Abstract
I consider the effect on MSTW partons distribution functions (PDFs) due to changes in the choices of theoretical procedure used in the fit. I first 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 all relatively small x values, the gluon distribution becoming smaller at high values of x and larger at small x, the preferred value of the coupling constant alpha _S(M_Z^2) falling, particularly at NNLO, and the fit quality deteriorates. I also consider lowering the kinematic cut on W^2 for DIS data and simultaneously introducing higher twist terms which are fit to data. This results in much smaller effects on both PDFs and alpha _S(M_Z^2) than the scheme change, except for quarks at very high x. I show that the structure function one obtains from a fixed input set of PDFs using the fixed flavour scheme and variable flavour scheme differ significantly for x sim 0.01 at high Q^2, and that this is due to the fact that in the fixed flavour scheme there is a slow convergence of large logarithmic terms of the form (alpha _Sln (Q^2/m_c^2))^n relevant for this regime. I conclude that some of the most significant differences in PDF sets are largely due to the choice of flavour scheme used.
Highlights
I first examine the number of active quark flavours used in the calculation of structure functions
In this article I have investigated whether the different theoretical choices in fits to data in order to determine partons distribution functions (PDFs) can influence the PDFs, the value of αS(MZ2 ) and the fit quality
I come to the strong conclusion that within the context of the MSTW2008 global fit the choice of a Fixed Flavour Number Scheme (FFNS) for heavy flavour production in deep inelastic scattering, as opposed to a GM-VFNS, leads to a lower αS
Summary
I first examine the number of active quark flavours used in the calculation of structure functions. The fit is often slightly better for the F2c(x, Q2) itself, but the total F2(x, Q2) is flatter in Q2 for x ∼ 0.01, and this worsens the fit to HERA inclusive structure function data For both GM-VFNS and FFNS, and at both NLO and NNLO, the fit quality to DIS data deteriorates by about 30 units when the fixed target Drell Yan data is added, showing that there is some tension in quarkantiquark decomposition between DIS and fixed-target Drell. VFNS fits for the DIS and DIS plus Drell Yan fits, but higher when the jet data is added, though the NNLO FFNS values are relatively slightly lower compared to GM-VFNS than the NLO values
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