Lepton Flavour!

Story of leptons begin with the discovery of electrons by Joseph John Thomson in the year 1897. After 40 years of his discovery of electron another generation of leptons was discovered namely muon (μ−). And then in the year 1975 another generation of leptons was discovered by Martin Lewis Perl. After the discovery of all these three leptons we may wonder whether we are faced with a lepton spectroscopy. In that case we should observe the following phenomena which we don't:

μ− → e− + γ

Now we can put some constraints so that this phenomena would not happen, at least the constraints would set some upper limit for such phenomena to happen. This constraints is the lepton flavour. Lepton flavour is a quantum number assumed to be conserved. Thus we have electron flavour Le such that Le(e−) = 1 and Le(e+) = −1; muon flavour Lμ such that Lμ(μ−) = 1 and Lμ(μ+) = −1; and tau flavour Lτ such that Lτ (τ−) = 1 and Lτ (τ+) = −1. Now if we look at the above phenomena and assume that lepton number is conserved the it can't happen.

--->Inspired from the book Gauge Theories in
Particle Physics, A practical introduction, volume 1. Author--> Ian J. R. Atchison | Anthony J.G. Hey