Lepton FCNC in Type III seesaw model

In Type III seesaw model, there are tree level flavor changing neutral currents (FCNC) in the lepton sector, due to mixing of charged particles in the leptonic triplet introduced to realize seesaw mechanism, with the usual charged leptons. In this work we study these FCNC effects in a systematic way using available experimental data. Several FCNC processes have been studied before. The new processes considered in this work include: lepton flavor violating processes tau -> Pl, tau -> Vl, V -> l (l') over bar, P -> l (l') over bar, M -> M'l (l') over bar and muonium-antimuonium oscillation. Results obtained are compared with previous results from l(i) -> l(j)l(k)(l) over bar (l), l(i) -> l(j)gamma, Z -> l (l') over bar and mu - e conversion. Our results show that the most stringent constraint on the e-to-tau FCNC effect comes from tau -> pi(0)e decay. tau -> rho(0)mu and tau -> pi(0)mu give very stringent constraints on the mu-to-tau FCNC effect, comparable with that obtained from tau -> mu(mu) over bar mu studied previously. The constraint on the e-to-mu FCNC effect from processes considered in this work is much weaker than that obtained from processes studies previously, in particular that from mu-e conversion in atomic nuclei. We find that in the canonical seesaw models the FCNC parameters, due to tiny neutrino masses, are all predicted to be much smaller than the constraints obtained here, making such models irrelevant. However, we also find that in certain special circumstances the tiny neutrino masses do not directly constrain the FCNC parameters. In these situations, the constraints from the FCNC studies can still play important roles.