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 α_s(M_Z). 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 α_s and PDF parameters. This method is based on a statistical relation that allows one to add the uncertainty produced by α_s, computed with some special PDF sets, in quadrature with the PDF uncertainty obtained for the fixed α_s value (such as the CTEQ6.6 PDF set). A series of four CTEQ6.6AS PDFs realizing this approach, for α_s values in the interval 0.116≤α_s(M_Z)≤0.120, is presented. Using these PDFs, the combined α_s and PDF uncertainty is assessed for theoretical predictions at the Fermilab Tevatron and Large Hadron Collider.