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Lepton-number-violating decays of heavy flavors induced by doubly-charged Higgs boson

We study lepton-number-violating (LNV) decays of heavy flavors ($τ$ lepton and top quark) induced by doubly-charged Higgs boson in the Higgs triplet model. It is found that the branching fractions of LNV $τ^- \to \ell^{+}M_1^-M_2^-$ decays are highly suppressed compared with the current experimental limits. On the other hand, for LNV top quark decays, the most optimistic branching ratios for $t \to b\ell^{+}\ell^{+}W^{-}$ turn out to be at the level of $\sim 10^{-7} - 10^{-8}$. The observation of these rare top quark decays would be a clear signal of LNV processes, and their non-observation would allow us to constraint the parameters of the Higgs triplet model.