Precise determination of αs from the C-parameter distribution

We present a global fit for αs(mZ), analyzing the available C-parameter data measured at center-of-mass energies between Q=35 and 207 GeV. The experimental data is compared to a N3LL'+O(αs3)+Ω1 theoretical prediction (up to the missing four-loop cusp anomalous dimension), which includes power corrections coming from a field-theoretical nonperturbative soft function. The dominant hadronic parameter is its first moment Ω1, which is defined in a scheme which eliminates the O(ΛQCD) renormalon ambiguity. The resummation region plays a dominant role in the C-parameter spectrum, and in this region a fit for αs(mZ) and Ω1 is sufficient. We find αs(mZ)=0.1123±0.0015 and Ω1=0.421±0.063 GeV with χ2/d.o.f.=0.988 for 404 bins of data. These results agree with the prediction of universality for Ω1 between thrust and C-parameter within 1-σ.