Thorne, RS; (2013) Theoretical Procedures and the effect on PDFs and α_S(M_Z^2). In: (Proceedings) XXI International Workshop on Deep-Inelastic Scattering and Related Subject -DIS2013. Proceedings of Science (In press).

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Abstract

I consider the effect on partons distribution functions (PDFs) of changes in the theoretical procedure used in a PDF fit. I consider using the 3-flavour fixed flavour number scheme instead of the standard general mass variable flavour number scheme used in the MSTW analysis. This results in the light quarks increasing at most x values, the gluon distribution becoming softer at high values of x and larger at small x, and the coupling constant \alpha_S(M_Z^2) falling, particularly at NNLO. The fit quality also deteriorates.I also consider lowering the kinematic cut on W² for DIS data and introducing higher twist terms which are fit to data. This results in much smaller effects on both PDFs and \alpha_S(M_Z^2), with changes generally smaller than uncertainties, except for quarks at very high x. I show that the fixed flavour scheme and variable flavour scheme structure functions differ significantly for x∼0.01 at high Q². I demonstrate that in the fixed flavour scheme there is a slow convergence of large logarithmic terms of the form (\alpha_S\ln(Q^2/m_c^2))^n in this regime. I conclude that some major differences in PDF sets are largely due to the choice of flavour scheme used.

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