Precision NNLO determination of alpha_s(M_Z) using an unbiased global parton set

We determine the strong coupling αs at NNLO in perturbative QCD using the global dataset input to the NNPDF2.1 NNLO parton fit: data from neutral and charged current deep-inelastic scattering, Drell–Yan, vector boson production and inclusive jets. We find alpha_s(M_Z)=0.1173+- 0.0007 (stat), where the statistical uncertainty comes from the underlying data and uncertainties due to the analysis procedure are negligible. We show that the distribution of αs values preferred by different experiments in the global fit is statistically consistent, without need for rescaling uncertainties by a “tolerance” factor. We show that if deep-inelastic data only are used, the best-fit value of αs is somewhat lower, but consistent within one sigma with the global determination. We find that the shift between the NLO and NNLO values of αs ∆α pert s = 0.0018, and we estimate the uncertainty from higher-order corrections to be ∆α NNLO s ∼ 0.0009.