table 1)
  • each family has three generations with two particles and two anti-particles in each
  • all of normal matter is made from the first generations of these families
  • the particles are held together (or apart) by four main forces (table 11)
Leptons Quarks
Generation Particles Antiparticles Particles Antiparticles
1 electron electron-
neutrino
positron anti-
neutrino
up down anti-
up
anti-
down
2 muon muon-
neutrino
anti-muon anti-muon-
neutrino
top bottom anti-
top
anti-
bottom
3 tau tau-
neutrino
anti-tau anti-tau-
neutrino
strange charm anti-
strange
anti-
charm
Forces felt electromagnetic, weak, (gravity) electromagnetic, weak, SNF, (gravity)
Table 10. The two families of particles that make up The Standard Model. Only the first generation particles are found in normal matter.
Use these links to find out more:    |  leptons  |  quarks  |  antimatter  |  forces  |

Family matters
The leptons and quarks are different from each other in a number of ways. Most notably:
  • leptons can exist on their own whereas quarks only exist in combination with other quarks making hadrons
  • quarks feel the strong nuclear force and leptons do not.
Beyond The Standard Model
The Standard Model is not a complete theory. It explains much of the structure of sub-atomic particles. However, it does not include the force of gravity and does not encompass The General Theory of Relativity.

One of the drives of modern physics beyond The Standard Model is to unify the fundamental forces. This would mean that each of the forces was just a special case of a single, unified force. Electromagnetism is an example of a unified force – even though we sometimes refer to electrostatics and magnetism as separate forces, they are both actually special cases of the single force of electromagnetism. And this force was unified with the weak force in the 1960s to produce the electroweak force.

Force Unified forces Particles it affects Exchange particle
Electromagnetic Electro-
weak
G
U
T
T
O
E
anything with charge virtual photon
Weak all fundamental particles W+, W-,
Z0
Strong nuclear force (SNF)             quarks gluon
(Gravity)                      anything with mass (graviton)
Table 11. The four forces of nature. Gravity is not described by The Standard Model. The darker squares are yet to be discovered.
Current research is aiming at unifying the electroweak force with the strong nuclear force to produce a Grand Unified theory (GUT). Eventually theorists hope to bring in gravity (which isn't a part of The Standard Model) to produce a Theory of Everything (TOE).
Question 21
a) What are the two families of fundamental particle in The Standard Model?

b) How many generations are there in each family?

c) How many generations are found in normal matter?

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