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Generation 4 (e, mu, tau, ?)

  1. Aug 18, 2007 #1
    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.
  2. jcsd
  3. Aug 18, 2007 #2
    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.
  4. Aug 18, 2007 #3


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    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.
  5. Aug 20, 2007 #4
    thank you so much for your answers :cool:
  6. Aug 20, 2007 #5
    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.
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