The presence of massive sterile neutrinos $N$ mixed with the active ones induces flavor violating processes in the charged lepton sector at the loop level. In particular, the amplitude of $H^0\to {\overline{\ell }}_i{\ell }_j$ is expected to be proportional to the product of heavy-light Yukawa couplings $y_i{y}_j=2s_{{\nu }_i}{s}_{{\nu }_j}{m}_N^2/v^2$, where $s_{{\nu }_{i,j}}$ express the heavy-light neutrino mixings. Here, we revisit these Higgs decays in the most generic extension of the neutrino sector, focusing on large values of $y_i$. We show that decoupling effects and a cancellation between the two dominant contributions to these processes makes the amplitude about 100 times smaller than anticipated. We find that perturbative values of $y_i$ giving an acceptable contribution to the $\rho$ parameter imply $\mathcal{B}(H^0\to {\overline{\ell }}_i{\ell }_j)<{10}^{-8}$ for any lepton flavors, a rate that is not accessible at current colliders.

• Received 4 September 2020
• Accepted 2 December 2020

DOI:https://doi.org/10.1103/PhysRevD.102.113006

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Published by the American Physical Society