We have revisited the computations of the flavor violating leptonic decays of the τ and µ leptons into three lighter charged leptons in the Standard Model with non-vanishing neutrino masses. We were driven by a claimed unnaturally large branching ratio predicted for the τ − → µ − + − ( = µ, e) decays [10], which was at odds with the corresponding predictions for the µ − → e − e − e + processes [8]. In contrast with the prediction in [10], our results are strongly suppressed and in good agreement with the approximation done in ref. [8], where masses and momenta of the external particles were neglected in order to deal with the loop integrals. However, as a result of keeping external momenta and masses in the computation of the dominant penguin and box diagrams-we even find slightly smaller branching fractions. Therefore, we confirm that any future observation of such processes would be an unambiguous manifestation of new physics beyond the Standard Model.

The observation of lepton flavor violating processes at colliders could be a clear signal of a nonminimal neutrino sector. We define a 5-parameter model with a pair of TeV fermion singlets and arbitrary mixings with the three active neutrino flavors. Then we analyze several flavor violating transitions ( → γ, ¯ or µ − e conversions in nuclei) and Z →¯ decays induced by the presence of heavy neutrinos. In particular, we calculate all the one-loop contributions to these processes and present their analytic expressions. We focus on the genuine effects of the heavy Majorana masses, comparing the results in that case with the ones obtained when the two heavy neutrinos define a Dirac field. Finally, we use our results to update the bounds on the heavy-light mixings in the neutrino sector.

The search for flavons with a mass of Oð1Þ TeV at current and future colliders might probe low-scale flavor models. We are interested in the simplest model that invokes the Froggatt-Nielsen mechanism with an Abelian flavor symmetry, which includes a Higgs doublet and a Froggatt-Nielsen complex singlet. Assuming a CP conserving scalar potential, there are a CP-even H F and a CP-odd A F flavons with lepton flavor violating (LFV) couplings. The former can mix with the standard-model-like Higgs boson, thereby inducing tree-level LFV Higgs interactions that may be at the reach of the LHC. We study the constraints on the parameter space of the model from low-energy LFV processes, which are then used to evaluate the flavon decay widths and the gg → ϕ → τμ (ϕ ¼ H F ; A F ) production cross section at hadron colliders. After imposing several kinematic cuts to reduce the standard model main background, we find that for m H F about 200-350 GeV, the decay H F → τμ might be at the reach of the LHC for a luminosity in the range 1-3 ab −1 , however, a luminosity of the order of 10 ab −1 would be required to detect the A F → τμ decay. On the other hand, a future 100 TeV pp collider could probe masses as high as Oð10Þ TeV if it reaches an integrated luminosity of at least 20 ab −1 . Therefore, the 100 TeV Collider could work as a flavon factory.