1. More leptons

2. The leptons The leptons we have met e+,e-, ,
Remember the leptons form a family.
They are all sister particles
They get involved in the weak interaction
Proton to neutron
Beta negative decay releases two leptons
e+,
Neutron to proton
Beta positive decay releases the other two leptons

3. PARTICLE SPIN Particles have intrinsic “spin”
This spin is not like spin in the “large world”.
Leptons have a fixed quantity of spin (measured in units of h/2π)
In these units leptons all have spin value of ½.

4. (of the electron family)
Leptons
(of the electron family)
Rest
Energy
(MeV/c2)
charge
Lepton number
(Le)
Spin
Electron
(e-)
0.511 -1 1
Positron
(e+) +1
Neutrino
(ve)
(0)
Antineutrino
( )

5. The muon
In the 1930’s a particle just like a heavy electron was discovered raining down in cosmic rays. It became known as the muon (μ). It was around 200 times heavier than the electron and it is unstable with a mean lifetime around 2.2 x 10-6 seconds.
This produced a bit of a problem!

6. The interesting thing is that it decays to give an electron and a normal antineutrino
But another particle is produced in this decay as well!

7. A new family of leptons e-
A new type of neutrino called a muon neutrino
Muon (μ)

8. An entire lepton family
The existence of two new particles meant that their anti particles must exist
muon
antimuon
Muon neutrino
Antimuon neutrino
This means we have an entire family of leptons, just like the electron family but heavier and unstable

9. Leptons
(of the muon family)
Rest
Energy
(MeV/c2)
charge
Lepton number
(Lμ)
Spin
muon
(μ-)
105.7 -1 1
antimuon
(μ+) +1
Muon Neutrino
(vμ)
(0)
Antimuon neutrino
( )

10. We have a new conservation rule
Our normal lepton number is conserved
We have a new conservation rule
Lμ (1) → (0) (0) (1)