Uncertainty induced by QCD coupling in the CTEQ global analysis of parton distributions

We examine the dependence of parton distribution functions (PDFs) on the value of the QCD coupling strength αₛ(MZ). We explain a simple method that is rigorously valid in the quadratic approximation normally applied in PDF fitting, and fully reproduces the correlated dependence of theoretical cross sections on αₛ and PDF parameters. This method is based on a statistical relation that allows one to add the uncertainty produced by αₛ, computed with some special PDF sets, in quadrature with the PDF uncertainty obtained for the fixed αₛ value (such as the CTEQ6.6 PDF set). A series of four CTEQ6.6AS PDFs realizing this approach, for αₛ values in the interval 0.116≤αₛ(MZ)≤0.120, is presented. Using these PDFs, the combined αₛ and PDF uncertainty is assessed for theoretical predictions at the Fermilab Tevatron and Large Hadron Collider.