Production of CP-even and CP-odd Higgs bosons at Muon colliders

In the $s$-channel Higgs-boson-exchange processes, the interference between the amplitudes for CP-even and CP-odd Higgs bosons is sizable, if the helicities of initial and final particles are properly fixed and if the mass difference between these bosons is not much larger than their decay widths. We discuss this interference effect in the process ${\mu }^{+}{\mu }^{-}$ $\to t\bar{t}$. Examining the effects gives us information on CP-parity for Higgs bosons and on the sign of a product of the coupling constants for $H{\mu }^{-}{\mu }^{+}$, $Ht\bar{t}$, $A{\mu }^{-}{\mu }^{+}$ and $At\bar{t}$ vertices. The feasibility of observing the interference effect in future muon colliders is evaluated in the framework of the minimal supersymmetric extension of the standard model as an example.