Charged lepton flavor violating Higgs decays at future $$e^+e^-$$ e + e - colliders

After the discovery of the Higgs boson, several future experiments have been proposed to study the Higgs boson properties, including two circular lepton colliders, the CEPC and the FCC-ee, and one linear lepton collider, the ILC. We evaluate the precision reach of these colliders in measuring the branching ratios of the charged lepton flavor violating Higgs decays $$H\rightarrow e^\pm \mu ^\mp $$ , $$e^\pm \tau ^\mp $$ and $$\mu ^\pm \tau ^\mp $$ . The expected upper bounds on the branching ratios given by the circular (linear) colliders are found to be $${\mathcal {B}}(H\rightarrow e^\pm \mu ^\mp ) < 1.2~(2.1) \times 10^{-5}$$ , $${\mathcal {B}}(H\rightarrow e^\pm \tau ^\mp ) < 1.6~(2.4) \times 10^{-4}$$ and $${\mathcal {B}}(H\rightarrow \mu ^\pm \tau ^\mp ) < 1.4~(2.3) \times 10^{-4}$$ at 95% CL, which are improved by one to two orders compared to the current experimental bounds. We also discuss the constraints that these upper bounds set on certain theory parameters, including the charged lepton flavor violating Higgs couplings, the corresponding parameters in the type-III 2HDM, and the new physics cut-off scales in the SMEFT, in RS models and in models with heavy neutrinos.