Complete O(αs4) QCD Corrections to Hadronic Z Decays

Corrections of order α_s⁴ for the axial singlet contributions for the decay rate of the Z boson into hadrons are evaluated in the limit of the heavy top quark mass. Combined with recently finished O(α_s⁴) calculations of the nonsinglet corrections, the new results directly lead us to the first complete O(α_s⁴) prediction for the total hadronic decay rate of the Z boson. The new O(α_s⁴) term in Z-decay rate leads to a significant stabilization of the perturbative series, to a reduction of the theory uncertainty in the strong coupling constant α_s, as extracted from these measurements, and to a small shift of the central value.