Indirect search for the fourth genration of quarks and leptons

摘要

It is known that the Standard Model (SM) does not predict the number of fermion families; N. The only restriction comes from the asymptotic freedom of QCD which requires the number of quarks to be less than 17 and, therefore, the number of SM families to be N8. Before 1990s, many authors published articles related to the extra SM families and their phenomenological consequences. In early 1990s, the LEP data yields N is almost 3 where the neutral lepton mass for each family is less than half the mass of the Z boson. Generally, this result is interpreted as the exact value of N, since one assumes that the neutrinos must have very small masses. If we disregard this incorrect assumption, the LEP data does not exclude the existence of extra SM families with heavy neutrinos. Meanwhile, few papers arguing the existence of the fourth SM family have been publishing. These arguments are based on the flavor democracy hypothesis. The study of FCNC in B decays can indirectly shed light on the SM4 family in quark sector and LFV is a good candidate to indirect search for 4th generation of leptons.