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 [Formula: see text] determines at which energy scale [Formula: see text] the QCD theory transitions from [Formula: see text] to [Formula: see text] in the variable flavor number scheme (VFNS) for the evolution of the parton distribution functions (PDFs) and strong coupling [Formula: see text] . 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 [Formula: see text] 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 [Formula: see text] to 4), we find a large [Formula: see text] dependence of the global fit [Formula: see text] at NLO, but this is significantly reduced at NNLO. For the transition across the bottom quark (from [Formula: see text] to 5), we have a reduced [Formula: see text] dependence of the [Formula: see text] 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.