QCD corrections to decay distributions of neutral Higgs bosons with (in)definite CP parity

We compute the order ${\ensuremath{\alpha}}_{s}$ QCD corrections to the density matrix for the decay of a neutral Higgs boson $\ensuremath{\varphi}$ with (in)definite $\mathrm{CP}$ parity into a quark-antiquark pair and the QED corrections for the decay into a pair of charged leptons. We classify and calculate single spin asymmetries and spin-spin correlations which are generated by the scalar and pseudoscalar Yukawa couplings. These spin effects can be traced in $\ensuremath{\varphi}\ensuremath{\rightarrow}{\ensuremath{\tau}}^{\ensuremath{-}}{\ensuremath{\tau}}^{+}$ and, for heavy Higgs bosons, in $\ensuremath{\varphi}\ensuremath{\rightarrow}t\overline{t}$. We also calculate resulting correlations among the final states and estimate, for the respective decay modes, the number of events needed to measure the Yukawa couplings with these correlations at the 3$\ensuremath{\sigma}$ level.