Uncertainties on alpha_S in global PDF analyses and implications for predicted hadronic cross sections

TL;DR

This paper quantifies the uncertainty in the strong coupling constant alpha_S from experimental data, explores its interplay with PDF uncertainties, and provides tools for combined uncertainty analysis in hadronic cross section predictions.

Contribution

It introduces a method to simultaneously account for alpha_S and PDF uncertainties in cross section calculations and provides eigenvector PDF sets with varying alpha_S values.

Findings

Uncertainty in alpha_S(M_Z^2) is about +0.0012/-0.0015 at NLO and ±0.0014 at NNLO.

Estimated theoretical uncertainty on alpha_S(M_Z^2) is about ±0.003 at NLO.

Provided PDF sets enable combined uncertainty analysis for collider cross sections.

Abstract

We determine the uncertainty on the strong coupling alpha_S due to the experimental errors on the data fitted in global analysis of hard-scattering data, within the standard framework of leading-twist fixed-order collinear factorisation in the MSbar scheme, finding that alpha_S(M_Z^2) = 0.1202^{+0.0012}_{-0.0015} at next-to-leading order (NLO) and alpha_S(M_Z^2) = 0.1171^{+0.0014}_{-0.0014} at next-to-next-to-leading order (NNLO). We do not address in detail the issue of the additional theory uncertainty on alpha_S(M_Z^2), but an estimate is +/-0.003 at NLO and at most +/-0.002 at NNLO. We investigate the interplay between uncertainties on alpha_S and uncertainties on parton distribution functions (PDFs). We show, for the first time, how both these sources of uncertainty can be accounted for simultaneously in calculations of cross sections, and we provide eigenvector PDF sets with different fixed alpha_S values to allow further studies by the general user. We illustrate the application of these PDF sets by calculating cross sections for W, Z, Higgs boson and inclusive jet production at the Tevatron and LHC.