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

Abstract

We examine the dependence of parton distribution functions (PDFs) on the\nvalue of the QCD coupling strength $\\alpha_{s}(M_{Z})$. We explain a simple\nmethod that is rigorously valid in the quadratic approximation normally applied\nin PDF fitting, and fully reproduces the correlated dependence of theoretical\ncross sections on $\\alpha_s$ and PDF parameters. This method is based on a\nstatistical relation that allows one to add the uncertainty produced by\n$\\alpha_s$, computed with some special PDF sets, in quadrature with the PDF\nuncertainty obtained for the fixed $\\alpha_s$ value (such as the CTEQ6.6 PDF\nset). A series of four CTEQ6.6AS PDFs realizing this approach, for $\\alpha_s$\nvalues in the interval $0.116 \\leq \\alpha_{s}(M_{Z}) \\leq 0.120$, is presented.\nUsing these PDFs, the combined $\\alpha_{s}$ and PDF uncertainty is assessed for\ntheoretical predictions at the Fermilab Tevatron and Large Hadron Collider.\n