Lepton FCNC in Type III Seesaw Model

TL;DR

This paper systematically studies lepton flavor-changing neutral currents in the Type III seesaw model, analyzing new processes with experimental data to constrain FCNC effects and compare them with previous results.

Contribution

It introduces analysis of new FCNC processes in the Type III seesaw model and compares constraints with prior studies, highlighting scenarios where FCNC constraints remain relevant.

Findings

$ au o ext{pi}^0 e$ decay provides the strongest $e$-to-$ au$ FCNC constraint.

$ au o ho^0 \mu$ and $ au o ext{pi}^0 \mu$ strongly constrain $ ext{$\mu$-to-$ au$}$ FCNC.

Constraints on $e$-to-$ ext$$-$ ext$$-FCNC are weaker than previous bounds.

Abstract

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 $\tau \to P l$, $\tau \to V l$, $V \to l \bar l'$, $P \to l \bar l'$, $M\to M' l \bar l'$ and muonium-antimuonium oscillation. Results obtained are compared with previous results from $l_i \to l_j l_k \bar l_l$, $l_i \to l_j \gamma$, $Z \to l \bar l'$ and $\mu - e$ conversion. Our results show that the most stringent constraint on the $e$-to-$\tau$ FCNC effect comes from $\tau \to \pi^0 e$ decay. $\tau \to \rho^0 \mu$ and $\tau \to \pi^0 \mu$ give very stringent constraints on the $\mu$-to-$\tau$ FCNC effect, comparable with that obtained from $\tau \to \mu \bar \mu \mu$ 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.