Revisiting lepton flavor violation: $\tau$ and meson decays

TL;DR

This paper revisits charged lepton flavor violation in the minimal type-I seesaw model, highlighting the significance of semileptonic tau and meson decays with updated data, and identifying promising channels for future experiments.

Contribution

It systematically compares semileptonic cLFV channels in the seesaw model using updated inputs, revealing new dominant decay modes and potential experimental signatures.

Findings

Tau to rho decay can dominate over purely leptonic tau decays in certain parameter regions.

Heavy-meson decays remain below current experimental sensitivity.

Some cLFV processes are within reach of next-generation experiments.

Abstract

The minimal type-I seesaw model provides a simple explanation of neutrino flavor oscillations and induces charged lepton flavor violation (cLFV). Despite extensive previous studies, semileptonic cLFV channels remain underexplored. Using updated form factors, decay constants, and oscillation data, we revisit $\tau$ and meson decay channels, performing a systematic comparison across the seesaw parameter space. Surprisingly, we find that decays such as $\tau \to \ell\rho$ can dominate over purely leptonic $\tau$-sector probes, including $\tau\to 3\ell$ and even $\tau\to\ell\,\gamma$, in certain regions. In contrast, heavy-meson decays remain far below experimental sensitivity. Considering global constraints on the seesaw parameters, we derive branching ratios for the relevant cLFV processes and identify those within potential reach of next-generation experiments.