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1cteq ss 11 B. Properties of the PDFs -- Definitions First, what are the Parton Distribution Functions? (PDFs) The PDFs are a set of 11 functions, f i (x,Q 2 ) where longitudinal momentum fraction momentum scale f 0 = g(x,Q 2 ) the gluon PDF f 1 = u(x,Q 2 ) the up-quark PDF f -1 = u(x,Q 2 ) the up-antiquark PDF f 2 = d and f -2 = d f 3 = s and f -3 = s etc parton index Exercise: What about the top quark? B1

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2cteq ss 11 B. Properties of the PDFs Second, what in the meaning of a PDF? We tend to think and speak in terms of “Proton Structure” u(x,Q 2 ) dx = the mean number of up quarks with longitudinal momentum fraction from x to x + dx, appropriate to a scattering experiment with momentum transfer Q. u(x,Q 2 ) = the up-quark density in momentum fraction This heuristic interpretation makes sense from the LO parton model. More precisely, taking account of QCD interactions, d proton = PDF C. B1

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3cteq ss 11 f i (x,Q 2 ) = the density of parton i w.r.t. longitudinal momentum fraction x longitudinal momentum fraction, carried by parton type i valence up quark density, valence down quark density, valence strange quark density B1

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4cteq ss 11 Example. DIS of electrons by protons; e.g., HERA experiments (summed over flavors!) in lowest order of QCD B1

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cteq ss 115 But QCD radiative corrections must be included to get a sufficiently accurate prediction. The NLO approximation will involve these interactions … From these perturbative calculations, we determine the coefficient functions C i (NLO), and hence write Approximations available today: LO, NLO, NNLO B1

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cteq ss 116 The Factorization Theorem For short-distance interactions, and the PDFs are universal ! We can write a formal, field-theoretic expression, although we can’t evaluate it because we don’t know the bound state |p>. B1 B. Properties of the PDFs

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cteq ss 117 Exercise. Suppose the parton densities for the proton are known, (A) In terms of the f i (x,Q 2 ), write the 11 parton densities for the neutron, say, g i (x,Q 2 ). (B) In terms of the f i (x,Q 2 ), write the 11 parton densities for the deuteron, say, h i (x,Q 2 ). B1 B. Properties of the PDFs

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cteq ss 118B1

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