The right generations
  The Standard Model has three generations of fermions and although it does not contain any explicit reason for this, the existence of additional generations is now very constrained by experiment. Present measurements are saturating perturbative unitarity limits. The main idea of this work is to show that those restrictions can be relaxed if the new generations experience different interactions. This new setup leads to the presence of additional stable degrees of freedom that give rise to a very rich phenomenology for cosmology, astrophysics and particle physics. The stability is a consequence of the conservation of new accidental baryon and lepton numbers. We present an explicit example by introducing a fourth generation charged under a new $SU(2)_R$ gauge interaction instead of the standard $SU(2)_L$. The simplest implementations lead to models that contain stable quarks, leptons and neutrinos. We show that these new particles can have a wide range of masses within a non-standard cosmological set-up. Indeed, the new neutrinos (and {\it neutral leptons}) constitute viable dark matter candidates if they are the lightest of these new particles. 
Alfredo Aranda, and Jose A. R. Cembranos
hep-ph