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

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.

Version accepted for publication in JHEP. New section added, conclusion unchanged. 20 pages + appendices (39 pages in total), 9 figures in the main text (+ 4 from appendices)

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