Is there any evidence for a 4th family of particles?

[Haelfix]

Not to go on a religious statement, but I don't like violating the EP.

The Sterile neutrino is a simple way to correct the minimal See-Saw mechanism, which is nice and elegant (again theoretical prejudice).

But here's my quick thoughts on why Sterile neutrinos are nice

1) They are present in my favorite GUT theory SO(10)

2) there's a missing entry in the minimal SDM, the right handed neutrino. Identification of this with a sterile would balance each family out in a historically known way (left-right and quark-lepton similarity provided the incentive for GIM)

A good paper on neutrino oscillations is hep-ph/0306075

Btw, if CPT is violated, that would be so cool!

 

[GString]

Several people have commented that one of the reasons that they do not think a fourth generation exists is that the fourth generation neutrino would have to be larger than M_Z/2. Since the three neutrinos that we know of have very tiny masses, this seems unnatural to some people. However, this really isn't as unnatural as it might seem. If the Yukawa matrix for Dirac neutrinos in a four-generation model happens to be close to rank 1, then this results in one large mass eigenstates and three small mass eigenstates. In the limit that the Yukawa matrix is rank 1, the result is one non-zero mass eigenstate and three eigenstates with zero mass. So, it would actually be somewhat natural to have three light neutrinos with a heavy fourth generation neutrino.