extension
More quarks model 2ext
Two of the families
Family Particle Fundamental
lepton electron yes
neutrino yes
hadron proton no
neutron no
delta - no
sigma - no
many more
Table 1
The two main families of particles that exist on their own are the leptons and hadrons. The leptons are fundamental particles but the hadrons are not. They are made up of quarks. The only hadrons found in normal matter are the proton and the neutron. However, from the mid 1940s to the 1960s, physicists started discovering many new hadrons, like the delta and sigma. They found these in cosmic rays and experiments using particle accelerators. Although these new hadrons are all very unstable and decay with half lives usually less than a billionth of a second, their existence meant that physicists needed to find a new model.
Gene-ration Quarks
1st up down
2nd top bottom
3rd strange charm
Table 2
Family structure
In the 1960s Murray Gell-Mann and George Zweig proposed the quark model. In this model, the hadrons are made from smaller particles – the quarks. The quarks are said to have different flavours and there are six of them: up, down, top, bottom, strange and charm. They showed that the hundreds of hadrons and all their different properties could be explained by using combinations of just these six quarks.
How did the families of hadrons lead to the proposal of quarks?
H5e
Nucleon Quarks Representation
Proton Up

Up

Down		 A rollover change of a proton
Neutron Up

Down

Down		 A rollover change of a neutron
Table 3
Normal matter
All of normal matter (including protons and neutrons) is made from only two flavours of quark – the up and down quarks. These are known as the first generation of quarks.
Different generations
The other four flavours of quark form the second and third generations, with two flavours in each new generation. These generations play little part in our normal lives. Most of the hadrons that are made from the second and third generation quarks do not exist for very long. Nevertheless, they make up the complete picture of the Standard Model. Also, they help physicists to understand the two hadrons found in normal matter - protons and neutrons - and how the up and down quark are bound together to make them.
The story so far
• Protons and neutrons are made of up and down quarks
• Up and down quarks make up the first generation of quarks
• There are three generations of quarks
• Only the first generation quarks are found in normal matter
Question M7

a) Which of these are fundamental particles:
     hadron, up quark, proton, electron, strange quark, neutron?

b) What are the six flavours of quark?

c) Which flavours are found in normal matter?
The story so far
  • There are two families of fundamental particles: quarks and leptons.
  • Protons and neutrons are made from three quarks
  • Electrons are leptons
  • Nuclei can change by alpha and beta decay
  • There are three forces in the story so far: gravity, electromagnetism and the strong nuclear force