Using spin correlations to distinguish Zh from ZA at the International Linear Collider

We investigate how to exploit the spin information imparted to the Z boson in associated Higgs production at a future linear collider as an aid in distinguishing between CP-even and CP-odd Higgs bosons. We apply a generalized spin-basis analysis which allows us to study the possibilities offered by nontraditional choices of spin projection axis. In particular, we find that the Z bosons produced in association with a CP-even Higgs via polarized collisions are in a single transverse spin state (>90% purity) when we use the Zh-transverse basis, provided that the Z bosons are not ultrarelativistic (speed <0.9c). This same basis applied to the associated production of a CP-odd Higgs yields Z's that are an approximately equal mixture of longitudinal and transverse polarizations. We present a decay angular distribution which could be used to distinguish between the CP-even and CP-odd cases. Finally, we make a few brief remarks about how this distribution would be affected if the Higgs boson turns out not to be a CP eigenstate.