The universality of leptonic sector and heavy neutral lepton(s)

In this paper the experimental tests on the universality of leptonic sector are viewed. All experiments are consistent with the requirement by the universality except the τ decays, which indicate a deviation from the requirement by two standard deviations. The authors point out that any improvement in measuring the branching ratios of pure leptonic decays and/or the lifetime of τ lepton will he helpful for clarifying the deviation but not in measuring the τ mass because it is comparatively accurate enough for testing the universality now. The authors also discuss some possible models involving heavy neutral lepton(s), in which some deviation of the universality occurs if analyzing experimental data without considering the heavy one(s)