I'm studying from these lecture notes by S. Pascoli. She mentions several times the leptonic flavour problem but she never actually defines it. I've looked a bit online but I only found vague expression of the problem. So I ask you,
- What is the flavour problem?
- In particular, what is the leptonic flavour problem?
As far as I've understood so far, it has to do with the masses, and the mixing but I don't have a clear understanding of what actually is the issue here.
I'd like to receive a rough explanation followed by a more rigourous treatment of the problem if it's possible. But I'd be ok with just the rough explanation as well if it helps me understand these issues and be able to go on reading about the problem on my own.