EVIDENCE FOR THE FOURTH GENERATION?

In a recent experiment, the measured lifetime of the tau lepton suggests that the e-µ universality may not hold in the case of the third-generation leptons. It is shown here that the universality of weak interactions can be restored by invoking the existence of a fourth generation of quarks and leptons, and by mixing the τ-lepton with the charged lepton of the fourth generation by a Cabibbo-like mixing angle. Agreement between the experimentally measured lifetime and the theoretical value as predicted by the standard SU (2) L × U (1) model on the basis of universality of weak interactions requires the cosine of the Cabbibo-like mixing angle (ϕ) to be | cos ϕ|=0.96±0.02.