Pseudoscalar Higgs boson in experiments at e ⁺ e ⁻ colliders

The potential of experiments at e ⁺ e ⁻ colliders to search for a signal of a new pseudoscalar Higgs boson and to study its CP properties is analyzed. A new Higgs boson coupling to the b quark and τ lepton, which is parameterized in a model-independent way as m _b /v(a + iγ₅ b), is chosen to be an indicator of its CP nature. We study the e ⁺ e ⁻ → τ⁺τ⁻vv, e ⁺ e ⁻ → b $\overline{b}$vv, and e ⁺ e ⁻ → e ⁺ e ⁻ b $\overline{b}$ processes highly sensitive to anomalous Higgs boson couplings due to the dominant contributions from the vector boson fusion processes W*W* → H and Z*Z* → H in the central region. It is shown that the study of polarization of particles in the final state could help to separate the contributions from the scalar and pseudoscalar Higgs bosons. For instance, the consideration of the cascade decays of τ leptons in the e ⁺ e ⁻ → τ⁺τ⁻vv process allows one to determine reliably the CP state of the Higgs boson as well as the value and the sign of the b parameter. As a result of our analysis, we develop a search strategy for a signal of a new pseudoscalar Higgs boson in future experiments at linear e ⁺ e ⁻ colliders, which will allow one to understand the CP nature of the Higgs boson and set the bounds on its couplings to fermions.