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QCD Corrections to Decay Distributions of Neutral Higgs Bosons with (In)definite CP Parity

We compute the order $α_s$ QCD corrections to the density matrix for the decay of a neutral Higgs boson $ϕ$ with (in)definite CP parity into a quark antiquark pair, respectively 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 $ϕ\to τ^-τ^+$ and, for heavy Higgs bosons, in $ϕ\to t\bar{t}$. 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 sigma level.