Impact of the heavy-quark matching scales in PDF fits

We investigate the impact of displaced heavy-quark matching scales in a global fit. The heavy-quark matching scale μ_m determines at which energy scale μ the QCD theory transitions from N_F to N_F + 1 in the variable flavor number scheme (VFNS) for the evolution of the parton distribution functions (PDFs) and strong coupling α_S(μ). We study the variation of the matching scales, and their impact on a global PDF fit of the combined HERA data. As the choice of the matching scale μ_m effectively is a choice of scheme, this represents a theoretical uncertainty; ideally, we would like to see minimal dependence on this parameter. For the transition across the charm quark (from N_F = 3 to 4), we find a large μ_m = μ_c dependence of the global fit χ² at NLO, but this is significantly reduced at NNLO. For the transition across the bottom quark (from N_F = 4 to 5), we have a reduced μ_m = μ_b dependence of the χ² at both NLO and NNLO as compared to the charm. This feature is now implemented in xFitter 2.0.0, an open source QCD fit framework. (orig.)