The lepton family model 1ext
The electron is the first member of the lepton family that most of us hear about. There is one other lepton that we find in normal matter: the neutrino. We call these two particles the first generation of the lepton family. They each have an associated anti-particle: the positron and the anti-neutrino.
What is antimatter
Two generations
However, in 1936, two American physicists discovered a new lepton. Carl Anderson and S.H. Neddermeyer were using a cloud chamber to study cosmic radiation. They noticed tracks that they could not explain using any known particles. The new tracks came from something like an electron but with 207 times its mass. They called this particle the muon. It is a second-generation lepton.
The muon
The muon has similar properties to an electron. However, it is very unstable. It decays with a half-life of 2 microseconds (2 millionths of a second). When it decays, it produces an electron and two different neutrinos. One of these is the neutrino that we have already met and one is a new, second generation, neutrino. Particle physicists are still trying to explain this decay.

What's in a name?

To distinguish between the two neutrinos, physicists name them after the other particle in their generation. So they become the electron-neutrino and the muon-neutrino respectively.

Table 1. The three generations of lepton. Roll over here to show just the electron's generation.
Second generation
The muon, the muon-neutrino and their anti-particles form a generation of particles that repeats the pattern of the first generation - the electron's generation. However, they are not the end of the story.
Another generation
In 1974, Martin Perl discovered a third generation lepton called the tau particle. Like the electron and the muon, the tau particle has an associated tau-neutrino and anti-particles. This third generation of leptons repeats the pattern of the other two generations but at a higher mass - about twice the mass of the muon generation.
The second and third generations of leptons are not part of ordinary matter and they have extremely short lives. It is, however, a satisfying symmetry that the leptons have three generations in the same way that the quarks have three generations.
The generations of quarks
The story so far
There are three generations of leptons
Electrons and neutrinos make up the first generation of leptons
The first generation leptons are the only ones found in normal matter
Leptons do not feel the strong nuclear force
Question M6

a) What does the term half life mean?

b) How many generations of lepton are there
   i. altogether?

   ii. in normal matter?

c) How many leptons are there in each generation?

The story so far
  • Atoms have a nucleus with the electrons outside it
  • The nucleus contains protons and neutrons
  • Protons and neutrons are members of the family of hadrons, which are not fundamental
  • Electrons are members of the lepton family - they are fundamental