Precision determination of the strong coupling constant within a global PDF analysis

Abstract

We present a determination of the strong coupling constant ${\alpha }_s(m_Z)$ based on the NNPDF3.1 determination of parton distributions, which for the first time includes constraints from jet production, top-quark pair differential distributions, and the $Z$ $p_T$ distributions using exact NNLO theory. Our result is based on a novel extension of the NNPDF methodology - the correlated replica method - which allows for a simultaneous determination of ${\alpha }_s$ and the PDFs with all correlations between them fully taken into account. We study in detail all relevant sources of experimental, methodological and theoretical uncertainty. At NNLO we find ${\alpha }_s(m_Z)=0.1185\pm 0.0005^{\text{(exp)}}\pm 0.0001^{\text{(meth)}}$, showing that methodological uncertainties are negligible. We conservatively estimate the theoretical uncertainty due to missing higher order QCD corrections (N${}^3$LO and beyond) from half the shift between the NLO and NNLO ${\alpha }_s$ values, finding $\Delta {\alpha }_s^{\mathrm{th}}=0.0011$.

23 pages, 10 figures.Final version, published in EPJC. New plots added to figs 3.5 and 3.6 and improved discussion of the d'Agostini bias. Discussion of ABMP clarified. Some references added

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