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It is argued that the parton distribution function Pa/A(x,→kT;ζ) is well defined and useful in quantum chromodynamics. Here ζ=(2PA·n)²(-n²) arises as a variable because of the use of the axial gauge n·A=0. An approximation for Pa/A(x,kT;ζ) is given which is obtained from an approximate solution of the Bethe-Salpeter equation in the axial-gauge ladder model.
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Davison E. Soper (1979) studied this question.
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