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Sept 2003PHYSTAT11 … of short-distance processes using perturbative QCD (NLO) The challenge of Global Analysis is to construct a set of PDF’s with good agreement between data and theory, for many disparate experiments. Global Analysis

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Sept 2003PHYSTAT12 Perfect agreement for all experiments simultaneously will not be possible because of systematic errors – both experimental and theoretical. Our philosophy is to make compromises between different experiments that are not perfectly compatible; to get satisfactory * agreement with every data set. * which needs to be defined The problem of balance. How much weight should be given to each of two competing experiments?

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Sept 2003PHYSTAT13 The program of Global Analysis is not a routine statistical analysis, because of systematic differences between experiments. We must sometimes use physics judgement in this complex real-world problem.

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Sept 2003PHYSTAT14 Parametrization At low Q 0, of order 1 GeV, P(x) has a few more parameters for increased flexibility. ~ 20 free shape parameters Q dependence of f(x,Q) is obtained by solving the QCD evolution equations (DGLAP).

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Sept 2003PHYSTAT15 CTEQ6 -- Table of experimental data sets H1 (a) 96/97 low-x e+p data ZEUS 96/97 e+p data H1 (b) 98/99 high-Q e-p data D0 : d 2 /d dpT

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Sept 2003PHYSTAT16 Global Analysis data from many disparate experiments

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Sept 2003PHYSTAT17 The Parton Distribution Functions

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Sept 2003PHYSTAT18 Different ways to plot the parton distributions Linear Logarithmic Q 2 = 10 (solid) and 1000 (dashed) GeV 2

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Sept 2003PHYSTAT19 In order to show the large and small x regions simultaneously, we plot 3x 5/3 f(x) versus x 1/3. {Integral = momentum fraction}

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Sept 2003PHYSTAT20 Comparison of CTEQ5 and CTEQ6 Adding new data and applying new methods of analysis — how much change results in the final PDF’s?

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Sept 2003PHYSTAT21 Comparison of CTEQ6 and MRST2002 blue curves : CTEQ6M black dots : MRTS2002 gluon and u quark at Q 2 = 10 GeV 2

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