Why is this interaction forbidden?

[Soffie]

So I'v been told that this interaction :

e- + e+ = mu- + e+

isn't allowed. Why not? I thought maybe because the mass of the muon is so much bigger, so would violate energy conservation, but the electron/positron could be moving relativistically and thus have enough energy.
Apparently the reason is because they're from different generations, but I have no idea what that means.

Cheers

 

[mfb]

Neglecting neutrino mixing, the lepton numbers are conserved individually for electrons, muons and taus. On the right you have a muon and no antimuon or neutrino, on the left you have nothing like that. Similarly, on the right you have a position without partner while on the left side there is only an electron/positron pair.