PDF Parametrization issues
Jon Pumplin · 2008
The role of parametrization dependence in the measurement of parton distributions is discussed, using the strangeness component in the CTEQ6.6 fit as an example. A method to impose theoretical constraints in PDF fitting is proposed. The Regge behavior of PDFs is examined. 1 Parametrization dependence In the standard PDF fitting paradigm, one parametrizes each flavor fa(x, Q0) at a small Q0 using currently a total of about 20 parameters. One then computes the PDFs fa(x, Q) for Q> Q0 by DGLAP evolution; followed by the cross section predictions for DIS(e,µ,ν), Drell-Yan, inclusive jets,... by perturbation theory; followed by a “χ 2 ” measure of agreement between the predictions and measurements. One minimizes χ 2 with respect to the parameters at Q0 to find Best Fit PDFs, and explores the neighborhood of the minimum via eigenvector sets to estimate an uncertainty range in which all of the data sets are described tolerably well. (Weight factors are included in the definition of χ 2 to keep experiments with a small number of data points from being unduly neglected.) Parametrization dependence is the systematic error caused by making specific choices of