State-of-the-Art Predictions for C-parameter and a Determination of α _s

The C-parameter event-shape distribution for e ⁺e ^- annihilation into hadrons is computed in the framework of SCET including input from fixed-order perturbation theory. We calculate all missing ingredients for achieving N ³LL resummation accuracy in the cross section, which is then matched onto O(α _s ³) fixed-order results. Hadronization power corrections are incorporated as a convolution with a nonperturbative shape function. Wide-angle soft radiation effects introduce an O(Λ _QCD) renormalon ambiguity in the cross section, which we cure by switching to the Rgap short-distance scheme. We also include hadron mass effects, but find their effect is rather small. Performing fits to the tail of the C-parameter distribution for many center of mass energies we find that the strong coupling constant is α _s(m _Z)=0.1121±0.0015, with χ ²/dof=0.99.