More leptons

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

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.

(of the electron family) Leptons (of the electron family) Rest Energy (MeV/c2) charge Lepton number (Le) Spin Electron (e-) 0.511 -1 1 ½ Positron (e+) +1 Neutrino (ve) (0) Antineutrino ( )

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!

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!

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

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

Leptons (of the muon family) Rest Energy (MeV/c2) charge Lepton number (Lμ) Spin muon (μ-) 105.7 -1 1 ½ antimuon (μ+) +1 Muon Neutrino (vμ) (0) Antimuon neutrino ( )

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