Determining the spacetime structure of bottom-quark couplings to spin-zero particles

This is the authors' abstract. We don't add key points for this paper.

We present a general argument that highlights the difficulty of determining the spacetime structure of the renormalizable bottom-quark Yukawa interactions of the Standard Model Higgs boson, or for that matter of any hypothetical spin-zero particle, at high energy colliders. The essence of the argument is that, it is always possible, by chiral rotations, to transform between scalar and pseudoscalar Yukawa interactions without affecting the interactions of bottom quarks with SM gauge bosons. Since these rotations affect only the b-quark mass terms in the Standard Model Lagrangian, any differences in observables for scalar versus pseudoscalar couplings vanish when mb→0, and are strongly suppressed in high energy processes involving the heavy spin-zero particle where the b quarks are typically relativistic. We show, however, that the energy dependence of, for instance, e⁺e^-→bb̄X (here X denotes the spin-zero particle) close to the reaction threshold may serve to provide a distinction between the scalar versus pseudoscalar coupling at electron-positron colliders that are being proposed, provided that the Xbb̄ coupling is sizeable. We also note that while various kinematic distributions for tt̄h are indeed sensitive to the spacetime structure of the top-Yukawa coupling, for a spin-zero particle X of an arbitrary mass, the said sensitivity is lost if mXmₜ.