Can There Be a Fourth Generation of Charged Leptons in the Standard Model?

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here I come with a question, maybe stupid.
everyone knows about electrons, muons, taus. Is a heavier charged lepton impossible to exist? why does SM deal with three generation of particles (each consisting in a charged lepton, a neutrino plus two quarks), and not more?

thanks everyone.
 
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wedge said:
here I come with a question, maybe stupid.
everyone knows about electrons, muons, taus. Is a heavier charged lepton impossible to exist? why does SM deal with three generation of particles (each consisting in a charged lepton, a neutrino plus two quarks), and not more?

thanks everyone.

Hi Wedge,

There are strong experimental evidences against a fourth generation of (rather light) fermions :
- the Z cross-section measured at LEP prooves there are only 3 light neutrinos. Of course, it is still possible to have a fourth neutrino much more heavier or non-interacting with Z (but is it still a similar generation ?).
- the so-called "unitarity triangle" mainly cronstrained by B physics measurements is a triangle...
But, of course, nothing prooves there are 3 and only 3 generations. The SM tells nothing about the number of generations, this is an input of the model.
 
Giving that the number of states is N*32, four generations is a kind of paradise for model builders: you have 128 states, a exact power of 2, and then Clifford algebras have a big role.

But there are only 3 light neutrinos.
 
There's still the neutrino 'puzzle' - to which there have been numerous rather outlandish suggestions; check the latest (I think) Scientific American for the details. Among the proto-theories are additional, non-generational and even more weakly-interacting neutrinos. So, you're unlikely to come up with another complete generation, but it's not out of the question for particles that don't belong in any generation to come up.