The mixing of new vectorlike leptons with leptons in the standard model can generate flavor violating couplings of $h$, $W$ and $Z$ between heavy and light leptons. Focusing on the couplings of the muon, the partial decay width of $h\to e_4^{\pm }{\mu }^{\mp }$, where $e_4$ is the new lepton, can be significant when this process is kinematically allowed. Subsequent decays $e_4^{\pm }\to Z{\mu }^{\pm }$ and $e_4^{\pm }\to W^{\pm }\nu$ lead to the same final states as $h\to Z{Z}^{*}\to Z{\mu }^{+}{\mu }^{-}$ and $h\to W{W}^{*}\to W\mu \nu$, thus possibly affecting measurements of these processes. We calculate $h\to e_4{\ell }_i\to Z{\ell }_i{\ell }_j$, where ${\ell }_{i,j}$ are standard model leptons, including the possibility of off-shell decays, interference with $h\to Z{Z}^{*}\to Z{\ell }_i{\ell }_i$, and the mass effect of ${\ell }_{i,j}$ which are important when the mass of $e_4$ is close to the mass of the Higgs boson. We derive constraints on masses and couplings of the heavy lepton from the measurement of $h\to 4\ell$. We focus on the couplings of the muon and discuss possible effects on $h\to Z{Z}^{*}$ from the region of parameters that can explain the anomaly in the measurement of the muon $g-2$.

• Received 4 July 2014

DOI:https://doi.org/10.1103/PhysRevD.90.034023